In practice the sextant is an instrument only six or eight inches in diameter. It is held in the right hand and the movable radial arm is adjusted with the left hand with the aid of a micrometer screw, and the reading of the scale is made accurate by the vernier arrangement. The ordinary observation—which every traveler has seen a navigator make from the ship's bridge just at midday—is carried out by holding the sextant in a vertical position directly in line of the sun, and sighting the visible horizon line, meantime adjusting the recording apparatus so as to keep the sun's limb seemingly in touch with the horizon. As the sun is constantly shifting its position the vernier must be constantly adjusted until the observation shows that the sun is at the very highest point. The instrument being clamped and the scale read, the latitude may be known when proper correction has been made for the so-called dip, for refraction, and where great accuracy is required for parallax.
"TAKING THE SUN" WITH THE SEXTANT.The instrument is held in the right hand, and levelled at the horizon; the left hand manipulating the micrometer screw which adjusts the radial arm carrying the index mirror (at top of figure). The result is read on the Vernier scale (arc at bottom of figure) with the aid of the magnifying glass.
"TAKING THE SUN" WITH THE SEXTANT.The instrument is held in the right hand, and levelled at the horizon; the left hand manipulating the micrometer screw which adjusts the radial arm carrying the index mirror (at top of figure). The result is read on the Vernier scale (arc at bottom of figure) with the aid of the magnifying glass.
"TAKING THE SUN" WITH THE SEXTANT.
The instrument is held in the right hand, and levelled at the horizon; the left hand manipulating the micrometer screw which adjusts the radial arm carrying the index mirror (at top of figure). The result is read on the Vernier scale (arc at bottom of figure) with the aid of the magnifying glass.
Dip, it may be explained, is due to the fact that the observation is made not from the surface of the water but from an elevation, which is greater or less according to the height of the bridge, and which therefore varies with each individual ship. The error of refraction is due to the refraction of the sun's light in passing through the earth's atmosphere, and will vary with the temperature and the degree of atmospheric humidity, both of which conditions must be taken into account. The amount of refractive error is very great if an objectlies near the horizon. Everyone is familiar with the oval appearance of the rising or setting sun, which is due to refraction. With the sun at the meridian, the refractive error is comparatively slight; and when a star is observed at the zenith the refractive error disappears altogether.
By parallax, as here employed, is meant the error due to the difference in the apparent position of the sun as viewed by an observer at any point of the earth's surface from what the apparent position would be if viewed from the line of the center of the earth, from which theoretical point the observations are supposed to be made. In the case of bodies so distant as the sun, this angle is an exceedingly minute one, and in the case of the fixed stars it disappears altogether. The sun's parallax is very material indeed from the standpoint of delicate astronomical observations, but it may be ignored altogether by the practical navigator in all ordinary observations. There is one other correction that he must make, however, in case of sun observations; he must add, namely, the amount of semi-diameter of the sun to his observed measurement, as all calculations recorded in theNautical Almanacrefer to the center of the sun's disk.
The observation of the sun's height, with the various corrections just suggested, suffices by itself to define the latitude of the observer. Something more is required, however, before he can know his longitude.How to determine this, was a problem that long taxed the ingenuity of the astronomer. The solution came finally through the invention of the chronometer, which is in effect an exceedingly accurate watch.
Time measurers of various types have, of course, been employed from the earliest times. The ancient Oriental and Classical nations employed the so-called clepsydra, which consisted essentially of receptacles from or into which water dripped through a small aperture, the lapse of time being measured by the quantity of water. At an undetermined later date sand was substituted for the water, and the hour glass with which, in some of its forms, nearly everyone is familiar, came into use. For a long time this remained a most accurate of time measurers, though efforts were early made to find substitutes of greater convenience. Then clocks operated by weights and pulleys were introduced; and, finally, after the time of the Dutchman Huygens, the pendulum clock furnished a timepiece of great reliability. But the mechanism operated by weight or pendulum is obviously ill-adapted to use on shipboard. Portable watches, in which coiled springs took the place of the pendulum, had indeed been introduced, but the mechanical ingenuity of the watchmaker could not suffice to produce very dependable time-keepers. The very idea of a watch that would keep time accurately enough to be depended upon for astronomical observations intended to determine longitude was considered chimerical.
Nevertheless the desirability of producing a portable time-keeper of great accuracy was obvious, and theefforts of a large number of experimenters were directed towards this end in the course of the eighteenth century. These efforts were stimulated by the hope of earning a prize of twenty thousand pounds offered by the British Government for a watch sufficiently accurate to determine the location of a ship with maximum error of half a degree, or thirty nautical miles, corresponding to two minutes of time, in the course of a transatlantic voyage. It affords a striking illustration of the relative backwardness of nautical science, and of the difficulties to be overcome, to reflect that no means then available enabled the navigator at the termination of a transatlantic voyage to be sure of his location within the distance of thirty nautical miles by any means of astronomical or other observation known to the science of the time.
