In the intervals of personal observation Percival was often giving lectures or writing on astronomical subjects for the publications of the Observatory, and for scientific societies and periodicals. The substance of most of these found their way into his books, which are summations or expositions of his conclusions. In December 1902, for example, he gave six lectures on “The Solar System” at the Massachusetts Institute of Technology, of which he was a non-resident professor, and they were published by Houghton, Mifflin & Company. Then in the autumn of 1906 he gave a course of eight lectures at the Lowell Institute in Boston on “Mars as the Abode of Life.” These were so crowded that they had to be repeated, were then printed as six papers in theCentury Magazine, and finally re-published by The Macmillan Company under the same title. Two years later, in the winter of 1909, he gave at the Massachusetts Institute of Technology, another course of six lectures on “Cosmic Physics: The Evolution of Worlds,” which were brought out in December by the same publisher with the latter half of the title. Although their names are so diverse, and far more is told of Mars in the book whose title contains its name, they all deal essentially with the same subject, the evolution of the planets and the development and end of life upon them. In the Preface to“Mars as the Abode of Life,”—for a preface, although printed at the beginning, is always written after the book is finished, and is the author’s last word to the reader, giving his latest thought as the work is being launched,—he tells us:[17]“Though dealing specifically with Mars, the theme of the lectures was that of planetary evolution in general, and this book is thus a presentation of something which Professor Lowell has long had in mind and of which his studies of Mars form but a part, the research into the genesis and development of what we call a world; not the mere aggregating of matter, but what that aggregation inevitably brings forth. The subject which links the Nebular Hypothesis to the Darwinian Theory, bridging the evolutionary gap between the two, he has called planetology, thus designating the history of the planet’s individual career. It is in this light that Mars is here regarded: how it came to be what it is and how it came to differ from the Earth in the process.”
At each opposition, in fact at every opposition during Percival’s life and long thereafter, Mars was observed at Flagstaff and more detail was discovered confirming what had been found before. He tells of a slight change in the estimated tilt in its axis; the fact that the temperature is warmer than was earlier supposed;[18]and he had found how to discover the gases by spectroscopic analysis applied according to an ingenious device of his own known as “Velocity Shift” and much used thereafter.[19]He tells also of aningenious and elaborate experiment with wires, and with lines on a wooden disk, which showed that such lines can be perceived at a greater distance and therefore of smaller size than had been supposed, so that the canals might have less width than had been assumed. It is, however, needless, in describing his planetary theory, to do more than allude to his evidence of Martian habitation drawn from the canals, with which the reader is already familiar. Curiously enough, however, it is interesting to note that on September 9, 1909, about the time when “The Evolution of Worlds” was going to press, a strange phenomenon appeared in Mars. Two striking canals were seen where none had ever been seen before, and the most conspicuous on that part of the disk. Moreover, they were photographed. After examining all the maps of canals made at Flagstaff and elsewhere, Percival discussed them in the Observatory Bulletin No. 45, and concluded that they must not only be new to us, but new to Mars since its previous corresponding season of two of our years before: “somethingextra ordinem naturae.” We may here leave Mars for the time, and turn to the more extensive study of the evolution of the planetary system.
The desire to rise from a particular case to a more general law was characteristic of his attitude of mind, constructiveand insatiable, and appears throughout these volumes. It may have been influenced by his great master Benjamin Peirce, who ever treated any mathematical formula as a special instance of a more comprehensive one. In such a subject as the evolution of the planets, especially of life on them, it involved dipping into many sciences, beyond the physical laws of matter; and he says in the same preface: “As in all theses, the cogency of the conclusion hangs upon the validity of each step in the argument. It is vital that each of these should be based on all that we know of natural laws and the general principles underlying them.” This did not mean that all his premises would be universally accepted, but that he found out all he could about them, convincing himself of their accuracy and of the validity of the conclusions he draws therefrom. That is all any man of science can do in a subject larger than his own special, and therefore limited, field.
But from the time of his resumption of research and the direction of the observatory in 1901, he was constantly enlarging his own field by the study of astrophysical subjects, and the methods for their determination. With this object he was initiating and encouraging planetary photography. He was constantly writing Dr. V. M. Slipher about procuring and using spectrographic apparatus and about the results obtained by him therefrom. By this process the rotations of planets were determined; and the spectra of the major ones—often reproduced in astronomical works—have been a puzzle to astrophysicists until their interpretation in very recent years. He was interested also in nebulae, especially in spiral ones, taking part in Dr. Slipher’s pioneering spectrographic work at the observatory, which showed that they were vast aggregations of stars of different spectral types,moving with great speed, and far beyond the limits of our universe. For over fifteen years the observatory was almost alone in this field of research, as well as in that of globular clusters. It is in fact, the discovery of the rapid motion of the spiral nebulae away from the solar system that has given rise to the conception of an expanding universe.
But these discoveries were still largely in the future, and to return to his books on the planetary system it may be noted that in the two larger and more popular ones the general planetary theory is expounded in the text, while the demonstrations of the more complex statements made, and the mathematical calculations involved, are relegated to a mass of notes at the end of the volume.
The first of his books on the solar system is the small volume bearing that title; but since all three of the books here described are several expositions of the same subject it may be well to treat his views on each topic in connection with the work in which he deals with it most fully. Indeed, “The Solar System” is not a general treatise, but rather a discussion of some striking points, and it is these which one thinks of in connection therewith.
