The years which have passed since Encke died have witnessed notable changes in the aspect of the science he loved so well. But we must look back over more than half a century, if we would form an estimate of the position of astronomy when Encke’s most notable work was achieved. At Seeberge, under Lindenau, Encke had been perfecting himself in the higherbranches of mathematical calculation. He took the difficult work of determining the orbital motions of newly-discovered comets under his special charge, and Dr. Bruhns tells us that every comet which was detected during Encke’s stay at Seeberge was subjected to rigid scrutiny by the indefatigable mathematician. Before long a discovery of the utmost importance rewarded his persevering labours. Pons had detected on November 26, 1818, a comet of no very brilliant aspect, which was watched first at Marseilles, and then at Mannheim, until December 29. Encke next took up the work, and tracked the comet until January 12. Combining the observations made between December 22 and January 12, he assigned to the body a parabolic orbit. But he was not satisfied with the accordance between this path and the observed motions of the body. When he attempted to account for the motions of the comet by means of an orbit of comparatively short period, he was struck by the resemblance between the path thus deduced and that of Comet I, 1805. Gradually the idea dawned upon him that a new era was opening for science. Hitherto the only periodical comets which had been discovered except Lexell’s—the ‘lost comet’—had travelled in orbits extending far out into space beyond the paths of the most distant known planets. But now Encke saw reason to believe that he had to deal with a comet travelling within the orbit of Jupiter. On February 5, he wrote to the eminent mathematician Gauss, pointing out the results of his inquiries, and saying that he only waited for the encouragement and authority of his former teacher to prosecute his researches to the end towards which they already seemed to point. Gauss, in reply, not only encouraged Encke to proceed, but counselled him as to the course he should pursue. The result we all know. Encke showed conclusively that the newly-discovered comet travels in a path of short period, and that it had already made its appearance several times in our neighbourhood.
From the date of this discovery, Encke took high rank among the astronomers of Europe. His subsequent labours by no means fell short of the promise which this, his first notable achievement, had afforded. If he effected less as an astronomical observer than many of his contemporaries, he was surpassed by few as a manipulator of those abstruse formulæ by which the planetary perturbations are calculated. It was to the confidence engendered by this skill that we owe his celebrated discovery of the acceleration of the motion of the comet mentioned above. Assured that he had rightly estimated the disturbances to which the comet is subjected, he was able to pronounce confidently that some cause continually (though all but imperceptibly) impedes the passage of this body through space, and so—by one of those strange relations which the student of astronomy is familiar with—the continually retarded comet travels ever more swiftly along a continually diminishing orbit.
Bruhns’ Life of Encke is well worth reading, not only by those who are interested in Encke’s fame andwork as an astronomer, but by the general reader. Encke the man is presented to our view, as well as Encke the astronomer. With loving pains the pupil of the great astronomer handles the theme he has selected. The boyhood of Encke, his studies, his soldier life in the great uprising against Napoleon in 1813, and his work at the Seeberge Observatory; his labours on comets and asteroids; his investigations of the transits of 1761 and 1769; his life as an academician, and as director of an important observatory; his orations at festival and funeral; and lastly, his illness and death, are described in these pages by one who held Encke in grateful remembrance as ‘teacher and master,’ and as a ‘fatherly friend.’
Not the least interesting feature of the work is the correspondence introduced into its pages. We find Encke in communication with Humboldt, with Bessel and Struve, with Hansen, Olbers, and Argelander; with a host, in fine, of living as well as of departed men of science.
(FromNature, March 10, 1870.)
More than a century ago scientific men were looking forward with eager interest to the passage of the planet Venus across the sun’s face in 1769. The Royal Society judged the approaching event to be ofsuch extreme importance to the science of astronomy that they presented a memorial to King George III., requesting that a vessel might be fitted out, at Government expense, to convey skilful observers to one of the stations which had been judged suitable for observing the phenomenon. The petition was complied with, and after some difficulty as to the choice of a leader, the good ship ‘Endeavour,’ of 370 tons, was placed under the command of Captain Cook. The astronomical work entrusted to the expedition was completely successful; and thus it was held that England had satisfactorily discharged her part of the work of utilising the rare phenomenon known as a transit of Venus.
A century passed, and science was again awaiting with interest the approach of one of these transits. But now her demands were enlarged. It was not one ship that was asked for, but the full cost and charge of several expeditions. And this time, also, science had been more careful in taking time by the forelock. The first hints of her requirements were heard some fourteen years ago, when the Astronomer-Royal began that process of laborious inquiry which a question of this sort necessarily demands. Gradually, her hints became more and more plain-spoken; insomuch that Airy—her mouthpiece in this case—stated definitely in 1868 what he thought science had a right to claim from England in this matter. When the claim came before our Government, it was met with a liberality which was a pleasing surprise after some former placid references of scientific people to their own devices. The sumof ten thousand five hundred pounds was granted to meet the cost of several important and well-appointed expeditions; and further material aid was derived from the various Government observatories.
And now let us inquire why so much interest is attached to a phenomenon which appears, at first sight, to be so insignificant. Transits, eclipses, and other phenomena of that nature are continually occurring, without any particular interest being attached to them. The telescopist may see half-a-dozen such phenomena in the course of a night or two, by simply watching the satellites of Jupiter, or the passage of our moon over the stars. Even the great eclipse of 1868 did not attract so much interest as the transit of Venus; yet that eclipse had not been equalled in importance by any which has occurred in historic times, and hundreds of years must pass before such another happens, whereas transits of Venus are far from being so uncommon.
The fact is, that Venus gives us the best means we have of mastering a problem which is one of the most important within the whole range of the science of astronomy. I use the term important, of course, with reference to the scientific significance and interest of the problem. Practically, it matters little to us whether the sun is a million of miles or a thousand millions of miles from us. The subject must in any case be looked upon as an extra-parochial one. But science does occasionally attach immense interest to extra-parochial subjects. And this is neither unwisenor unreasonable, since we find implanted in our very nature—and not merely in the nature of scientific men—a quality which causes us to take interest in a variety of matters that do not in the least concern our personal interests. Nor is this quality, rightly considered, one of the least noble characteristics of the human race.
That the determination of the sun’s distance is important, in an astronomical sense, will be seen at once when it is remembered that the ideas we form of the dimensions of the solar system are wholly dependent on our estimate of the sun’s distance. Nor can we gauge the celestial depths with any feeling of assurance, unless we know the true length of that which is our sole measuring-rod. It is, in fact, our basis of measurement for the whole visible universe. In some respects, even if we knew the sun’s distance exactly, it would still be an unsatisfactory gauge for the stellar depths. But that is the misfortune, not the fault, of the astronomer, who must be content to use the measuring-rod which nature gives him. All he can do is to find out as nearly as possible its true length.
When we come to consider how the astronomer is to determine this very element—the sun’s distance—we find that he is hampered with a difficulty of precisely the same character.
The sun being an inaccessible object, the astronomer can apply no other methods to determine its distance—directly—than those which a surveyor would use in determining the distance of an inaccessible castle, orrock, or tree, or the like. We shall see presently that the ingenuity of astronomers has, in fact, suggested some other indirect methods. But clearly the most satisfactory estimate we can have of the sun’s distance is one founded on such simple notions and involving in the main such processes of calculation as we have to deal with in ordinary surveying.