The problem was finally solved by an ingenious British carpenter named John Harrison, who devoted his life to the undertaking, and who came finally to be the most successful of watchmakers. Harrison first achieved distinction by inventing the compensating pendulum—a pendulum made of two metals having a different rate of expansion under the influence of heat, so adjusted that change in one was compensated by a different rate of change in the other. Up to the time of this discovery, even the best of pendulum clocks had failed of an ideal degree of accuracy owing to the liability to change of length of the pendulum—and so, of course, to corresponding change in the rate of its oscillation—with every alteration of temperature. Another means of effecting the desired compensation wassubsequently devised by Mr. Graham, through the use of a well of mercury in connection with the pendulum, so arranged that the expansion of the mercury upward in its tube would compensate the lengthening of the pendulum itself under effect of heat, and vice versa; but the Harrison pendulum, variously modified in design, remains in use as a highly satisfactory solution of the problem.
Harrison early conceived the idea that it might be possible to apply the same principle to the balance-wheel of the watch. This problem presented very great practical difficulties, but by persistent effort these were finally overcome, and a balance-wheel produced, which, owing to the unequal expansion and contraction of its two component metals under changing temperature, altered its shape and so maintained its rate of oscillation almost—though never quite—regardless of changing conditions of temperature.
In 1761 Harrison produced a watch which was tested on a British ship in a trip to the West Indies in that and the succeeding year, and which proved to be a time-keeper of hitherto unexampled accuracy. The inventor had calculated that the watch, when carried into the tropics, would vary its speed by one second per day with each average rise of ten degrees of temperature. Making allowance for this predicted alteration, it was found that the watch was far within the limits of variation allowed by the conditions of the test above outlined. It had varied indeed only five seconds during the journey across the ocean. On the return trip the watch was kept in a place near the stern of theship, for the sake of dryness, where, however, it was subjected to a great deal of joggling, which led to a considerably greater irregularity of action; but even so its variation on reaching British shores was such as to cause a maximum miscalculation of considerably less than thirty nautical miles.
Although Harrison seemed clearly enough to have won the prize, there were influences at work that interfered for a time with full recognition of his accomplishment. Presently he received half the sum, however, and ultimately, after having divulged the secret of his compensating balance and proved that he could make other watches of corresponding accuracy, he received the full award.
Minor improvements have naturally been made in the watch since that time, but the essential problem of making a really reliable portable timepiece was solved by the compensating balance-wheel of Harrison. The ship's chronometer of to-day is merely a large watch, with an escapement of particular construction, mounted on gimbals so that it will maintain a practically horizontal position.
Modern ships are ordinarily provided with at least three of these time-keepers in order that each may be compared with the others, and a more accurate determination of the time made than would be possible from observation of a single instrument; inasmuch as no absolutely accurate time-keeper has ever been constructed. Two chronometers would obviously be not much better than one, since there would be no guide as to whether any variation between them had beencaused by one running too fast or the other too slowly. But with a third chronometer to check the comparison, it is equally obvious that a dependable clue will be given as to the exact time.
It is to be understood of course that the variation of any of the chronometers will be but slight if they are good instruments. Moreover the tendency to vary in one direction or the other of each individual instrument will be known from previous tests. Such tests are constantly made at the Royal Observatory in England and elsewhere, and the best chronometers bear certificates as to their accuracy and as to their rate of variation. It may be added that a chronometer or other timepiece is technically said to be a perfect instrument, not when it has no variation at all—since this has proved an unattainable ideal—but when its variation is slight, is always in one direction, and is perfectly or almost perfectly uniform.
In the reference made above to the testing of Harrison's watch, it was stated that that instrument varied by only a certain number of seconds in the course of the westerly voyage across the Atlantic, and that its variation was somewhat greater on the return voyage. This implies, clearly, that some method was available to test the watch in the West Indies, without waiting for the return to England. At first thought this may cause no surprise, since the local time can of course be known anywhere through meridian observations; buton reflection it may seem less and less obvious as to just what test was available through which the exact difference in time between Greenwich, at which the watch was originally tested, and local time at the station in the West Indies could be determined. There are, however, several astronomical observations through which this could be accomplished, and in point of fact the comparative times and hence the precise longitudes at many points on the Western Hemisphere—and indeed of all portions of the civilized globe—were accurately known before the day of the chronometer.
One of the simplest and most direct means of testing the time of a place, as compared with Greenwich time, is furnished by observation of the occultation of one of the moons of Jupiter. By occultation is meant, as is well known, the eclipse of the body through passing into the shadow of its parent planet. This phenomenon, causing the sudden blotting out of the satellite as viewed from the earth, occurs at definite and calculable periods and is obviously quite independent of any terrestrial influence. It occurs at a given instant of time and would be observed at that instant by any mundane witness to whom Jupiter was at that time visible. If then an observer noted the exact local time at which occultation occurred, and compared this observed time with the Greenwich time at which such occultation was predicted to occur, as recorded in astronomical tables, a simple subtraction or addition will tell him the difference in time between his station and the meridian at Greenwich; and this difference of time can be translated into degrees of longitude by merely reckoning fifteen degreesfor each hour of time, and fractions of the hour in that proportion.
It will be noted that this observation has value for the purpose in question only in conjunction with certain tables in which the movements of Jupiter and its satellite are calculated in advance. This is equally true of the various other observations through which the same information may be obtained—as for example, the observation of a transit of Mars, or the measurement of apparent distance between the moon and a given fixed star. Before the tables giving such computations were published it was quite impossible to determine the exact longitude of any transatlantic place whatsoever. We have already pointed out that Columbus had only a vague notion as to how far he had sailed when he discovered land in the West. The same vagueness obtained with all the explorations of the immediately ensuing generations.