In considering the origin of the planets he had become much interested in the meteors, shooting stars, meteoric streams and comets, all or almost all of which he regarded as parts of the solar system, revolving about the Sun in elliptic orbits, often so eccentric as to appear parabolas.[20]The old idea that comets came from outer space and therefore travelled in hyperbolas can, he points out, be true of few, if any, of them. “Very few, three or four perhaps, hint athyperbolas. Not one is such beyond question.” Many of them are associated with the meteoric streams with which everyone is familiar at certain seasons of the year. Indeed seventy-six of these associations were then known, and comets sometimes break up into such streams.
Now if the comets are travelling in orbits around the Sun they must be throughout their course within its control, and not within that of some other star; and therefore he computes how far the Sun’s control extends. Taking for this purpose our nearest star, α Centauri, a double with a total mass twice that of the Sun, at a distance of 275,000 astronomical units, in other words that number of times our distance from the Sun, he finds that the point at which its attraction and that of the Sun become equal is 114,000 of these units. This he calls the extent of the Sun’s domain, certainly an area large enough for any, or almost any, comet known.[21]
He then turns to some of the planets,—Mercury to show the effect of tidal action in slowing the rotation of a planet or satellite, and causing it to turn the same face always to its master.[22]This involved a highly interesting comparison of Newton’s theory of the tides, long generally accepted, but not taking enough account of the planet’s rotation, and that of Sir George Darwin based upon the effect of such rotation. The general conceptions are even more different than the results, and the later theory is less concerned with the tides in oceans, which probably affect only our Earth, than with those of a planet in a fluid or viscous condition, which may still continue to some extent after the surface has becomepartly solidified. He therefore studies the tide raising force, and the tendency to retardation of rotation, by the Sun on the planets, and by these on their satellites while still in a fluid state, tabulating some very striking results.
What he says about Mars is more fully dealt with in his other writings; and the same is true of Saturn’s rings, except for the reference to the calculation by Edward Roche of the limit of possible approach by a fluid satellite to its planet without being disrupted, and for the fact that this limit in Saturn’s case falls just beyond the outer edge of the rings. In discussing Saturn’s satellites he brings out a curious analogy between the order of distribution of these attendants of the three best known major planets and the order of the planets themselves about the Sun. In each case the largest of the bodies so revolving is nearly in the centre of the line, as in the case of Jupiter among the planets; the second largest the next, or not far, beyond, as in the case of Saturn; while there is another maximum farther in, for as the Earth is larger than any planet on either side until Jupiter is reached, so a like order is found in the satellites of Jupiter, Saturn and Uranus. In other words, the size in each case rises with increasing distance, falls off, then rises again to the largest and thence declines. This he believed cannot be an accidental coincidence, but the result of a law of development as yet unexplained.
To the ordinary reader the most novel thing he says about Jupiter relates to its family of comets, for no less than thirty-two of these bodies have their aphelia, or greatest distance from the Sun, near its orbit. Moreover, their ascending nodes—that is the place where their paths if inclined to the plane of the ecliptic pass through it—are close to its orbit.At some time, therefore, in the vast ages of the past they must have passed close to the planet, and if so have had their orbits greatly changed by its attraction. He considers the various effects Jupiter may have upon a comet, and shows—contrary to the opinion of Professor H. A. Newton—that any such body moving by the attraction of the Sun would be going too fast for Jupiter to capture completely. Then he takes up other effects of deflection. The comet’s speed may be accelerated and its direction changed even so much as to drive it out of the solar system; it may be retarded so that its path is contracted and the aphelion drawn nearer to the planet’s orbit. After calculating the possible conditions and analyzing the actual orbits of Jupiter’s family, he comes to the provisional conclusion that these comets have been drawn from the neighborhood. “It is certain,” he says, “that Jupiter has swept his neighborhood.... If we consider the comet aphelia of short-period comets, we shall notice that they are clustered about the path of Jupiter and the path of Saturn, thinning out to a neutral ground between, where there are none. Two-thirds of the way from Jupiter’s orbit to Saturn’s, space is clear of them, the centre of the gap falling at 8.4 astronomical units from the sun....
“Jupiter is not the only planet that has a comet family. All the large planets have the like. Saturn has a family of two, Uranus also of two, Neptune of six; and the spaces between these planets are clear of comet aphelia; the gaps prove the action.
“Nor does the action, apparently, stop there. Plotting the aphelia of all the comets that have been observed, we find, as we go out from the Sun, clusters of them at first, representing, respectively, Jupiter’s, Saturn’s, Uranus’, and Neptune’sfamily;[23]but the clusters do not stop with Neptune. Beyond that planet is a gap, and then at 49 and 50 astronomical units we find two more aphelia, and then nothing again till we reach 75 units out.
“This can hardly be accident; and if not chance, it means a planet out there as yet unseen by man, but certain sometime to be detected and added to the others. Thus not only are comets a part of our system now recognized, but they act as finger-posts to planets not yet known.”
We shall hear more of this last suggestion hereafter.