There is, in this respect, no mystery about the solution of the famous problem. Unfortunately, there is enormous difficulty.
When a surveyor has to determine the distance of an inaccessible object, he proceeds in the following manner. He first very carefully measures a base-line of convenient length. Then from either end of the base-line he takes the bearing of the inaccessible object—that is, he observes the direction in which it lies. It is clear that, if he were now to draw a figure on paper, laying down the base-line to some convenient scale, and drawing lines from its ends in directions corresponding to the bearings of the observed object, these lines would indicate, by their intersection, the true relative position of the object. In practice, the mathematician does not trust to so rough a method as construction, but applies processes of calculation.
Now, it is clear that in this plan everything depends on the base-line. It must not be too short in comparison with the distance of the inaccessible object; for then, if we make the least error in observing the bearings of the object, we get an important error in the resulting determination of the distances. The reader can easilyconvince himself of this by drawing an illustrative case or two on paper.
The astronomer has to take his base-line for determining the sun’s distance, upon our earth, which is quite a tiny speck in comparison with the vast distance which separates us from the sun. It had been found difficult enough to determine the moon’s distance with such a short base-line to work from. But the moon is only about a quarter of a million of miles from us, while the sun is more than ninety millions of miles off. Thus the problem was made several hundred times more difficult—or, to speak more correctly, it was rendered simply insoluble unless the astronomer could devise some mode of observing which should vastly enhance the power of his instruments.
For let us consider an illustrative case. Suppose there was a steeple five miles off, and we had a base-line only two feet long. That would correspond as nearly as possible to the case the astronomer has to deal with. Now, what change of direction could be observed in the steeple by merely shifting the eye along a line of two feet? There is a ready way of answering. Invert the matter. Consider what a line of two feet long would look like if viewed from a distance of five miles. Would its length be appreciable, to say nothing of its being measurable? Yet it is just such a problem as the measurement of that line which the astronomer would have to solve.
But even this is not all. In our illustration only one observer is concerned, and he would be able to useone set of instruments. Suppose, however, that from one end of the two-feet line an observer using one set of instruments took the bearings of the steeple; and that, half a year after, another observer brought another set of instruments and took the bearing of the steeple from the other end of the two-feet line, is it not obvious how enormously the uncertainty of the result would be increased by such an arrangement as this? One observer would have his own peculiar powers of observation, his own peculiar weaknesses: the other would have different peculiarities. One set of instruments would be characterised by its own faults or merits, so would the other. One series of observations would be made in summer, with all the disturbing effects due to heat; the other would be made in winter, with all the disturbing effects due to cold.
The observation of the sun is characterised by all these difficulties. Limited to the base-lines he can measure on earth, the astronomer must set one observer in one hemisphere, another in the other. Each observer must have his own set of instruments; and every observation which one has made in summer will have to be compared with an observation which the other has made in winter.
Thus we can understand that astronomers should have failed totally when they attempted to determine the sun’s distance without aid from the other celestial bodies.
It may seem at first sight as though nothing the other celestial bodies could tell the astronomer wouldbe of the least use to him, since these bodies are for the most part farther off than the sun, and even those which, approach nearest to us are still far beyond the limits of distance within which the simple plan followed by surveyors could be of any service. And besides, it might be supposed that information about the distance of one celestial body could be of no particular service towards the determination of the distance of another.
But two things aid the astronomer at this point. First of all, he has discovered the law which associates together the distances of all the planets from the sun; so that if he can determine the distance of any one planet, he learns immediately the distances of all. Secondly, the planets in their motion travel occasionally into such positions that they become mighty indices, tracing out on a natural dial-plate the significant lesson from which the astronomer hopes to learn so much. To take an instance from the motions of another planet than the one we are dealing with. Mars comes sometimes so near the earth that the distance separating us from him is little more than one-third of that which separates us from the sun. Suppose that, at such a time, he is seen quite close to a fixed star. That star gives the astronomer powerful aid in determining the planet’s distance. For, to observers in some parts of the earth, the planet will seem nearer to the star than he will to observers elsewhere. A careful comparison of the effects thus exhibited will give significant evidence respecting the distance of Mars. And we see that the star has served as a fixed mark upon the vastnatural dial of the heavens, just as the division-marks on a clock-face serve to indicate the position of the hands.
Now we can at once see why Venus holds so important a position in this sort of inquiry. Venus is our nearest neighbour among the planets. She comes several millions of miles nearer to us than Mars, our next neighbour on the other side. That is the primary reason of her being so much considered by astronomers. But there is another of equal importance. Venus travels nearer than our earth to the sun. And thus there are occasions when she gets directly between the earth and the sun. At those times she is seen upon his face, and his face serves as a dial-plate by which to measure her movements. When an observer at one part of the earth sees her on one part of the sun’s face, another observer at some other part of the earth will see her on another, and the difference of position, if accurately measured, would at once indicate the sun’s distance. As a matter of fact, other modes of reading off the indications of the great dial-plate have to be adopted. Before proceeding to consider those modes, however, we must deal with one or two facts about Venus’s movements which largely affect the question at issue.
Let us first see what we gain by considering the distance of Venus rather than that of the sun.
At the time of a transit Venus is of course on a line between the earth and the sun, and she is at somewhat less than a third of the sun’s distance from us. Thuswhatever effect an observer’s change of place would produce upon the sun would be more than trebled in the case of Venus. But it must not be forgotten that we are to judge the motions of Venus by means of the dial-plate formed by the solar disc, and that dial-plate is itself shifted as the observer shifts his place. Venus is shifted three times as much, it is true; but it is only the balance of change that our astronomer can recognise. That balance is, of course, rather more than twice as great as the sun’s change of place.
So far, then, we have not gained much, since it has been already mentioned that the sun’s change of place is not measurable by any process of observation astronomers can apply.
It is to the fact that we have the sun’s disc, whereby to measure the change, that we chiefly trust; and even that would be insufficient were it not for the fact that Venus is not at rest, but travels athwart the great solar dial-plate. We are thus enabled to make a time measurement take the place of a measurement of space. If an observer in one place sees Venus cross the sun’s face at a certain distance from the centre, while an observer at another place sees her follow a path slightly farther from the centre, the transit clearly seems longer to the former observer than to the latter.
This artifice of exchanging a measurement of time for one of space—orvice versâ—is a very common one among astronomers. It was Edmund Halley, the friend and pupil of Sir Isaac Newton, who suggested its application in the way above described. It will be noticedthat what is required for the successful application of the method is that one set of observers should be as far to the north as possible, another as far to the south, so that the path of Venus may be shifted as much as possible. Clearly the northern observers will see her path shifted as much to the south as it can possibly be, while the southern observers will see the path shifted as far as possible towards the north.