It was not until about the middle of the sixteenth century that Mercator and his successors brought the art of map-making to perfection; and the celebrated astronomical tables of the German Mayer, which served as the foundation for calculations of great importance to the navigator, were not published until 1753. The firstNautical Almanac, in which all manner of astronomical tables to guide the navigator were included, was published at the British Royal Observatory in 1767.
At the present time, a navigator would be as likely to start on a voyage without compass and sextant as without charts and aNautical Almanac. Indeedwere he to overlook the latter the former would serve but a vague and inadequate purpose. Yet, as just indicated, this invaluable adjunct to the equipment of the navigator was not available until well toward the close of the eighteenth century. But of course numerous general tables had been in use long before this, else—to revert to the matter directly in hand—it would not have been possible to make the above-recorded test in the case of Harrison's famous watch in the voyage of 1761–62.
In the days before the chronometer was perfected, almost numberless methods of attempting to determine the longitude of a ship at sea were suggested. There were astronomers who advocated observation of the eclipse of Jupiter's satellites; others who championed the method of so-called lunars—that is to say, calculation based on observation of the distance of the moon at a given local time from one or another of certain fixed stars arbitrarily selected by the calculator. Inasmuch as the seaman could always regulate even a faulty watch from day to day by observation of the meridian passage of the sun, it was thought that these observations of Jupiter's satellite or of the moon would serve to determine Greenwich time and therefore the longitude at which the observation was made with a fair degree of accuracy. But in practice it is not easy to observe the eclipse of Jupiter's satellite without a fair telescope; and it was soon found that the tables for calculatingthe course of the moon were by no means reliable, hence theoretically excellent methods of determining longitude by observation of that body proved quite unreliable in practice.
It was with the chief aim of bettering our knowledge of the moon's course through long series of very accurate observations that the Royal Observatory at Greenwich was founded. Perhaps it was not unnatural under these circumstances that certain of the Astronomers Royal should have advocated the method of lunars as the mainstay of the navigator. In particular Maskelyne, who was in charge of the Observatory in the latter part of the eighteenth century, was so convinced of the rationality of this method that he was led to discredit the achievements of Harrison's watches, and for a long time to exert an antagonistic influence, which the watchmaker resented bitterly and it would appear not without some show of reason.
Ultimately, however, the accuracy of the watch, and its indispensableness in the perfected form of the chronometer, having been fully demonstrated, the method of lunars became practically obsolete and the mariner was able to determine his longitude with the aid of sextant, chronometer, andNautical Almanac, by means of direct observation of the altitude of the sun by day and of sundry fixed stars by night, a much simpler calculation sufficing than was required by the older method.
As the sun is the chief time-measurer, whose rate of passage in a seeming circumnavigation of the heavens is recorded by the dial of clock, watch, or chronometer,it would seem as if the simplest possible method of determining longitude would be found through observation of the sun's meridian passage. The user of the sextant on shipboard always makes, if weather permits, a meridian observation of the sun, and such observation gives him an accurate gauge of the altitude of the sun at its highest point and hence of his own latitude. By adjusting the arm of the sextant with which this observation is made, the observer is able to determine the exact point reached by the sun in its upward course with all requisite accuracy.
But, unfortunately for his purpose, the sun does not poise for an instant at the apex of its upward flight and then begin its descent. On the contrary, its orbit being circular, the course of the sun just at its highest point is approximately horizontal for an appreciable length of time, and while the observer therefore has adequate opportunity to measure with accuracy the highest point reached, he cannot possibly make sure, within the limits of a considerable fraction of a minute, as to the precise moment when the center of the sun is on the meridian. He can, indeed, determine this point with sufficient accuracy for rough calculations, but modern navigation demands something more than rough calculations, inasmuch as a variation in time of one minute represents one-quarter of a degree of longitude, or fifteen nautical miles at the equator, and such uncertainty as this would imply can by no means be permitted in the safe navigation of a ship that may be passing through the water at the rate of a nautical mile in less than three minutes.
It follows that meridian observation of the sun, owing to the necessary inaccuracy of such observation, is not the ideal method. In point of fact the sun may be observed for this purpose to much better advantage when it is at a considerable distance from the meridian, since then its altitude above the horizon at a given moment is the only point necessary to be determined. The calculation by which the altitude of the sun may be translated into longitude is indeed more complicated in this case; but while spherical trigonometry is involved in the calculation, the tables supplied by theNautical Almanacenable the navigator to make the estimate without the use of any knowledge beyond that of the simplest mathematics.
While these observations tell the navigator his exact location in degrees of latitude and longitude, such knowledge does not of course reveal the distance traversed unless the precise length of the degree itself is known; and this obviously depends upon the size of the earth. Now we have seen that the earth was measured at a very early date by Greek and Roman astronomers, but of course their measurements, remarkable though they were considering the conditions under which they were made, were but rough approximations of the truth. Numerous attempts were made to improve upon these early measurements, but it was not until well into the seventeenth century that a really accurate measurement was made between two points on theearth's surface, the difference between which, as measured in degrees and minutes, was accurately known.
In June of the year 1633, the Englishman Robert Norman made very accurate observations of the altitude of the sun on the day of the summer solstice (when of course it is at its highest point in the heavens); the observation being made with a quadrant several feet in diameter stationed at a point near the Tower of London. On the corresponding day of the following year he made similar observations at a point something like 125 miles south of London, in Surrey. The two observations determined the exact difference in latitude between the two points in question.