In both “Mars as the Abode of Life” and “The Evolution of Worlds,” he accepts the proposition that our present solar system began with a collision with some dark body from interstellar space, as had been suggested by Chamberlin and Moulton a few years before. He points out that stars which have finished contracting, grown cold and ceased to be luminous, must exist, and although we cannot see them directly we know about some of them,—such as the dark companion of Algol, revolving around it and cutting off two-thirds of its light every three days. Many dark wanderers there must be, and thenovae, as he says, are sometimes, at least, due to a collision with such a body,—not necessarily an actual impact, but an approach so near that the star is sprung asunder by the tidal effect. In such a case the opposite sides of the victim would be driven away from it, and if it was rotating would form spirals. Now we know that the apparently empty spaces in our solar system still contain a vast number of little meteoric particles, which as judged from their velocity do not fall from outer space,but are members of our system travelling in their own orbits around the sun. As he puts it, “Could we rise a hundred miles above the Earth’s surface we should be highly sorry we came, for we should incontinently be killed by flying brickbats. Instead of masses of a sunlike size we should have to do with bits of matter on the average smaller than ourselves[24]but hardly on that account innocuous, as they would strike us with fifteen hundred times the speed of an express train.” That these meteorites are moving in the same direction as the Earth he shows by an ingenious calculation of the proportion that in such a case would be seen at sunrise and sunset, which accords with the observed facts. Moreover, their chemical composition shows that they were once parts of a great hot body from which they have been expelled.
The meteorites that are seen because they become hot and luminous in traversing our atmosphere, and occasionally fall upon the Earth, are the remnants of vastly larger numbers formerly circling about the sun, but which, by collision and attraction, were, as he describes, gathered into great masses, thus forming the planets. The force of gravity gradually compacted these fragments closer and closer together, thereby generating heat which if the body were homogeneous would be in proportion to the square of its mass. The larger the planet therefore the more heat it would generate, and owing to the fact that mass is in proportion to the cube and its radiating surface to the square of the diameter the slower it would radiate, and thus lose,its heat, so that the larger ones would be hotter and remain hot longer than the smaller ones.
Some of the planets may once have been white-hot, and luminous of themselves, some were certainly red-hot, some only darkly warm; all growing cooler after the amount radiated exceeded the amount generated. Now by the difference in the heat generated and retained by the larger and smaller bodies he explains the diverse appearance of those whose surfaces we know, the Earth, Mars and the Moon. As the surface cools it forms a crust, but if the interior still remains molten it will continue to contract, the crust will be too large for it and crinkle, like the skin of a dried apple; and this will be more true of a large than a small body. “In like manner is volcanic action relatively increased, and volcanoes arise, violent and widespread, in proportion; since these are vents by which the molten matter under pressure within finds exit abroad.” By a calculation, which agrees with the formula of Laplace, he finds that the effective internal heat of the Earth might be 10,000 degrees Fahrenheit, enough to account for all the phenomena; and for Mars only 2,000, which is below the melting point of iron, and would not cause volcanic action. Now the observations of Mars at Flagstaff show that there can be no mountains on it more than two or three thousand feet high, and that the surface is singularly flat.
But here he met a difficulty; for the Moon ought to be flatter still if it had evolved in the ordinary way, whereas it has enormous volcanic cones, craters 17,000 feet high, some exceeding 100 miles in diameter, and a range of mountains rising to nearly 30,000 feet. An explanation he finds in the analysis of the action of the tides in the Earth-Moonsystem by Sir George Darwin, who showed that when traced backward it “lands us at a time when the Moon might have formed a part of the Earth’s mass, the two rotating together as a single pear-shaped body in about five hours.... For in that event the internal heat which the Moon carried away with it must have been that of the parent body—the amount the Earth-Moon had been able to amass. Thus the Moon was endowed from the start of its separate existence with an amount of heat the falling together of its own mass could never have generated. Thus its great craters and huge volcanic cones stand explained. It did not originate as a separate body, but had its birth in a rib of Earth.”[25]
The Flagstaff site having been selected for the purpose of planetary observation yielded facts less easily detected elsewhere. Mercury, for instance, is so near the Sun that it could be observed in the dark only a short time after sunset and before sunrise, an obstacle that gave rise to errors of fact. Schiaparelli led the way to better results by observing this planet in broad daylight. Up to that time it had been supposed to rotate on its axis in about twenty-four hours, and therefore to have a day and night like those of the Earth, but daylight observation showed him markings constant on its illuminated face, and therefore that it turns nearly the same side to the Sun. Before knowing his conclusions, and therefore independently, the study of Mercury was taken up at Flagstaff in 1896, and the result was a complete corroboration of his work. It showed that, as in the case of the Moon with the Earth, tidal action on the still partially fluid mass had slowed its rotation until it has littlewith regard to the central body around which it revolves. He discovered also other facts about Mercury, which Schiaparelli had not, that its size, mass and density had not been accurately measured.
A similar discovery about the period of rotation had been made in the case of Venus. For more than two centuries astronomers had felt sure that this period was just under twenty-four hours, figured, indeed, to the minute. But again it was Schiaparelli who doubted, and once more by observing the planet at noon; when he noted that the markings on the disk did not change from day to day, and concluded that the same side was always pointed at the Sun. At Flagstaff in 1896 his observations were verified and the inference later confirmed by the spectroscope, which was, indeed, first brought to the Observatory for that purpose. Thus Venus, which from its distance from the Sun, its size and density, is most like the Earth, turns out to be in a totally different condition, one face baked by unending glare, the other chilled in interstellar night, and as he puts it: “To Venus the Sun stands substantially stock-still in the sky,— ... No day, no seasons, practically no year, diversifies existence or records the flight of time. Monotony eternalized,—such is Venus’ lot.”[26]
On the movements and physical condition of the Earth it was needless to dwell, and he passed to the asteroids. He describes how they began to be discovered at the beginning of the last century by searching for a planet that would fill a gap in Bode’s law. This, a formula of arithmetical progressionfor the distances of the planets from the Sun, has proved not to be a law at all, especially since the discovery of Neptune which is much nearer than the formula required; but for nearly a century it had a strong influence on astronomic thought, and the gap in the series between Mars and Jupiter was searched for the missing link. Two were found, then two more, about the middle of the last century another, and then many, smaller and smaller, until by the time Percival wrote six hundred were known, and their number seems limitless. Only the four first found, he remarks, exceed a hundred miles in diameter, the greater part being hardly over ten or twenty. But here he points out a notable fact, that they are not evenly distributed throughout this space; and although massed in a series growing thicker toward its centre there are many gaps, even close to the centre, where few or no asteroids are found. Now it is the large size and attraction of Jupiter by which Percival explains the presence of asteroids with gaps in their ranks, instead of a planet, in the space between it and Mars; but we shall hear much more of this subject when we come to his work on Saturn’s rings and the order in the distribution of the planets.