One thing, however, is to be remembered. A transit lasts several hours, and our observers must be so placed that the sun will not set during these hours. This consideration sometimes involves a difficulty. For our earth does not supply observing room all over her surface, and the region where observation would be most serviceable may be covered by a widely-extended ocean. Then again, the observing parties are being rapidly swayed round by the rotating earth and it is often difficult to fix on a spot which may not, through this cause, be shifted from a favourable position at the beginning of the transit to an unfavourable one at the end.
Without entering on all the points of difficulty involved by such considerations as these, I may simply indicate the fact that the astronomer has a problem of considerable complexity to solve in applying Halley’s method of observation to a transit of Venus.
It was long since pointed out by the French astronomer Delisle that the subject may be attacked another way—that, in fact, instead of noticing how much longer the transit lasts in some places than in others, the astronomer may inquire how much earlier it begins or ends in some places than in others.
Here is another artifice, extremely simple in principle, though not altogether so simple in its application. My readers must bear with me while I briefly describe the qualities of this second method, because in reality the whole question of the transit, and all the points which have to be attended to in the equipment and placing of the various observing parties, depend on these preliminary matters. Without attending to them—or at least to such primary points as I shall select—it would be impossible to form a clear conception of the circumstances with which astronomers have to deal. There is, however, no real difficulty about this part of the subject, and I shall only ask of the reader to give his attention to it for a very brief space of time.
Suppose the whole of that hemisphere of the earth on which the sun is shining when the transit is about to begin were covered with observers waiting for the event. As Venus sweeps rapidly onwards to the critical part of her path, it is clear that some of these observers will get an earlier view of the commencement of the transit than others will; just as at a boat-race, persons variously placed round a projecting corner of the course see the leading boat come into view at different times. Some one observer on the outer rim of the hemisphere would be absolutely the first to see the transit begin. Then rapidly other observers would see the phenomenon; and in the course of a few minutessome one observer on the outer rim of the hemisphere—almost exactly opposite the first—would be absolutely the last to see the transit begin. From that time the transit would be seen by all for several hours—I neglect the earth’s rotation, for the moment—but the end of the transit, like the beginning, would not be seen simultaneously by the observers. First one would see it, then in succession the rest, and last of all an observer almost exactly opposite the first.
Now, here we have had to consider four observers who occupy exceptional positions. There is (1) the observer who sees the transit begin earliest, (2) the one who sees it begin latest, (3) the one who sees it end earliest, and (4) the one who sees it end latest. Let us consider the first two only. Suppose these two observers afterwards compared notes, and found out what was the exact difference of time between their respective observations. Is it not clear that the result would at once afford the means of determining the sun’s distance? It would be the simplest of all possible astronomical problems to determine over what proportion of her orbit Venus passed in the interval of time which elapsed between these observations; and the observers would now have learned that that portion of Venus’s orbit is so many miles long, for they know what distance separated them, and it would be easy to calculate how much less that portion of Venus’s orbit is. Thus they would learn what the length of her whole orbit is, thence her distance from the sun, and thence the sun’s distance from us.
The two observers who saw the transit end earliest and latest could do the like.
Speaking generally, and neglecting all the complexities which delight the soul of the astronomer, this is Delisle’s method of utilising a transit. It has obviously one serious disadvantage as compared with the other. An observer at one side of the earth has to bring his observations into comparison with those made by an observer at the other side of the earth. Each uses the local time of the place at which he observes, and it has been calculated that for the result to be of value there must not be an error of a single second in their estimates of local time. Now, does the reader appreciate the full force of this proviso? Each observer must know so certainly in what exact longitude he is, that his estimate of the time when true noon occurs shall not be one second wrong! This is all satisfactory enough in places where there are regular observatories. But matters are changed when we are dealing with such places as Woahoo, Kerguelen Land, Chatham Island, and the wilds of Siberia.
In the transit3of 1874 there are many such difficulties to be encountered. In fact, it is almost impossible to conceive a transit the circumstances of which are more inconvenient. On the other hand, however, the transit is of such a nature that if once the preliminary difficulties are overcome, we can hope more from its indications than from those of any other transit which will happen in the course of the next few centuries.
The transit will begin earliest for observers in the neighbourhood of the Sandwich Islands, latest for observers near Crozet Island, far to the south-east of the Cape of Good Hope. It ends earliest for observers far to the south-west of Cape Horn, latest for observers in the north-eastern part of European Russia. Thus we see that, so far as the application of our second method is concerned, the suitable spots are not situated in the most inviting regions of the earth’s surface. As the transit happens on December 8, 1874, the principal northern stations will be very bleak abodes for the observers. The southern stations are in yet more dreary regions,—notwithstanding the fact that the transit occurs during the summer of the southern hemisphere.
For the application of Halley’s method we require stations where the whole transit will be visible; and as the days are very short at the northern stations in December, it is as respects these that we encounter most difficulty. However, it has been found that many places in Northern China, Japan, Eastern Siberia, and Manchouria are suitable for the purpose. The best southern stations for this method lie unfortunately on the unexplored Antarctic continent and the islands adjacent to it; but Crozet Island, Kerguelen Land, and some other places more easy of access than the Antarcticcontinent, will serve very well. Indeed, England has so many stations to occupy elsewhere that it is doubtful whether she will care to undertake the dangerous and difficult task of exploring the Antarctic wastes to secure the best southern stations. The work may fairly be left to other nations, and doubtless will be efficiently carried out.
What England will actually undertake has not yet been fully decided upon. We may be quite certain that she will send out a party to Woahoo or Hawaii to observe the accelerated commencement of the transit. She will also send observers to watch the retarded commencement, but whether to Crozet Island, Kerguelen Land, Mauritius, or Rodriguez is uncertain. Possibly two parties will be sent out for this purpose, and most likely Rodriguez and Mauritius will be the places selected. It had been thought until lately that the sun would be too low at some of the places when the transit begins, but a more exact calculation of the circumstances of the transit has shown this to be a mistake. Both Crozet Island and Kerguelen Land are very likely to be enveloped in heavy mists when the transit begins—that is, soon after sunrise—hence the choice of Mauritius and Rodriguez as the most suitable station.
England will also be called on to take an important part in observing the accelerated end of the transit. A party will probably be sent to Chatham Island or Campbell Island, not far from New Zealand. It had been thought that at the former island the sun wouldbe too low; but here, again, a more exact consideration of the circumstances of the transit has led astronomers to the conclusion that the sun will be quite high enough at this station.
The Russian observers are principally concerned with the observation of the retarded end of the transit, nearly all the best stations lying in Siberia. But there are several stations in British India where this phase can be very usefully observed; and doubtless the skilful astronomers and mathematicians who are taking part in the survey of India will be invited—as at the time of the great eclipse—to give their services in the cause of science. Alexandria, also, though inferior to several of the Indian stations, will probably be visited by an observing party from England.
It will be seen that England will thus be called on to supply about half-a-dozen expeditions to view the transit. All of these will be sent out in pursuance of Delisle’s mode of utilising a transit, so that, for reasons already referred to, it will be necessary that they should be provided with instruments of the utmost delicacy, and very carefully constructed.4They will have to remain at their several stations for a long time before the transit takes place—several months, at least—so that they may accurately determine the latitude of the temporary observatories they will erect. This is a work requiring skilled observers and recondite processesof calculation. Hence it is that the cost of sending out these observing parties is so considerable.