Norman then undertook a laborious survey, that he might accurately measure the precise distance in miles and fractions thereof that corresponded to these known degrees of latitude. He made actual measurements with the chain for the most part, but in a few places where the topography offered peculiar difficulties he was obliged to depend upon the primitive method of pacing.
The modern surveyor, equipped with instruments for the accurate measuring of angles, not differing largely in principle from the quadrant of the navigator, would consider Norman's method of measurement a very clumsy one. He would measure only a single original base line of any convenient length, but would make that measurement with very great accuracy, using, perhaps, a rod packed in ice that it might not vary in length by even the fraction of an inch through changes in temperature. An accurate base line thussecured, he would depend thereafter on the familiar method of triangulation, in which angles are measured very accurately, and from such measurement the length of the sides of the successive triangles determined by simple calculation. In the end he would thus have made the most accurate determination of the distance involved, without having actually measured any portion thereof except the original base line. Notwithstanding the crudity of Norman's method, however, his estimate of the actual length of a degree of the earth's surface was correct, as more recent measurements have demonstrated, within twelve yards—a really remarkable result when it is recalled that the total length of the degree is about sixty nautical miles.
Inasmuch as the earth is not precisely spherical, but is slightly flattened at the poles, successive degrees of latitude are not absolutely uniform all along a meridian, but decrease slightly as the poles are approached. The deviation is so slight, however, that for practical purposes the degree of latitude may be considered as an unvarying unit. But obviously such is not the case with a degree of longitude. The most casual glance at a globe on which the meridian lines are drawn, shows that these lines intersect at the poles, and that the distance between them is, in the nature of the case, different at each successive point between poles and equator. It is only at the equator itself that a degree of longitude represents 1/360 of the earth's circumference. Everywhere else the parallels of latitude cut the meridians in what are termed small circles—that is to say, circles that do not represent circumference lines in theplane of the earth's center. Therefore while all points on any given meridian of longitude are equally distant in terms of degrees and minutes of arc from the meridian of Greenwich, the actual distances from that meridian of the different points as measured in miles will depend entirely upon their latitude.
At the equator each degree of longitude corresponds to (approximately) sixty miles, but in the middle latitudes traversed for example by the transatlantic lines, a degree of longitude represents only half that distance; and in the far North the meridians of longitude draw closer and closer together until they finally converge, and at the poles all longitudes are one.
It follows, then, that the navigator must always know both his latitude and his longitude in order to estimate the exact distance he has sailed. We have seen that a single instrument, the sextant, enables him to make the observations from which both these essentials can be determined. We must now make further inquiry as to the all important guide without the aid of which his observations, however accurately made, would avail him little. This guide, as already pointed out, is found in the set of tables known as theNautical Almanac.
Had the earth chanced to be poised in space with its axis exactly at right angles to its plane of revolution, many computations of the astronomer would be greatly simplified. Again, were the planetary course circularinstead of elliptical, and were the earth subject to no gravitational influences except that of the sun and moon, matters of astronomical computation would be quite different from what they are. But as the case stands, the axis of the earth is not only tipped at an angle of about twenty-three degrees, but is subject to sundry variations, due to the wobbling of the great top as it whirls.
Then the other planets, notably the massive Jupiter, exert a perverting influence which constantly interferes with the regular progression of the earth in its annual tour about the sun. A moment's reflection makes it clear that the gravitation pull of Jupiter is exerted sometimes in opposition to that of the sun, whereas at other times it is applied in aid of the sun, and yet again at various angles. In short, on no two successive days—for that matter no two successive hours or minutes—is the perturbing influence of Jupiter precisely the same.
What applies to the earth applies also, of course, to the varying action of Jupiter on the moon and to the incessantly varied action of the moon itself upon the earth. All in all, then, the course of our globe is by no means a stable and uniform one; though it should be understood that the perturbations are at most very slight indeed as compared with the major motions that constitute its regular action and lead to the chief phenomena of day and night and the succession of the seasons.
Relatively slight though the perturbations may be, however, they are sufficient to make noteworthy changesin the apparent position of the sun and moon as viewed with modern astronomical instruments; and they can by no means be ignored by the navigator who will determine the position of his ship within safe limits of error. And so it has been the work of the practical astronomers to record thousands on thousands of observations, giving with precise accuracy the location of sun, moon, planets, and various stars at given times; and these observations have furnished the basis for the elaborate calculations of the mathematical astronomers upon which the tables are based that in their final form make up theNautical Almanac, to which we have already more than once referred.
These calculations take into account the precise nature of the perturbing influences that are exerted on the earth and on the moon on any given day, and hence lead to the accurate prediction as to the exact relative positions of these bodies on that day. Stated otherwise, they show the precise position in the heavens which will be held at any given time by the sun for example, or by the important planets, as viewed from the earth. How elaborate these computations are may be inferred from the statement that the late Professor Simon Newcomb used about fifty thousand separate and distinct observations in preparing his tables of the sun. Once calculated, however, these tables of Professor Newcomb are so comprehensive as to supply data from which the exact position of the sun can be found for any day between the years 1200B.C.and 2300A.D., a stretch of some thirty-five centuries.