Jupiter, he tells us, having a mass 318 times that of the Earth, and a volume 1400 times as large, is much less dense, not much more than water, in short still fluid; and as it has a tremendous spin, rotating in less than ten hours, it is more oblate than the Earth; that is, the diameter at its equator is larger in proportion to that from pole to pole. The observations at Flagstaff brought out some interesting facts: first, that the dark belts of cloud that surround it are red, lookingas if the planet within were still molten;[27]second, that the bright central belt lies exactly upon its equator, without regard to, and hence independent of, its tilt toward the Sun, and that the belts of cloud on each side appear at the planet’s morning just as they left it in the evening. All which shows that Jupiter’s cloud formation is not due to the Sun, but to its own internal heat, an interpretation of the phenomena that has a direct bearing on his explanation of the Earth’s carboniferous age.
Saturn is still less dense, even more oblate; but its most extraordinary feature is of course the rings. Assumed by the early astronomers to be solid and continuous, they were later shown to have concentric intervals, and to be composed of discrete particles. They have usually been supposed flat, but when the position of the planet was such that they were seen on edge knots or beads appeared upon them; and in 1907 these were studied critically at Flagstaff, when it was found that the shadows of the rings on the planet were not uniform, but had dark cores; these thicker places lying on the outer margin of each ring where it came to one of the intervals. These phenomena he explained in the same way as the distribution of the intervals among the asteroids.[28]
About Uranus and Neptune he tells us in this book little that was not known, and save for their orbits, masses and satellites not much was known of their condition. But later, in 1911, the spectroscope at Flagstaff determined the rotation period of Uranus, afterwards precisely duplicated atthe Lick; and later still the spectral bands in the vast atmosphere of the giant planets were identified as due to methane, or marsh, gas.[29]
After the planets had been formed through the aggregation of revolving fragments driven off by the catastrophic collision from the Sun, and after they had attained their maximum heat in the process, they began, he says, to go through six stages:
I. The Sun-Stage, when they were white-hot and gave out light. This could have been true only of the largest ones if any.
II. The Molten Stage, when they were still red-hot, but not enough to give light, in which are now the four great outer planets.
III. The Solidifying Stage, when a crust formed, and the surface features of the planet began to assume their character. Here the science of geology takes its start with the metamorphic rocks, and it is the dividing line between the inner, smaller, and the outer, larger, planets.
IV. The Terraqueous Stage, when the surface has become substantially stable, there are great oceans gradually diminishing in size, and land gradually increasing. This is the stage of the sedimentary rocks, the time when the planet passes from its own supply of heat to dependence upon that of the sun; the stage when life begins, and the one in which the Earth is now.
V. The Terrestrial Stage, when the oceans have disappeared, and water is scarce, the one in which Mars is now.
VI. The Dead Stage, where are already the Moon and the satellites of other planets.
On the question of the origin of life Percival took the mechanistic view: “Upon the fall of the temperature to the condensing point of water, occurred another event in the evolution of our planet, the Earth, and one of great import to us: life arose. For with the formation of water, protoplasm (the physical basis of all plants and animals) first became possible, what may be called the life molecule then coming into existence. By it, starting in a simple, lowly way, and growing in complexity with time, all vegetable and animal forms have since been gradually built up. In itself the organic molecule is only a more intricate chemical combination of the same elements of which the inorganic substances which preceded it are composed.... There is now no more reason to doubt that plants grew out of chemical affinity than to doubt that stones did. Spontaneous generation is as certain as spontaneous variation, of which it is, in fact, only an expression.”
Life, he believed, began in the oceans soon after they had cooled below the boiling point, and spread all over them; seaweeds and trilobites existed in France, Siberia and the Argentine, their nearest relatives being now confined to the tropics; coral reefs, now found only in warm equatorial seas, have left their traces within eight degrees of the pole. This looks as if in paleozoic times the oceans were uniformly warm. The same record he finds in the plants of the carboniferous age. Gigantic ferns and other cryptogams grew to an immense size, with vast rapidity and withoutstopping, for there are no annual rings of growth, no signs of the effect of seasons, no flowers, and little or no color. “Two attributes of the climate this state of things attests. First, it was warm everywhere with a warmth probably surpassing that of the tropics of to-day; and, second, the light was tempered to a half-light known now only under heavy clouds. And both these conditions were virtually general in locality and continuous in time.” In the later volume he adds, to corroborate the general darkness, that many of the earlier trilobites, who lived in shallow water, were blind, while others had colossal eyes.