The only English party which will apply Halley’s method of observation is the one which will be stationed at Mauritius, under Lord Lindsay. This part of their work will be comparatively easy, the method only requiring that the duration of the transit should be carefully timed. In fact, one of the great advantages of Halley’s method is the smallness of the expense it involves. A party might land the day before the transit, and sail away the day after, with results at least as trustworthy as those which a party applying Delisle’s method could obtain after several months of hard work. It is to this, rather than any other cause, that the small expense of the observations made in 1769 is to be referred. And doubtless had it been decided by our astronomical authorities to apply Halley’s method solely or principally, the expense of the transit-observations would have been materially lessened. There would, however, have been a risk of failure through the occurrence of bad weather at the critical stations; whereas now—as other nations will doubtless avail themselves of Halley’s method—the chance that the transit-observations will fail through meteorological causes is very largely diminished. Science will owe much to the generosity of England in this respect.
It is, indeed, only recently that the possibility of applying Halley’s method has been recognised. It had been thought that the method must fail totally in1874. But on a more careful examination of the circumstances of the transit, a French astronomer, M. Puiseux, was enabled to announce that this is not the case. Almost simultaneously I published calculations pointing to a similar result; but having carried the processes a few steps further than M. Puiseux, I was able to show that Halley’s method is not only available in 1874, but is the more powerful method of the two.
Unfortunately, there is an element of doubt in the inquiry, of which no amount of care on the part of our observers and mathematicians will enable them to get rid. I refer to the behaviour of Venus herself. It is to the peculiarity we are now to consider that thequasi-failure of the observations made in 1769 must be attributed. It is true that Mr. Stone, the first-assistant at the Greenwich Observatory, has managed to remove the greater part of the doubts which clouded the results of those observations. But not even his skill and patience can serve to remove the blot which a century of doubt has seemed to throw upon the most exact of the sciences. We shall now show how much of the blame of that unfortunate century of doubt is to be ascribed to Venus.
At a transit, astronomers confine their attention to one particular phase—the moment, namely, when Venus just seems to lie wholly within the outline of the sun’s disc. This at least was what Halley and Delisle both suggested as desirable. Unfortunately, Venus had notbeen consulted, and when the time of the transit came she declined to enter upon or leave the sun’s face in the manner suggested by the astronomers. Consider, for example, her conduct when entering on the sun’s face:—
At first, as the black disc of the planet gradually notched the edge of the sun’s disc, all seemed going on well. But when somewhat more than half of the planet was on the sun’s face, it began to be noticed that Venus was losing her rotundity of figure. She became gradually more and more pear-shaped, until at last she looked very much like a peg-top touching with its point the edge of the sun’s disc. Then suddenly—‘as by a lightning flash,’ said one observer—the top lost its peg, and then gradually Venus recovered her figure, and the transit proceeded without further change on her part until the time came for her to leave the sun’s face, when similar peculiarities took place in a reversed order.
Here was a serious difficulty indeed. For when was the moment of true contact? Was it when the peg-top figure seemed just to touch the edge of the sun? This seemed unlikely, because a moment after the planet was seen well removed from the sun’s edge. Was it when the rotund part of the planet belonged to a figure which would have touched the sun’s edge if the rotundity had been perfect elsewhere? This, again, seemed unlikely, because at this moment the black band connecting Venus and the sun was quitewide. And, besides, if this were the true moment of contact, what eye could be trusted to determine the occurrence of a relation so peculiar? Yet the interval between this phase and the final or peg-top phase lasted several seconds—as many as twenty-two in one instance in 1769—and the whole success of the observation depended on exactness within three or four seconds at the outside.
We know that Venus will act in precisely the same manner in 1874. If we had been induced to hope that improvements in our telescopes would diminish the peculiarity, the observations of the transit of Mercury, in November 1868, would have sufficed to destroy that hope, for even with the all but perfect instruments of the Greenwich Observatory, Mercury assumed the peg-top disguise in the most unpleasing manner.
It may be asked, then, What do astronomers propose to do in 1874 to prevent Venus from misleading them again as she did in 1769? Much has already been done towards this end. Mr. Stone undertook a series of careful researches to determine the law according to which Venus may be expected to behave, or to misbehave herself; and the result is, that he has been able to tell the observers exactly what they will have to look for, and exactly what it is most important that they should record. In 1769, observers recorded their observations in such doubtful terms, owing to their ignorance of the real significance of the peculiarities they witnessed, that the mathematicians who had tomake use of those observations were misled.Hinc illæ lacrymæ.Hence it is that an undeserved reproach has fallen upon the ‘exact science.’
The amount of the error resulting from the misinterpretation of the observations made in 1769 was, however, very small indeed, when its true character is considered. It is, indeed, easy to make the error seem enormous. The sun’s distance came out some four millions of miles too large, and that seems no trifling error. Then, again, the resulting estimate of the distance of Neptune came out more than a hundred million miles too great; while even this enormous error was as nothing when compared with that which resulted when the distances of the fixed stars were considered.
But this is an altogether erroneous mode of estimating the effect of the error. It would be as absurd to count up the number of hairs’ breadth by which the geographer’s estimates of the length and breadth of England may be in error. In all such matters it is relative and not absolute error we have to consider. A microscopist would have made a bad mistake who should over-estimate the length of a fly’s proboscis by a single hair’s breadth; but the astronomer had made a wonderfully successful measurement of the sun’s distance who deduced it within three or four millions of miles of the true value. For it is readily calculable that the error in the estimated relative bearing of the sun as seen from opposite sides of the earth corresponds to the angle which a hair’s breadth subtends when seen from a distance of 125 feet.
The error was first detected when other modes of determining the sun’s distance were applied by the skilful astronomers and physicists of our own day. We have no space to describe as fully as they deserve the ingenious processes by which the great problem has been attacked without aid from Venus. Indeed, we can but barely mention the principles on which those methods depend. But to the reader who takes interest in astronomy, we can recommend no subject as better worth studying than the masterly researches of Foucault, Leverrier, and Hansen upon the problem of the sun’s distance.
The problem has been attacked in four several ways. First, the tremendous velocity of light has been measured by an ingenious arrangement of revolving mirrors; the result combined with the known time occupied by light in travelling across the earth’s orbit immediately gives the sun’s distance. Secondly, a certain irregularity in the moon’s motion, due to the fact that she is most disturbed by the sun when traversing that half of her path which is nearest to him, was pressed into the service with similar results. Thirdly, an irregularity in the earth’s motion, due to the fact that she circles around the common centre of gravity of her own mass and the moon’s, was made a means of attacking the problem. Lastly, Mars, a planet which, as we have already mentioned, approaches us almost as nearly as Venus, was found an efficient ally.
The result of calculations founded on these methodsshowed that the sun’s distance, instead of being about 95,000,000 miles, is little more than 91,500,000 miles. And recently a re-examination of the observations made upon Venus in 1769 led Mr. Stone to believe that they point to a similar result.