Such a statement makes it clear how very crude andvague the deductions must have been from the observations of navigators, however accurately made, prior to the time when such tables as those of theNautical Almanachad been prepared. Fully to appreciate this, it is necessary to understand that the observations supplied in such profusion for the use of the mathematical astronomer are in themselves subject to errors that might seriously vitiate the results of the final computation. They must, therefore, be made with the utmost accuracy, and with instruments specially prepared for the purpose. The chief of these instruments is not the gigantic telescope but the small and relatively simple apparatus known as a transit instrument. This constitutes essentially a small telescope poised on very carefully adjusted trunnions, in such a way that it revolves in a vertical axis, bringing into view any celestial body that is exactly on the meridian, and bodies in this position only. To make observation of the transit—that is to say the passage across the meridian line—of any given body more accurate, the transit instrument has stretched vertically across the center of its field of vision a spider web, or a series of parallel spider webs; in order, in the latter case, that the mean time of several observations may be taken.
So exceedingly difficult is it to manufacture and mount an instrument of requisite nicety of adjustment, that the effort has almost baffled the ingenuity of the mechanic. Sir George Airy, in making a transit instrument for use at the Royal Observatory at Greenwich, required the trunnions on which it was to be mounted to be ground truly cylindrical in form within a variationof one thirty-two-thousandth of an inch as determined by a delicate spirit level. Even when all but absolute decision has been obtained, however, it is quite impossible to maintain it, as the slightest variation of temperature—due perhaps to the application of the hand to one of the pillars on which the trunnions rest—may disturb the precise direction of the spider webs and so militate against absolute accuracy of observation. The instrument must, therefore, be constantly tested and its exact range of errors noted and allowed for.
To devote so much labor to details, merely in the effort to determine the precise moment at which a star or planet crosses the meridian, would seem to be an absurd magnification of trifles. But when we reflect that the prime object of such observations is to supply practical data which will be of service in enabling navigators on all the seas of the globe to bring their ships safely to port, the matter takes on quite another aspect. We have here, obviously, another and a very striking illustration of the close relationship that obtains between the work of the theoretical devotee of science and that of the practical man of affairs.
Though the navigator, thanks to his compass, sextant, andNautical Almanac, may determine with a high degree of precision his exact location, yet even the best observations do not enable him to approach a coast without safeguarding his ship by the use of anotherpiece of mechanism calculated to test the depth of the waters in which he finds himself at any given moment. In its most primitive form—in which form, by the bye, it is still almost universally employed—this apparatus is called the lead,—so called with much propriety because it consists essentially of a lump of lead or other heavy weight attached to a small rope. Knots in the rope enable the sailor who manipulates the lead to note at a glance the depth to which it sinks. Most ocean travelers have seen a sailor heaving the lead repeatedly at the side of the ship and noting the depth of the water, particularly as the ship approached the Long Island shore.
While this simple form of lead suffices for ordinary purposes, when the chief information sought is as to whether the water is deeper than the draft of the ship, it is at best only a rough and ready means of testing the depth in relatively shallow waters. For deeper waters and to test with greater accuracy the depths of uncharted regions, and in particular to determine the character of the sea bottom at any given place, more elaborate apparatuses are employed. One of the most useful of these is the invention of the late Lord Kelvin. In this the lead is replaced by a cannon ball, perforated and containing a cylinder which is detached when the weight reaches the bottom and is drawn to the surface filled with sand or mud, the cannon ball remaining at the bottom. In another form of patent lead, a float becomes detached so soon as the weight strikes the bottom and comes at once to the surface, thus recording the fact that the bottom has been reached,—a factnot always easy to appreciate by the mere feel of the line when the water is fairly deep.
It is obvious that however well informed the navigator may be as to his precise latitude and longitude, he can feel no safety unless he is equally well informed as to the depth of the water, the proximity of land, the presence or absence of shallows in the region, and the like. He must, therefore, as a matter of course, be provided with maps and charts on which these things are recorded. From the days when navigation first became a science, unceasing efforts have been made to provide such maps and charts for every known portion of the globe. Geographical surveys, with the aid of the method of triangulation, have been made along all coasts, and elaborate series of soundings taken for a long distance from the coast line, and there are now few regions into which a ship ordinarily sails, or is likely to be carried by accident, for which elaborate charts, both of coast lines and of soundings, have not been provided. The experienced navigator is able to direct his ship with safety along coasts that he visits for the first time, or to enter any important harbor on the globe without requiring the services of a local pilot,—albeit the desire to take no undue risk makes it usual to accept such services.
Time was, however, when maps and charts were not to be had, and when in consequence the navigator who started on his voyages of exploration was undertaking a feat never free from hazard. Until the time of Mercator there was not even uniformity of method among map makers in the charting of regions that had beenexplored. The thing seems simple enough now, thanks to the maps with which every one has been familiar since childhood. But it required no small exercise of ingenuity to devise a reasonably satisfactory method of representing on a flat surface regions that in reality are distributed over the surface of a globe. The method devised by Mercator, and which, as everyone knows, is now universally adopted, consists in drawing the meridians as parallel lines, giving therefore a most distorted presentation of the globe, in which the distance between the meridians at the poles—where in reality there is no distance at all—is precisely as great as at the equator. To make amends for this distortion, the parallels of latitude are not drawn equidistant, as in reality they practically are on the globe, but are spaced farther and farther apart, as we advance from the equator toward either pole. The net result is that an island in the arctic region would be represented on the map several times as large as an island actually the same size but located near the equator. Doubtless most of us habitually conceive Alaska and Greenland to be vastly more extensive regions than they really are, because of our familiarity with maps showing this so-called "Mercator's projection."