Various theories have been advanced to explain the carboniferous age, which he reviews, showing why they do not account for the facts. His own is that while the oceans were still hot a vast steaming must have gone up from them, forming clouds of great density that would keep the sun’s heat and light out, and the warmth of the Earth in. “In paleozoic times, then, it was the Earth itself, not the Sun, to which plant and animal primarily stood beholden for existence. This gives us a most instructive glimpse into one planetologic process. To the planet’s own internal heat is due the chief fostering of the beginnings of life upon its surface.”[30]
But he points out that a time must have come when the Earth, and especially its seas, had cooled, the envelope of dense cloud had gradually been pierced, and the sun’s rays let in. Then began the sharp alternation of day and night, the changes in the seasons and the diversity of climates, when the palms descended to the tropics, and the flora and fauna as we know them started to develop. This is the periodwhen the Sun was dominant, or the Sun-Sustained Stage, the one in which we live.
Later the Earth went through another experience of which the facts are well known, but the date and cause have puzzled astronomers and geologists alike, for it lies in the twilight zone between the regions they illuminate. It is the Glacial Periods. He discusses the theory of Croll, once largely accepted but now abandoned, that these periods were due to a change in the eccentricity of the Earth’s orbit, combined with a progression of the equinoxes, which so altered the seasons that the northern hemisphere would have summers hot but too short to melt the snow and ice accumulated in the long cold winters. In fact Percival had already reviewed this theory some years before in a paper presented to the American Philosophic Society (Proc. Vol. XXXIX, No. 164) in which he showed that the eccentricity and inclination of axis in Mars are very close to those Croll had attributed to the Earth, and yet a glacial period does not exist there. In the case of Mars it is the southern hemisphere that should be glaciated, but in fact, although that pole has the larger extent of snow in winter this sometimes disappears wholly in the summer, which is never true at the northern pole. If, indeed, the amount of ice formed were much larger it would not be melted, so that the amount of water falling and frozen, and not the eccentricity or inclination of the axis, would be the cause of an ice age.
But he had another reason for rejecting Croll’s theory, and, indeed, for disbelieving in a general ice age altogether. It was that the glaciation does not appear to proceed from the pole, but from various distinct centres, moving from them in all directions, north as well as south; while someplaces, like northern Siberia, that one would expect to be covered with ice, were not so covered. Nor was the greater cold confined to the northern hemisphere, for on some mountains at the equator, and even at the south pole, there was more ice and snow than there is to-day. His explanation is that certain parts of the Earth’s surface were for some reason raised higher than they are now; and from the snow mountains or plateaus so formed the sheets of ice flowed down.
The remainder of the book on “Mars as the Abode of Life”—and it is the larger part of it—contains the reasons for believing that Mars is inhabited, the canals artificial, and that the Earth will in like manner gradually lose its supply of water. But this argument need not be retraced here, because with it the reader has already been made familiar. “The Evolution of Worlds” ends with a chapter entitled “Death of a World”; for to him the whole theory of planetary evolution is a vast drama, albeit with a tragic close. He describes four ways in which a planet, and all life thereon, may be destroyed. Three of these are: the effect of tidal action that would bring the same face always toward the Sun; the loss of water and atmosphere; and the cooling and final extinction of the Sun. All these things he cheerfully reminds us are sure to happen, but at a time enormously distant. The other is a collision with a star—“That any of the lucent stars, the stars commonly so called, could collide with the Sun, or come near enough to amount to the same thing, is demonstrably impossible for aeons of years. But this is far from the case for a dark star. Such a body might well be within a hundredth of the distance of the nearest of our known neighbors.... Our senses couldonly be cognizant of its proximity by the borrowed light it reflected from our own Sun.” A collision of this kind might happen at any time, but he consoles us by saying that “judged by any scale of time we know, the chance of such occurrence is immeasurably remote.” In an earlier part of the book he describes what its advent would be:
“We can calculate how much warning we should have of the coming catastrophe. The Sun with its retinue is speeding through space at the rate of eleven miles a second toward a point near the bright star Vega. Since the tramp would probably also be in motion with a speed comparable with our own, it might hit us coming from any point in space, the likelihood depending upon the direction and amount of its own speed. So that at the present moment such a body may be in any part of the sky. But the chances are greatest if it be coming from the direction toward which the Sun is travelling, since it would then be approaching us head on. If it were travelling itself as fast as the Sun, its relative speed of approach would be twenty-two miles a second.
“The previousness of the warning would depend upon the stranger’s size. The warning would be long according as the stranger was large. Let us assume it the mass of the Sun, a most probable supposition. Being dark, it must have cooled to a solid, and its density therefore be much greater than the Sun’s, probably something like eight times as great, giving it a diameter about half his or four hundred and thirty thousand miles. Its apparent brightness would depend both upon its distance and upon its intrinsic brightness or albedo, and this last would itself vary according to its distance from the Sun.... We shall assume, therefore,that its brilliancy would be only that of the Moon, remembering that the last stages of its fateful journey would be much more resplendently set off.
“With these data we can find how long it would be visible before the collision occurred. As a very small telescopic star it would undoubtedly escape detection. It is not likely that the stranger would be noticed simply from its appearance until it had attained the eleventh magnitude. It would then be one hundred and forty-nine astronomical units from the Sun or at five times the distance of Neptune. But its detection would come about not through the eye of the body, but through the eye of the mind. Long before it could have attracted man’s attention to itself directly its effects would have betrayed it. Previous, indeed, to its possible showing in any telescope the behavior of the outer planets of the system would have revealed its presence. The far plummet of man’s analysis would have sounded the cause of their disturbance and pointed out the point from which that disturbance came. Celestial mechanics would have foretold, as once the discovery of another planet, so now the end of the world. Unexplained perturbations in the motions of the planets, the far tremors of its coming, would have spoken to astronomers as the first heralding of the stranger and of the destruction it was about to bring. Neptune and Uranus would begin to deviate from their prescribed paths in a manner not to be accounted for except by the action of some new force. Their perturbations would resemble those caused by an unknown exterior planet, but with this difference that the period of the disturbance would be exactly that of the disturbed planet’s own period of revolution round the Sun.