Doubtless, however, we must wait for the transit of Venus in 1874 before forming a final decision as to the estimate of the sun’s distance which is to take its place in popular works on astronomy during the next century or so. Nothing but an unlooked-for combination of unfavourable circumstances can cause the failure of our hopes. Certainly, if we should fail in obtaining satisfactory results in 1874, the world will not say that the generosity of the English Government has been in fault, since it would be difficult to find a parallel in the history of modern science to the munificence of the grant which has been made this year for expeditions to observe a phenomenon whose interest and importance are purely scientific.
(FromSt. Paul’s, October 1869.)
It would have been deemed a strange thought in the days of the Tudors to suggest that England’s greatness would one day depend,—or seem to depend,—on her stores of coal, a mineral then regarded as only anunpleasant rival of the wood-log for household fires. When Shakespeare put into the mouth of Faulconbridge the words—
This England never did, nor never shall,Lie at the proud foot of a conqueror,But when it first did help to wound itself,
This England never did, nor never shall,Lie at the proud foot of a conqueror,But when it first did help to wound itself,
This England never did, nor never shall,Lie at the proud foot of a conqueror,But when it first did help to wound itself,
This England never did, nor never shall,
Lie at the proud foot of a conqueror,
But when it first did help to wound itself,
he would have thought it a singular proviso that England should be watchful of her coal stores if she would preserve her position among the nations. And yet there is a closer connection between the present greatness of Britain and the mighty coal cellars underlying certain British counties than we are commonly prepared to acknowledge. Saxon steadiness and Norman energy have doubtless played their part in placing Britain in the position she now holds; but whatever may have been the case in past ages of our history, it is certain that at present there is much truth in Liebig’s assertion that England’s power is in her coal. The time may come again, as the time has been, when we shall be less dependent on our coal stores,—when bituminous bankruptcy will not be equivalent to national bankruptcy; but if all our coal mines were at this moment rendered unworkable, the power of England would receive a shock from which it would be ages in recovering.
I have quoted an assertion made many years since by Baron Liebig. The assertion was accompanied by another not less striking. ‘Civilisation,’ he said, ‘is the economy of power; and English power is coal.’ It is on this text that I propose now to comment. There hasrecently been issued a Blue Book, bearing in the most important manner on the subject of England’s coal-supply. For five years fifteen eminent Commissioners have been engaged in examining the available evidence respecting the stores of coal contained in the various coal-fields of Great Britain. Their inquiries were commenced soon after the time when the fears of the country on this subject were first seriously awakened; and were directed specially to ascertain how far those fears were justified by the real circumstances of the case. It will be well to compare the various opinions which were expressed before the inquiries were commenced, with the results which have now been obtained.
In the first place it should be noticed that the subject had attracted the attention of men of science many years ago. Some forty years5have passed sinceDr. Buckland, in one of the Bridgewater Treatises, pointed to the necessity for a careful examination of our coal stores, lest England should drift unawares into what he called ‘bituminous bankruptcy.’ At that time the quantity of coal raised annually in England amounted to but about forty millions of tons. Ten years later the annual yield had risen to about fifty millions of tons; and then another warning voice was raised by Dr. Arnold. Ten more years passed, and the annual yield had increased to 83,635,214 tons, when Mr. Hull made the startling announcement that our coal stores would last us but about two centuries, unless some means were adopted to check the lavish expenditure of our black diamonds.
But it was undoubtedly the address of Sir W. Armstrong to the British Association, in 1863, which first roused the attention of the country to the importance of the subject. ‘The greatness of England,’ he said, ‘depends much upon the superiority of her coal, in cheapness and quality, over that of other nations. But we have already drawn from our choicest mines a far larger quantity of coal than has been raised in all other parts of the world put together; and the time is not remote when we shall have to encounter the disadvantages of increased cost of working and diminished value of produce.’ Then he summed up the state ofthe case as he viewed it. ‘The entire quantity of available coal existing in these islands has been calculated to amount to 80,000 millions of tons, which, at the present rate of consumption, would be exhausted in 930 years; but with a continued yearly increase of 2¾ millions of tons would only last 212 years.’
Other statements were not wanting, however, which presented matters in a more favourable light. Mr. Hussey Vivian, M.P., expressed the opinion that South Wales alone could supply all England with coals for 500 years. Mr. R. C. Taylor, of the Geological Society, said that our coal stores would suffice for 1,700 years. And there were some who adopted a yet more sanguine view of our position.
On the other hand, Mr. Edward Hull, of the Geological Survey, calculated that with an increase of but one million and a half of tons per annum—considerably less than even the average increase for the preceding decade6—our coals would last us but a little more than 300 years. Mr. Stanley Jevons, in his masterly treatise on ‘The Coal Question,’ adopted a mode of considering the increase, which has led to an even more unpleasant conclusion than any hitherto obtained. He observed that the quantity of coal raised in successive years is not merely increasing, but the amount of increase is itself increasing. ‘We, of course, regard not,’ he said, ‘the average annual arithmeticalincrease of coal consumption between 1854 and 1863, which is 2,403,424 tons, but the average rate per cent. of increase, which is found by computation to be 3·26 per cent.’ That is to say, for every hundred tons of coal consumed in one year, 103¼ tons, or thereabouts, would be consumed in the next—taking one year with another. Without entering into technicalities, or niceties of calculation, it is easy to show the difference between this view of the matter and a view founded only on the average increase during so many years. Consider 10,000 tons of coal sold in one year, then Mr. Stanley Jevons points out that instead of that amount, 10,326 would be sold in the next; and so far we may suppose that the other view would agree with his. But in the next, or third year (always remembering, however, that we must take one year with another), the increase of 326 tons would not be merely doubled, according to Mr. Stanley Jevons; that is, the consumption would not be only 10,652 tons:—the 10,000 of the second year would be replaced by 10,326 tons in the third year, and the remaining 326 would be increased by 3¼ tons for each hundred, or by rather more than 10½ tons; so that in all there would be 10,662¼ tons, instead of 10,652. Now the difference in this third year seems small, though when it is applied to about nine thousand times 10,000 tons it is by no means small, amounting in fact to 95,000 tons; but when the principle is extended to sequent years its effects assume paramount importance. The small increase is as the small increase of a farthing for thesecond horseshoe-nail in the well-known problem. The effects, after a few years have passed, correspond to the thousands of pounds by which the last shoe-nails of that problem increase the cost of the horse. As Mr. Leonard Lemoran points out in the paper mentioned in the above note, if the assumed rate per cent. of increase continue, ‘we should draw in the year 1900 from our rocks more than 300 millions of tons, and in 1950 more than 2,000 millions.7About 300,000 miners are now (1866) employed in raising rather more than 92 millions of tons of coals; therefore more than eight million miners would be necessary to raise the quantity estimated as the produce of 1950. One-third of the present population of Great Britain would be coal miners.’ Or as Mr. Jevons himself sums up our future, ‘If our consumption of coal continue to multiply for 110 years at the same rate as hitherto, the total amount of coal consumed in the interval would be 100,000 millions of tons.’ Now as Mr. Hull estimated the available coal in Great Britain, within a depth of 4,000 feet, at 83,000 millions of tons, it followed that, adopting Mr. Jevons’s mode of calculation, a century would exhaust‘all the coal in our present workings, as well as all the coal seams which may be found at a depth of 1,500 feet below the deepest working in the kingdom.’ It should be added, however, that Mr. Stanley Jevons mentioned 200,000 millions of tons as the probable limit of the coal supplies of Great Britain.