Of course maps are also made that hold to the true proportions, representing the lines of latitude as equidistant and the meridians of longitude as lines converging to a point at the poles. But while such a map as this has certain advantages—giving, for example, a correct notion of the relative sizes of polar and other land masses—it is otherwise confusing inasmuch asplaces that really lie directly in the north and south line cannot be so represented except just at the middle of the map, and it is very difficult for the ordinary user of the map to gain a clear notion as to the actual points of the compass. A satisfactory compromise may be effected, however, by using Mercator's projection for maps showing wide areas, while the other method is employed for local maps.
While the average man, even with well developed traveling instincts, would perhaps prefer always to feel that he is sailing in well charted waters and along carefully surveyed coasts, there have been in every generation men who delighted in taking risks, and for whom half the charm of a voyage must always lie in its dangers. Such men have been the pioneers in exploring the new regions of the globe. That there was no dearth of such restless spirits in classical times and even in the dark ages, records that have come down to us sufficiently attest. For the most part, however, their names have not been preserved to us. But since the ushering in of the period which we to-day think of as the beginning of modern times, records have been kept of all important voyages of discovery, and at least the main outlines of the story of the conquest of the zones are familiar to everyone.
Some of the earliest explorers, most notable among whom was the Italian Marco Polo, traveled eastward from the Mediterranean and hence journeyed largelyby land. But soon afterward, thanks to the introduction of the compass,—which instrument Marco Polo has sometimes been mistakenly accredited with bringing from the East,—the adventurers began to cast longing eyes out toward the western horizons. Among the first conspicuous and inspiriting results were the discoveries of the groups of islands known as the Cape Verdes and the Azores. The Canary Islands were visited by Spaniards even earlier, and became the subject of controversy with the other chief maritime nation of the period, the Portuguese.
When the controversy was adjusted the Spaniards were left in possession of the Canaries, but the Portuguese were given by treaty the exclusive right to explore the coast of Africa. Following up sundry tentative efforts, the daring Portuguese navigator, Bartholomeo Dias, in the year 1487, passed to the southern-most extremity of Africa, which he christened the Cape of Good Hope. At last, then, it had been shown that Africa did not offer an interminable barrier to the passage to the fabled land of treasures in the East. Before anyone had ventured to follow out the clues which the discovery of the Cape had presented, however, Columbus had seemingly solved the problem in another way by sailing out boldly into the West and supposedly coming to the East Indies in 1492.
The western route was barred to the Portuguese but the eastern one remained open to them, and before the close of the century Vasco da Gama had set out on the voyage that ultimately led him to India by way of the Cape (1497–1500A.D.). Twenty years later anotherPortuguese navigator, Magellan by name, started on what must ever remain the most memorable of voyages, save only that of Columbus. Magellan rounded the southern point of South America and in 1521 reached the Philippines, where he died. His companions continued the voyage and accomplished ultimately the circumnavigation of the globe; and in so doing afforded the first unequivocal practical demonstration, of a character calculated to appeal to the generality of uncultured men of the time, that the world is actually round.
Two routes from Europe to the Indies had thus been established, but both of them were open to the objection that they necessitated long detours to the South. To the geographers of the time it seemed more than probable that a shorter route could be established by sailing northward and coasting along the shores either of Europe to the East or—what seemed more probable—of America to the West. Toward the close of the sixteenth century the ships of the Dutch navigators had penetrated to Nova Zembla, and a few years later Henry Hudson visited Spitzbergen, thus inaugurating the long series of arctic expeditions. Then Hudson, still sailing under the Dutch flag, made heroic efforts to find the fabled northwest passage, only to meet his doom in the region of the Bay that has since borne his name.
This was in the year 1610. For long generations thereafter successors of Hudson were to keep up thefutile quest; and when finally it had been clearly established that no northwest passage to the Pacific could be made available, owing to the climate, the zest for arctic exploration did not abate, but its goal was changed from the hypothetical northwest passage to the geographical pole.
Henry Hudson had in his day established a farthest North record of about the eighty-second parallel of latitude—leaving only about five hundred miles to be traversed. But three centuries were required in which to compass this relatively small gap. Expedition after expedition penetrated as far as human endurance under given conditions could carry it. Some of the explorers returned with vivid tales of the rigors of the arctic climate; others fell victim to conditions that they could not overcome. But the seventeenth, eighteenth, and nineteenth centuries passed and left the "Boreal Center" undiscovered.
Toward the close of the nineteenth century the efforts of explorers seemed to be redoubled and one famous expedition after another established new records of "farthest North." The names of Nansen, the Duke of the Abruzzi, and Peary, became familiar to a generation whose imagination seemed curiously in sympathy with that lure of the North which determined the life activities of so many would-be discoverers. So when in the early Autumn of 1909 it was suddenly announced that two explorers in succession had at last, in the picturesque phrasing of one of them, "penetrated the Boreal Center and plucked the polar prize," the popular mind was stirred as it has seldom been by any otherevent not having either a directly personal or an international political significance.