“Our exterior sentinels might fail thus to give us warning of the foreign body because of being at the time in the opposite parts of their orbits. We should then be first apprised of its coming by Saturn, which would give us less prefatory notice.
“It would be some twenty-seven years from the time it entered the range of vision of our present telescopes before it rose to that of the unarmed eye. It would then have reached forty-nine astronomical units’ distance, or two-thirds as far again as Neptune. From here, however, its approach would be more rapid. Humanity by this time would have been made acquainted with its sinister intent from astronomic calculation, and would watch its slow gaining in conspicuousness with ever growing alarm. During the next three years it would have ominously increased to a first magnitude star, and two years and three months more have reached the distance of Jupiter and surpassed by far in lustre Venus at her brightest.
“Meanwhile the disturbance occasioned not simply in the outer planets but in our own Earth would have become very alarming indeed. The seasons would have been already greatly changed, and the year itself lengthened, and all these changes fraught with danger to everything upon the Earth’s face would momentarily grow worse. In one hundred and forty-five days from the time it passed the distance of Jupiter it would reach the distance of the Earth. Coming from Vega, it would not hit the Earth or any of the outer planets, as the Sun’s way is inclined to the planetary planes by some sixty degrees, but the effects would be none the less marked for that. Day and night alone of our astronomic relations would remain. It would be like going madand yet remaining conscious of the fact. Instead of following the Sun we should now in whole or part, according to the direction of its approach, obey the stranger. For nineteen more days this frightful chaos would continue; as like some comet glorified a thousand fold the tramp dropped silently upon the Sun. Toward the close of the nineteenth day the catastrophe would occur, and almost in merciful deliverance from the already chaotic cataclysm and the yet greater horror of its contemplation, we should know no more.”[31]
Naturally Percival’s observations of Mars, and still more the conclusions he drew from them, provoked widespread attention among astronomers, some of whom were convinced, while some withheld judgment and others were very frankly disbelievers. This did not amaze him, for he felt that new ideas made their way slowly, and had always done so. He met objections, argued his case and expected ultimate acceptance of his views. Perhaps not less naturally the popular interest was also great. Newspapers as well as periodicals all over America, in England, France, Germany and other countries, published and discussed his views, especially, of course, on the existence of intelligent beings on Mars and their artificial canals upon its surface. Marconi was reported as saying that within a few years we should be in wireless communication with them.
Meanwhile his life had been going on at the usual furious pace; lecturing here and there; writing for scientific journals, mostly, but not wholly, on planets, satellites etc.; managing his own property and his father’s estate; keeping in constant touch with his computers in Boston and his observers at Flagstaff, worrying over the health of one of them whom he urges to take a vacation and recruit; and also standing his watch as observer himself. A watch it was,“Jupiter before dinner and Mars at 4A.M.” There was also a large correspondence with astronomers and others who were interested in his work. To one of the latter he writes on December 14, 1907: “In answer to your note of Dec. 5, which has been forwarded to me here, I beg to say that the best and final education must always be given by one’s self.”
Although the canals had already been photographed, he was not yet free from the doubters of the actuality of his observations, for on May 15th of that year we find him writing to Professor Simon Newcomb—then at the height of his great reputation who had suggested that the comparative continuity of the canals was an optical illusion, a long letter giving the reasons for believing that this could not be so, but that they must be as observed.[32]The proof of this he was seeking to make more clear, and in this same year he sent Dr. Slipher, with Professor Todd of Amherst College, on an expedition to the Andes to take more photographs of Mars, which appeared in theCenturyfor December.
But it was not all work. The hospitality of the Observatory was kept up; visiting astronomers and friends lent a gayety to the place. Mr. George Agassiz, for example, long his friend in many labors, was there for many months in 1907 and 1909, helping greatly in his observations;[33]the late Professor Edward S. Morse at sundry times, and ProfessorRobert W. Willson in 1909 and 1914. He was also in kindly relations with his neighbors, who were “courteous enough to ask me to talk, and I am deep in addresses.” In fact some of them were constantly urging him to stand for Senator from the State. He was interested also in children, and in March, 1908, he is sending word to Dr. Slipher about a little girl from Texas eight years old who is to pass through Flagstaff, and asks permission to look through his big telescope as she “just loves astronomy.” He was fond of telling about his meeting a negro tending chickens to whom he suggested keeping a watch on them the next day because they would go to roost about eleven o’clock; and they did, for there was an eclipse of the sun. Some days later he met the negro again, who expressed astonishment at his knowing in advance that the chickens would go to roost, and asked if he had known it a week before. Yes, he had known it then. “Did you know it a month before?” “Yes, I knew it a month before.” “Did you know it a year before?” “Yes, I knew it a year before.” “But those chickens weren’t born then!” Had he lived to the present day he might have discovered a resemblance to some tendencies in ideas about the present depression.