The opinion of Mr. Jevons respecting the probable rate of increase of our consumption was not accepted by the generality of those who examined the subject in 1865 and 1866. There were some, indeed, who considered that the assumption was ‘absurd in every point of view.’ In one sense, indeed, Mr. Jevons himself would have been ready to admit that his estimates would not be justified by the result. The observed rate of increase could not possibly be maintained beyond a certain epoch, simply because there would not be enough men to work the coal mines to the extent required. But, regarding the increase as indicating the requirements of the kingdom, it would matter little whether the necessary supply failed for want of coal or for want of the means of raising the coal. In other words, removing the question from the arena of geological dispute, and considering only the requirements of the country, we should have this disagreeable conclusion forced upon us, if Mr. Jevons’s estimate is just, that England will not be able, a century, or even half a century hence, to get as many coals from her subterranean cellars as she will then require. She may have the coals, but she will not have men enough to bring them to bank.
It is, perhaps, in this aspect, that the question assumes its chief interest for us. Rightly understood, the statements of Mr. Jevons were of vital importance;so important, indeed, that the nation might have looked forward to the results of the Commission much as a patient would await the physician’s report of the result of a stethoscopic examination. The power of the nation residing—for the nonce at least—in her coal, the enforced consumption of coal at a rate which cannot be maintained (from whatever cause), means to all intents and purposes the decline and approaching demise of England’s power as a nation. Furthermore, apart from all inquiries such as the Commissioners undertook to make, the mere statement of the successive annual yields was to be looked upon as of vital interest, precisely as the progressive waste of a consumptive patient’s strength and substance suggests even more serious apprehensions than the opinion of the physician.
I have said that many eminent authorities held that the rate of increase assumed by Mr. Jevons would not actually prevail. But some went farther, and questioned whether the average annual arithmetical increase of the lately passed years would continue even for the next few years after the publication of Mr. Jevons’s work. ‘Such a continued increase as that, which has taken place during the last five years,’ wrote an excellent practical authority, ‘cannot continue for the next ten years,’—far less, therefore, that increasing rate of increase which Mr. Jevons had assumed. The same writer went farther even than this. For, after pointing out that the exportation of coal would probably be soon reduced, rather than undergo, as during the past, a steady increase, he added that‘on every side there were evidences of the most decided character, warranting the supposition that the annual exhaustion of our coal fields would not at any period much exceed the hundred million tons which it had nearly reached’ (in 1866).
One of the most interesting questions, then, which the Commissioners were called upon to decide was, whether, at least during the period of their labours, the anticipations of Mr. Jevons would be fulfilled or not. It is easy to compare his anticipations with those above quoted; or rather, it is easy to determine whether Mr. Jevons’s theory of an increasing increase, or the theory of a uniform average increase, accords best with the experience of the last five years. To make the comparison fairly we must adopt the figures on which his own estimate was founded. We have seen that he rejected the annual increase of 2,403,424 deduced from the records of the nine preceding years, and adopted instead an increase of 3¼ per cent. year by year, taking one year with another. His own calculations gave for this year 1871 a consumption of 118 millions of tons,—an enormous increase on the annual consumption when he wrote. According to the view he rejected, the consumption for the year 1871 is easily computed, though slightly different results will be obtained, according to the year we choose to count from. The annual increase above mentioned gives an increase of 24,034,240 tons in ten years, and if we add this amount to the consumption in 1861 (83,635,214 tons) we obtain for the year 1871 a consumption of 107,669,454 tons. On the otherhand, if we add eight years’ increase to the consumption of 1863 (88,292,515 tons), we obtain 107,519,907 tons.8It will be seen that there is an important difference between the consumption for 1871, as estimated according to Mr. Jevons’s view, and according to the average rate of increase in the nine preceding years. As the matter stood in 1865, the great question concerning the consumption of the year 1871 would have been,—whether it would be nearer 118 millions, the estimate of Mr. Jevons; or to 107½ millions, the estimate, according to the annual rate of increase; or, lastly, to a number of tons, not much, if at all, exceeding 100 millions?
The answer of the Commissioners comes in no doubtful terms. Judging from the consumption during the four years ending in 1870, the estimated consumption for the year 1872 is no less than 115 millions, an amount approaching Mr. Jevons’s estimate much more nearly than could be desired. Indeed, if we consider the imperfect nature of the statistics on which he founded his calculations, the agreement between his estimate and the observed result must be regarded as surprisingly close. Remembering the conclusion to which Mr. Jevons came with respect to the period for which our coal stores would last, and noticing the close agreement thus far between his anticipations and the result, we can well understand the warning tone of the report issued by the Commissioners.‘Every hypothesis,’ they say, ‘must be speculative, but it is certain that if the present rate of increase in the consumption of coal be indefinitely continued, even in an approximate degree, the progress towards the exhaustion of our coal will be very rapid.’ Let it be remembered that the Commission was issued at the instance of those who took the more sanguine view, and that it included within its ranks such eminent authorities as Sir William Armstrong, Sir Robert Murchison, Professor Ramsay, Mr. John Hunt, and others of like experience in the subject under inquiry.
If, in the next place, we compare Mr. Jevons’s estimate of the quantity of coal available for use with the result obtained by the Commissioners, we find little to restore our confidence in the extent of time during which our coal stores may be expected to last. We have seen that 200,000 millions of tons had been supposed to be available; but the Commissioners find that ‘we now have an aggregate of 146,480 millions of tons, which may be reasonably expected to be available for use.’ Again, it had been supposed that our coal mines could be worked to a depth of 4,000 feet, or to an even greater depth. ‘The difficulties in the way of deep mining,’ wrote Mr. Lemoran in 1866,‘are mere questions of cost. It is important to notice that the assumption of 4,000 feet as the greatest depth to which coal can be worked, on account of the increase of temperature, is purely voluntary. The increase has been calculated at a rate for which there is no authority; and while we are saying our coal-beds cannot be worked below 4,000 feet, a colliery in Belgium has nearly approached that depth, and no inconvenience is experienced by the miners.’ But the Commissioners state that at a depth of only 2,419 feet in the Rosebridge mine (the deepest in England), the temperature is 94 degrees of Fahrenheit, or within four degrees of blood heat. ‘The depth at which the temperature of the earth would amount to blood heat,’ they add, ‘is about 3,000 feet.’ They express a belief that by the ‘long wall system’ of working (a system as yet seldom adopted in the chief northern mines) it will be possible to reach a depth of 3,420 feet before this heat is attained; but it is by no means certain that this will prove to be the case.
On the other hand, it will be well to regard the more promising aspect of the question.