The two men whose claims to have discovered the pole were thus announced in such spectacular fashion, were Dr. Frederick A. Cook, of Brooklyn, and Lieutenant Commander Robert E. Peary, of the United States Navy. Dr. Cook claimed to have reached the pole, accompanied only by two Eskimo companions, on the twenty-first day of April, 1908. Commander Peary reported that he had reached the pole, accompanied by Mr. Matthew H. Henson and four Eskimos, on the seventh day of April, 1909.
The controversy that ensued regarding the authenticity of these alleged discoveries is not likely to be forgotten by any reader of our generation. Its merits and demerits have no particular concern for the purely scientific inquirer. At best, as Professor Pickering of Harvard is reported to have said, "the quest of the pole is a good sporting event" rather than an enterprise of great scientific significance. It suffices for our present purpose, therefore, to know that Dr. Cook's records, as adjudged by the tribunal of the University of Copenhagen, to which they were sent, were pronounced inadequate to demonstrate the validity of his claim; whereas Peary and Henson were adjudged by the American Geographical Society, after inspection of the records, to have accomplished what was claimed for them. What has greater interest from the present standpoint is the question, which the controversy brought actively to the minds of the unscientific public, as to how tests are made which determine, in the mindof the explorer himself, the fact of his arrival at the pole.
The question has, indeed, been largely answered in the earlier pages of this chapter, in our discussion of the sextant and theNautical Almanac; for these constitute the essential equipment of the arctic explorer no less than of the navigators of the seas of more accessible latitudes. There is one important matter of detail, however, that remains to be noted. This relates to the manner of using the sextant. On the ocean, as we have seen, the navigator levels the instrument at the visible horizon; but it is obvious that on land or on the irregular ice fields of the arctic seas no visible horizon can be depended upon as a basis for measuring the altitude of sun or stars. So an artificial horizon must be supplied.
The problem is solved by the use of a reflecting surface, which may consist of an ordinary mirror or a dish of mercury. The glass reflector must be adjusted in the horizontal plane with the aid of spirit levels; mercury, on the other hand, being liquid, presents a horizontal surface under the action of gravitation. Unfortunately mercury freezes at about 39 degrees below zero; it is therefore often necessary for the arctic explorer to melt it with a spirit lamp before he can make use of it. These, however, are details aside from which the principles of use of glass and mercury horizon are identical. The method consists simply in viewing the reflected image of the celestial body—which in practice in the arctic regions is usually the sun—and so adjusting the sextant that the direct image coincides with thereflected one. The angle thus measured will represent twice the angular elevation of the body in question above the horizon,—this being, as we have seen, the information which the user of the sextant desires.
Of course the explorer makes his "dash for the pole" in a season when the sun is perpetually above the horizon. As he approaches the pole the course of the sun becomes apparently more and more nearly circular, departing less and less from the same altitude. Hence it becomes increasingly difficult to determine by observation the exact time when the sun is at its highest point. But it becomes less and less important to do so as the actual proximity of the pole is approached; and as viewed from the pole itself the sun, circling a practically uniform course, varies its height in the course of twenty-four hours only by the trifling amount which represents its climb toward the summer solstice. Such being the case, an altitude observation of the sun may be made by an observer at the pole at any hour of the day with equal facility, and it is only necessary for him to know from his chronometer the day of the month in order that he may determine from theNautical Almanacwhether the observation really places him at ninety degrees of latitude. Nor indeed is it necessary that he should know the exact day provided he can make a series of observations at intervals of an hour or two. For if these successive observations reveal the sun at the same altitude, it requires noAlmanacand absolutely no calculation of any kind to tell him that his location is that of the pole.
The observation might indeed be made with a fairdegree of accuracy without the use of the sextant or of any astronomical equivalent more elaborate than, let us say, an ordinary lead pencil. It is only necessary to push the point of the pencil into a level surface of ice or snow and leave it standing there in a vertical position. If, then, the shadow cast by the pencil is noted from time to time, it will be observed that its length is always the same; that, in other words, the end of the shadow as it moves slowly about with the sun describes a circle in the course of twenty-four hours. If the atmospheric conditions had remained uniform, so that there was no variation in the amount of refraction to which the sun's rays were subjected, the circle thus described would be almost perfect, and would in itself afford a demonstration that would appeal to the least scientific of observers.
An even more simple demonstration might be made by having an Eskimo stand in a particular spot and marking the length of his shadow as cast on a level stretch of ice or snow. Just twelve hours later let the Eskimo stand at the point where a mark had been made to indicate the end of the shadow, and it would be found that his present shadow—cast now, of course, in the opposite direction—would reach exactly to the point where he had previously stood. The only difficulty about this simple experiment would result from the fact that the sun is never very high as viewed from the pole and therefore the shadow would necessarily be long. It might therefore be difficult to find a level area of sufficient extent on the rough polar sea. In that case another measurement similar in principle could bemade by placing a pole upright in the snow or ice and marking on the pole the point indicated by the shadow of an Eskimo standing at any convenient distance away. At any interval thereafter, say six or twelve hours, repeat the experiment, letting the man stand at the same distance from the pole as before, and his shadow will be seen to reach to the same mark.