Nor were his thoughts confined to this country, for in August, 1905, he writes to a friend: “I go to Japan this autumn, but how and when I have not yet decided.” His old interest remained, and in April 1908, he arranged an exhibition in Boston by a Shinto priest of walking over hot coals and up a ladder of sword blades. “The place,” he says, “was full and the audience gratified at being asked. While in the distance people outside the pale stood on carts and boys even to the tops of far off houses, one perched onthe tip of a chimney. Dr. Suga cut himself slightly but not seriously. He did very well considering, though it was not possible of course for a poor lone priest to come up to what he might have done in Japan. The rite was beautifully set forth and the setting of the whole enclosure worthy the most artistic people in the world. Policemen kept out the crowd and stared aghast, and altogether it was a relished function.”
He probably would have been greatly grieved had he been told that he would never revisit the land where he had spent so much of his earlier life and thought; but astronomy was now his dominant occupation, and was constantly presenting new questions to engross his attention and fill his time. Yet in the years when Mars was not in opposition this did not prevent, indeed it rather stimulated, visits to Europe, where he saw his astronomical friends, and lectured on his discoveries; for he was a member of the National Astronomic Societies of France and Germany, had received from the former in 1904 the Janssen medal for his researches on Mars, and in 1907 Mr. Lampland that of the Royal Photographic Society of Great Britain for the work on the planets. We find him across the ocean in the summer of 1906, lunching with Sir Robert Ball in Cambridge, Deslandres and Flammarion in Paris, and “pegging away” there at his lectures.
Two years later, on June 10, 1908, he married Miss Constance Savage Keith, and they went abroad at the end of the month. When in London they met his first cousin, A. Lawrence Rotch, the meteorologist, who like him had established and directed, at his own expense, an observatory for the study of his subject; in this case on Blue Hill nearBoston. Percival wanted to photograph measurable lines to see how they appeared in a camera from the air. So he went up with his cousin in a balloon, and obtained photographs of the paths in Hyde Park which came out very well. His wife also went up with them; and, what with his reputation, the ascent in a balloon and their recent marriage, the event was too much for a reporter to resist; and there appeared in a newspaper an imaginary picture of an astronomer and a bride in a wedding dress taking their honeymoon in the basket of a balloon. They travelled together in England, Switzerland, Germany and France, and she recalls, when he was giving a lecture at the Sorbonne, a sudden exclamation from a Frenchman directly behind her: “Why! He is even clever in French!”
Mrs. Lowell has written an account of the diligence, the enthusiasm, the hardships of Percival and his colleagues, and the spirit of Flagstaff:
“In October, soon after our return from Europe, I discovered that the scientist’s motto is—“Time is sacred.” I was to meet him on the train for Flagstaff leaving the South Station at 2P.M.; anxious to impress him with my reputation for being punctual, I boarded the train about ten minutes before two. Percival came into the car, holding his watch in his hand, just about two minutes before two. He turned to me: “What time were you here?” I answered triumphantly: “Oh, I got here about ten minutes ago.” His reply was: “I consider that just as unpunctual as to be late. Think how much could have been accomplished in ten minutes!” I have never forgotten that remark. Percival never wasted minutes.
“Late in the afternoon of the third day, as we were nearingFlagstaff, through the dusk we could see that there had been a heavy fall of snow, so deep that when the train stopped our Pullman, being far in the rear, was where the snow—not having been shovelled—was almost level with the upper step. The men from the Observatory were there, and their first words were ‘Seeing Good.’ Percival jumped into the deep snow, and taking Mr. E. C. Slipher with him, drove to the telescope.
“Astronomers take much for granted so far as the details of domestic life are concerned, and I made up my mind to be a help and not a hindrance. Dr. V. M. Slipher’s wife came to the rescue, and under her supervision things were soon adjusted even to a hot supper and preparation for breakfast the next morning. She was, and always is, a wonder. Though the wife be not an astronomer a happy asset is it if she can appreciate her husband’s work, his sacrifices and self-denials. Many times have I seen their frost-bitten ears and thumbs; hungry and tired men, but never complaining—patience personified. They are slaves to the laws that rule the celestial.
“The house we lived in on Mars Hill was a long rambling one, both roof and sides shingled. Inside all but two rooms were finished, and partitioned. Two were papered; one of them I papered because no paper hanger happened to be in town. Occasionally Percival would come in to see how the work was progressing, and help by steadying the ladder or stirring the paste. The sitting room—or den, as it was referred to more often—was lined with half logs from which the bark had not been stripped. In the ceiling were logs used as beams. During the evening, when all was quiet, one might hear insects busily working out some scheme oftheir own. Open spaces were beamed and, as the logs did not exactly fit, through the spaces trade-rats would descend from the attic.
“To love nature, and the one for whom one works, it matters not where one is; that is what one realizes when on Mars Hill. One learns to go without things. They seem of such minor importance to that for which the men are seeking; one gets ashamed of oneself to think otherwise. Each man moves with a definite purpose, indefatigable workers, no thought of themselves when skies are clear, always watching, cold or torrid heat makes no difference, work goes on just the same.
“I became deeply impressed with the necessity of obedience to laws. I said once to Percival that I had been asked if it were true that he was an atheist, a non-believer. His answer was that he believed in keeping the laws; what chaos would happen if they were not. Often he would quote passages from the Bible—[Genesis I, 14-20]. The laws made on Mount Sinai, he said, are still the same laws to obey. To live in the atmosphere of such men accomplishing great things, deprived of many material comforts, makes one feel humble and spurs one on to ‘Help and not to hinder.’