We must not forget, in the first place, that in all matters of statistical research there is room for misapprehension unless careful attention be paid, not merely to the observed facts, but to the circumstances with which those facts are more or less intimately associated. If we consider, for example, the progress of the consumption of our coal during the past fifteen years, we find that a law of increase exists, which is, as we have seen, easily expressed, and which, after being tested by a process resembling prediction, has been singularly confirmed by the result. But if we inquire into the various causes of the great increase in the consumption of coals, we find that while those causes have been increasing in activity—so to speak—to a degree quite sufficient to explain the observed consumption, they areyet such as in their very nature must needs be unable to pass beyond a certain range of increase. Thus the population of Great Britain has been steadily increasing, and at present the annual increase is itself increasing. Then the amount of coal used in inland communication is increasing, not only on account of the gradual extension of the railway network, but also on account of the increase of population, of commerce, and so on. Again, our commerce with other countries has increased with great rapidity since the year 1860, when the French treaty came into operation, and it will continue to increase with the increase of our population, of our means of communication within our own country as well as with foreign countries, and so on. But all these causes of increase are now growing in activity at a rate which must inevitably diminish. Our population cannot increase beyond a certain extent, because the extent of the country will suffice for but a certain number of inhabitants. If emigration do not prevent increase beyond that number, other causes will, or else a much more serious evil than the exhaustion of all our coal stores awaits the country. Again, the requirements of inland communication will before long be so far met that no such rapid extension as is now in progress will be called for. After convenient communication has been established between all parts of the country—whether the process require the formation of new lines or of new services—no important increase can be required. As regards our commerce, its increase depends necessarily on the increase at present goingon in the requirements of the country. Year by year Britain has a larger population, and the average requirements of each member of the population are also increasing. But we have seen that the increase of her population is necessarily limited; and although the increase of the requirements of her people may not be (strictly speaking) limited, yet it is manifest that, inasmuch as that increase depends on causes which are themselves approaching a limit, its rate must, after a time, continually diminish. Let it be understood that, when I speak of the requirements of the population, I do not mean only what they must obtain from other countries. The commerce of a country is the expression of the activity with which the nation is ‘earning its living,’ so to speak, and in a given population there is a limit to what is necessary for this purpose, precisely as there is a limit to the sum which an individual person in any given state of life requires for the maintenance of a given family. Indeed, although such comparisons are not always safe, we may in this case compare what may be called the commercial requirements of the nation with the requirements of the head of the family,—a merchant suppose. There are no limits to the degree of wealth which a merchant maydesireto gain, but unquestionably there are limits to the income necessary to maintain his house and family and mercantile position. Supposing he were extending his gains far beyond his actual requirements, it would by no means imply his approaching ruin that there was a demonstrable limit to this extension. And in likemanner, it would seem that, apart from the limits set by nature to the extension of our population, it need by no means be assumed that if our commerce showed signs of approaching a limit, the downfall of England’s power would be at hand.
In fact, we cannot accept Mr. Jevons’s figures for distant epochs without first inquiring whether it is likely that at those epochs the circumstances on which the consumption of our coal depends will be correspondingly changed. Supposing that 120 millions of tons of coals suffice for the requirements of our present population, we can scarcely believe that 1,440 millions will be needed in 1950, unless we suppose that the population of Britain will be twelve times greater than at present; or that the population will be even greater than this, since the consumption of coal upon our railways could scarcely be expected to increase in proportion to the population. Now no one believes that Britain will number 300 millions of inhabitants in 1950, or in 2950; the country could not maintain half that number, even though all her available stores of coal and iron, and other sources of commercial wealth were increased a hundredfold.
It is a mistake, indeed, to extend the results of statistical research very far beyond the time to which the facts and figures belong. It would be easy to multiply instances of the incorrectness of such a process. To take a single case.—When cholera has been extending its ravages in this country, the statistics of mortality from that cause, if studied with reference to four or fivesuccessive weeks, have indicated a law of increase, which is very readily expressed so as to accord with the mortality during those weeks, and perhaps two or three following weeks. But if such a law were extended indefinitely it might be found to imply nothing short of the complete desolation of the country by cholera, within the space of a few months. Thus, if the deaths (from cholera) in five successive weeks were 20, 27, 35, 47, and 63,—numbers corresponding with the general characteristics of cholera mortality in the earlier stages of a visitation,—the weekly mortality a year later, estimated according to the observed percentage of increase, would be more than 173 millions! Now this method of estimation, though leading to this preposterous conclusion as respects a more distant epoch, would probably lead to tolerably correct results for the next week or two after that in which 63 persons died,—the estimated numbers being 84 and 110 for the next two weeks respectively.
It seems to me, therefore, that we are not justified, by the observed seeming fulfilment of Mr. Jevons’s anticipations, in concluding that a hundred years hence the consumption of coals will be 2,000 millions of tons, or that the total consumption during the next 110 years will be 100,000 millions of tons. We might almost as safely infer that because a growing lad requires such and such an increase of food year by year, the grown man will need a similar rate of increase, and the septuagenarian require so many tons and hogsheads of solid and liquid foodper diem.
At present it does not seem possible to arrive at any definite conclusions respecting the probable consumption of coal in years to come. The range of observation is not sufficiently extended. It seems clear, indeed, that the epoch is not near at hand when the present law of increase will be modified. This is shown by the agreement of the observed results during the past five years with the anticipations of Mr. Jevons. It would be altogether unsafe to predict that the yearly consumption will not rise to 150 or 200 or even 250 millions of tons per annum, or to point to any definite stage at which the present increasing rate of increase will be changed first into uniform (or arithmetical) increase, and thence into a decreasing rate of increase. But it appears to me that no question can exist that these changeswilltake place. We might even go farther, and regard it as all but certain that the time will come when there will be no annual increase. Nay, unless the history of this country is to differ from the history of all other nations which have attained to great power, the time might be expected to arrive when there will be, year by year, a slow diminution in the commercial activity of Britain, and a corresponding diminution in the exhaustion of her coal stores. There is room for an amazing increase in Britain’s power and greatness, room also for an unprecedented continuance of these attributes, while yet the coal stores of the country remain well supplied.
Let us conceive, for instance, that the greatest annual consumption of coal during the future years of England’s existence as a great nation, should be set at threetimes her present annual consumption, or at 350 millions of tons. Few will regard this as an unduly low estimate when they remember that it is exceedingly unlikely that the present population of Britain will ever be tripled, and that a triple population could be commercially far more active (in relation to its numbers) than the present population, with no greater consumption of coal per head. Now, to begin with, if this enormous annual consumption began immediately, we should yet (with Mr. Jevons’s assumption as to the quantity of available coal) have 570 years’ lease of power instead of 110. But, as a matter of fact, so soon as we have recognised the principle that there is a limit to the increase of annual consumption, we are compelled to believe that that limit will be approached by a much gentler gradient, so to speak, than the same consumption as attained on Mr. Jevons’s assumption. According to his view, in fact, an annual consumption of 350 millions of tons per annum will be attained early in the twentieth century; but according to the theory which sets such a consumption as the highest ever to be attained, we should place its attainment several hundreds of years later. This is a vague statement, I admit, but the very fact on which I am mainly insisting is this, that the evidence at present in our hands is insufficient as a basis of exact calculation. Now, if we set 500 years hence as the time when the annual consumption of coal will have reached the above enormous amount, we should set the total consumption during those centuries at about one-half that due to an annualconsumption of 350 millions of tons. In that case there would still remain coal enough to supply the country for 320 years at the same tremendous rate. In all, on these suppositions, 820 years would be provided for. These would be years of commercial activity far exceeding that of our own day—in fact, they would be years during which Britain would be accumulating wealth at a rate so enormous that at the end of the era she would be not wholly unprovided with the means of supporting her existence as a nation, apart from all reference to her mineral stores. It is, indeed, utterly inconceivable, I think, that Great Britain and her people will ever beableto progress at the rate implied by these suggestions. To conceive of Great Britain as arriving at ruin within a thousand years by the over-rapid exhaustion of her coal stores, is, in fact, equivalent to supposing that she will attain in the interval to a wholly unprecedented—I had almost said a wholly incredible—degree of wealth and power.
As regards the evidence which has been adduced respecting the extent of the available coal supply, it is to be remarked that, on the whole, the result cannot be regarded as unfavourable. The more sanguine views entertained five or six years ago have not, indeed,, been fully justified. Yet our coal supply has been shown to be enormous, even when considered with reference to the continually increasing exhaustion.
But it must be admitted that the question of the depth to which our coal mines may be conveniently or even possibly worked, has an unpleasantly doubtfulaspect. Of the stores which the Commissioners regard as available a vast proportion must be mined out from depths far exceeding any which have been at present reached in England. It is not as yet clear how far the increase of depth will add to the cost and risk of working; nor do I propose to discuss a subject which can only be adequately dealt with by those who possess practical knowledge of the details of colliery-working. I will content myself by quoting some remarks on the subject, in an inaugural address delivered by Mr. George Elliot (one of the Royal Commissioners) before the North of England Institute of Mining Engineers in 1868. ‘The great depth,’ he remarked, ‘at which many of our pits are worked, and the vast extent of their lateral ramifications, make it more than ever necessary that we should secure the best mode of rendering the supply of pure air certain, regular, and safe. It is maintained that ventilating by machinery ensures these desiderata; that the nicety with which mechanical appliances may be regulated, the delicate adjustment of power of which they are capable, and the complete safety with which they may be worked, place them far before the system they are intended to supersede. The extent of our coal supply will be materially increased by the improvement of which this is a type.... It is probable that the ordinary means of ventilation—whether by furnace or fan—may be aided by a change in the force or agency employed for the purposes of haulage and other independent work. As an instance of my meaning, I may mention that theapparatus which I have introduced in South Wales, and which, by means of compressed air used as a motive power instead of steam, draws trams and pumps water with complete success, is found to generate ice in an atmosphere which is naturally hot and oppressive. The mechanical usefulness of these new air-engines seems capable of indefinite extension; while, as their cooling properties form a collateral advantage arising out of their use, it is at least possible that they may prove valuable auxiliaries to the more regular means of ventilation in extending the security and promoting the healthfulness of our mines.The difficulties of ventilation once surmounted, the extent of coal at our disposal is incalculably increased.‘
In the address just quoted there are some striking suggestions as to the possibility of working those coal fields which extend below the sea on our east and west coasts, especially in the counties of Durham, Northumberland, and Cumberland. Mr. Elliot remarks that ‘for all practical purposes these fields are as entirely within the reach of the mining engineer as the ordinary workings out of which coal is hewed.’ It is known that in many districts the coal strata extend ten or twelve miles beyond the shore; and Mr. Elliot believes that by sinking ventilating shafts in the German Ocean the coal below may be safely worked. The idea seems somewhat daring; yet, after the feats of engineering which have been achieved in our day, there seems no valid reason for doubting that at least when the pressure of a failing coal supply begins to be felt, the means will be foundfor rendering these immense submarine coal stores available. As to the difficulty of transport, Mr. Elliot remarks that, according to his estimates, ‘transport would neither be more costly nor more laborious than it has been in days gone by to convey coal the same distance after it was brought to the surface inland.’ The enormous importance of the subject is shown by the fact that ‘out of the minerals obtainable in Durham alone, one-third,’ Mr. Elliot tells us, ‘may be held to lie under the sea, and that all coalfields having a similar inclination of strata, and bordering on the ocean, will be similarly enlarged. This at once disposes,’ he adds, ‘of some of the fears expressed as to the duration of our coal supply; and while I am quite aware that these theories may be challenged, they are not put forward without due deliberation, and I am content to stake my professional reputation on their practicability.’
With regard to the future of this country, it appears to me that little anxiety need be entertained. Apart from the considerations I have urged, which seem to indicate that our consumption cannot long increase at the same rate as at present, it seems not unreasonable to anticipate that within the next few decades science will find the means of economising our coals in more ways than one. It does not indeed appear likely that any form of fuel will ever take the place of coal; but a portion of the work now derived from the consumption of coal may be expected to be derived in future years from some of the other substances now coming into use. It may be hoped, also, that science may suggest meansfor bringing coals to the surface with less waste, and even at less cost, than at present. And in other ways the process of exhaustion may be more or less effectively checked.
But while we may thus look somewhat confidently forward, as I judge, to the future of our country, serious questions are suggested as to the future of the human race. The period during which a nation flourishes, long as it seems by comparison with the life of man, yet sinks into insignificance when compared with the period during which civilised men will bear sway upon the earth. The thousands of years during which the coal stores of the earth may be expected to last will pass away, and then the descendants of those now living on the earth will have to trust to other force-supplies than those which we are now using so lavishly. It may seem fanciful to look so far forward, and yet by comparison with the periods which the astronomer deals with in considering the future of our earth, thousands of years are as nothing. As I have said elsewhere, ‘those thousands of years will pass as surely as the thousands which have already passed, and the wants entailed by wastefulness in our day will then be felt, and none the less that for so many years there had been no failure in the supplies contained within the great subterranean storehouse.’ It behoves us to consider thoughtfully the wants even of those distant eras. If the greatest good for the greatest number is to be regarded as the true rule for the conduct of intelligent beings, then unquestionably mere distance in point of time should not prevent us from anticipating the requirements of those remote descendants of ours. We should regard the consciousness of this duty and its performance as signs by which the superiority of our own over less civilised times is partly manifested. As man is in dignity higher than non-intelligent animals, in that he alone provides of his own forethought for the wants of his children, so our generation would be raised in dignity above preceding generations if it took intelligent charge of the wants of its remote descendants. We ourselves are now employing stores of force laid up for us by the unconscious processes of Nature in long past ages. As Professor Tyndall has finely said, we are utilising the Sun of the Carboniferous Epoch. The light ‘which streamed earthwards from the sun’ was stored up for us by the unconscious activity of ‘organisms which living took into them the solar light, and by the consumption of its energy incessantly generated chemical forces.’ The vegetable world of that old epoch ‘constituted the reservoir in which the fugitive solar rays were fixed, suitably deposited, and rendered ready for useful application.’ What the vegetable world did for us unconsciously during the Carboniferous Epoch, the scientific world of our epoch must do for our remote descendants. While we are consuming the stores of force laid up in past ages for our benefit, we must invent the means for obtaining directly from the solar rays fresh and inexhaustible supplies of motive energy.