“Servants we often had to do without. They would come out with us, and then after a few days, learning of the nearness to the Pacific coast, the lure of California would bring from them some lame excuse to leave, at once! To obtain others, when none were to be had in the town, I would have to go to Los Angeles. Finally, after several had left, I persuaded Percival to let me try to do the cooking; and later he would refer to that time as happy peaceful days. With the help of the kind wives, Mrs. Slipher and Mrs.Lampland, I learned much, how to make bread and soup,—two very essential articles in our household,—and to get up camping outfits and quick meals for unexpected guests.
“Lonesome, monotonous—never. Distant as Mars Hill may be from large cities, something of interest was happening continually. The State Normal School of Arizona is in the town, and on certain nights classes of students were brought up the hill to look through the telescope. Flagstaff is on the main line of the Santa Fe. There were three incoming trains from the East each day, and as many from the West, and many people stop off there to visit the different points of interest, the Lowell Observatory being one.
“In August, 1910, a group of astronomers, representing the International Union for Coöperation in Solar Research, debarked from the train, on their way to Pasadena; Professor Herbert H. Turner from England among them. He it was who many years later suggested for Percival’s ‘Planet X’ the name Pluto. The group, of about thirty, arrived by the first morning train and stayed at the Observatory until the last train left at night. The one thing that I was successful in getting enough of for lunch and dinner was watermelon. It proved a happy hit; for a year or two afterward, when telling how much they enjoyed their visit, the watermelons were spoken of as being such a treat. It was a hot day and the melons were cold; probably that explained their enthusiasm.
“One Christmas we invited all the children of Flagstaff to come to the Observatory for a Christmas tree and supper. Percival dressed as Santa Claus and spoke to them down the chimney; then he came down into the Library where they were gathered about the tree, and gave a presentand candy to every child. That was twenty-seven years ago. When I was in Flagstaff this spring, the little child I had held in my lap while Percival read ‘The Night Before Christmas’ came to speak to me and told me never would she forget that Christmas, and that her two little children repeatedly asked her to tell them the story of that Christmas and all that happened at the Santa Claus party on Mars Hill.”
In a recent letter to Mrs. Lowell, Dr. Lampland also gives a glimpse into Percival’s life at Flagstaff; and though written to refresh her recollections she preferred to insert it as it stands.
“Fresh in memory and pleasant to recall are your many visits to Flagstaff and your activities at the Observatory, where you were designing and supervising architect, carrying through the additions to the director’s residence, the garage, and the new administration building. And I also remember your valued help to us in connection with the house in which we live and your telegram ‘Mr. Lowell gives benediction and sanction to plans. Proceed.’”
He then goes on to tell of Percival’s friends from both West and East, and continues:
“You remember he was an enthusiastic gardener and always had a garden here at the Observatory. He had great success with many flowers and I recall especially fine displays of hollyhocks, zinnias, and a considerable variety of bulbs. Gourds, squashes and pumpkins were also great favorites. You will remember one year the especially fine collection of gourds and that bumper crop of huge pumpkins, many prize specimens being sugar fed. At times Dr. Lowell could be seen in the short intervals he took foroutdoor recreation, busy with his little camel’s hair brush pollenizing some of the flowers. And perhaps you will remember the little record book lying on the back veranda containing his observations of the daily growth of the diameter of the gourds, all measured carefully with little calipers. Then the frequent, almost daily, walks on the mesa. Certainly he knew all the surrounding country better than anyone here. He would refer to the different places such as Wolf Canyon, Amphitheatre Canyon, Indian Paint Brush Ridge, Holly Ravine, Mullein Patch, etc. In these walks he seemed to be constantly observing something new and of course trees, flowers, and wild life always interested him. Trees were an endless source of interest to him and he took many trips to more distant localities for these studies. Cedars or junipers seemed to be favorite subjects for study, though other varieties or kinds were not overlooked. An oak and an ash were named after him, new species that were discovered on the Observatory mesa and in Sycamore Canyon.
“At every season of the year he always found something in wild life to fascinate him, and you will remember his observations and notes of butterflies, birds, squirrels, rabbits, coyotes, deer and other inhabitants of the mesa. These friends must never be disturbed or harmed. But it was permissible to hunt with a camera! And he himself delighted with his kodak, photographing footprints, etc., and often attempting to get exposures of the creatures themselves. The Observatory grounds were a sanctuary for wild life.
“For many of us an interesting side of eminent personages is to know something about their activities, such for exampleas reading, outside of their professional occupations. In Dr. Lowell’s case you should find ample opportunity to treat a subject that will not admit of monotony. It would seem that practically every field of knowledge interested him. For the lighter reading as a relaxing and restful diversion you will remember the full bookshelves of detective stories, travel, exploration, etc. Accounts of adventure and discoveries, if well written, were welcome to his list of miscellaneous reading. The Latin classics were always near at hand, and widely and well had he read them, and much were they prized as friends in his later life.
“As you know, it is not easy for the observing astronomer to lead a strictly regular life in that the hours at the telescope often make it necessary to use, for the much needed rest, part of the daily hours usually given to work. His intense occupation with his research problems, however, was broken with great regularity for short intervals before lunch and dinner. These times of recreation were given to walks on the mesa or work in the garden. When night came, if he was not occupied at the telescope, he was generally to be found in his den. It was not always possible for him to lay aside his research problems at this time of the day, but he did have some wholesome views on the necessity of recreation and a necessary amount of leisure to prevent a person from falling into the habit of the ‘grind.’ To those who came to his den the picture of some difficult technical work near his chair, such as Tisserand’sMechanique Celestewill be recalled, though he might at the time be occupied with reading of a lighter character. And occasionally during the evening he might be seen consulting certain difficult parts upon which he was pondering....