Various other simple experiments of similar character may be devised, any of which would appeal to the most untutored intelligence as exhibiting phenomena of an unusual character. Absolutely simple as these experiments are, they are also, within the limits of their accuracy, absolutely demonstrative. There are only two places on the globe where the shadow of the upright pencil would describe a circle, or where the man's shadow would be of the same length at intervals of twelve hours, or would reach to the same height on a pole in successive hours. These two regions are of course the poles of the earth. It may reasonably be expected that explorers who reach the poles will make some such experiments as these for the satisfaction of their untrained associates, to whom the records of the sextant would be enigmatical. But for that matter even an Eskimo could make for himself a measurement by using only a bit of a stick held at arm's length—as an artist measures the length of an object with his pencil—that would enable him to make reasonably sure that the sun was at the same elevation throughout the day—subject, however, to the qualification that the polar ice was sufficiently level to provide a reasonably uniform horizon.
While, therefore, it appears that the one place of all others at which it would be exceedingly easy to determine one's position from the observation of the sun is the region of the pole, it must be borne in mind that the low elevation of the sun, and the extreme cold may make accurate instrumental observations difficult; and it is conceivable that the explorer who had the misfortune to encounter cloudy weather, and who therefore gained only a brief view of the sun, might be left in doubt as to whether he had really reached the goal of his ambition. Fortunately, however, the explorers who thus far claim to have reached the pole record uninterruptedly fair weather, enabling observations to be taken hour after hour. Under these circumstances, there could be no possibility of mistake as to the general location, although perhaps no observation, under the existing conditions, could make sure of locating the precise position of the pole within a few miles.
A curious anomaly incident to the unique geographical location of the pole is that to the observer stationed there all directions are directly south. Yet of course all directions are not one, and the query may arise as to how an explorer who has reached the pole may know in what direction to start on his return voyage. The answer is supplied by the compass, which—perforce pointing straight south—indicates the position of the magnetic pole and so makes clear in which direction lies the coast of Labrador. Moreover if the explorer is provided with reliable chronometers, which of course record the time at a given meridian—say that of Greenwich—these will enable him to determine by the simplestcalculation what particular region lies directly beneath the sun at any given time. If, for example, his chronometer shows five o'clock Greenwich time, he knows that the sun's position, as observed at the moment, marks the meridian five hours (i.e., 75° of longitude) west of Greenwich.
While the arctic region appears thus to have given up its last secret, this is not as yet true of the antarctic. The expedition of Lieutenant (now Sir Ernest) Shackleton, in 1908, approached within about one hundred and eleven miles of the South Pole. The intervening space—less than two degrees in extent—represents, therefore, the only stretch of latitude on the earth's surface that has not been trodden by man's foot or crossed by his ships. More than one expedition is being planned to explore this last remaining stronghold, and in all probability not many years—perhaps not many months—will elapse before the little stretch of ice that separated Lieutenant Shackleton from the South Pole will be crossed, and man's conquest of the zones will be complete.
THEREis no doubt that the use of sails for propelling boats is as old as civilization itself. We know that the Egyptians used sails at least 4,000 years before the Christian era. They did not depend entirely upon the sails, however, but used oars in combination with them. Steering was done with single or double oars lashed to the stern and controlled by ropes or levers. This method of steering remained in use until late in the Middle Ages, the invention of the rudder being one of the few nautical inventions made during the centuries immediately following that unproductive period of history known as the Dark Age.
Following the Egyptians, the Phœnicians were the greatest maritime nation of ancient times, but unfortunately they have left no very satisfactory and authentic records describing their boats. In all probability, however, their ships were galleys having one or two banks of oars, fitted with sails similar to those of the Egyptians.
If our knowledge of Phœnician boats is meager, our knowledge of Greek boats, particularly the fighting craft, is correspondingly full. From the nature of its geographical location Greece was necessarily a maritimenation, and it was here that boat-building reached a very high state of development during the period of Greek predominance. Large ships fitted with sails and having several banks of rowers were used habitually in commerce and war, and it was here also that the management of sails became so well understood that oars were often dispensed with except as auxiliaries.
It was in Greece that the custom of having several banks of oars superimposed reached its highest development, but the fabulous number of such banks credited by some authors seems to be entirely without foundation. It is possible that as many as seven banks were used, although the evidence in favor of more than five is very slight.
The writings of Callixenos describe a ship said to have been used by Ptolemy Philopater, which was a forty-banker. This ship is described as 450 feet long, 57 feet broad, carrying a crew of about 7,000 men, of whom 4,000 were rowers. This description need not be taken seriously, as there is no proof that boats of such proportions were ever attempted in ancient times. But it is certain that the Greeks did build large vessels, some of them at least one hundred and fifty feet long—perhaps even larger than this. The tendency of shipbuilders during the later Greek period was to build large, unwieldy boats, which used sails under favorable circumstances, but depended entirely upon oars for manœuvering in battle.
The Romans used similar vessels of large size until the time of the battle of Actium, where the clumsy, many-banked ships of Antony and Cleopatra were destroyedby the lighter single- or double-banked vessels of Augustus. Augustus had adopted the low, swift, handy vessels of a piratical people, the Liburni, who had learned in their sea fights against all kinds of vessels that the lighter type of boat could be used most effectively. Structurally the hulls of these boats were not unlike modern wooden vessels.
While the various types of vessels were being developed in the Mediterranean region, a race of mariners far to the north were perfecting boats in which they were destined to overrun the Western seas from the tropics to the arctic circle. These people, the Norsemen, left few written descriptions that give a good idea of the construction of their boats, which were sufficiently seaworthy to enable the Danes to cross the Atlantic and colonize America. But thanks to one of their peculiar burial customs some of their smaller boats have been preserved and brought to light in recent years. It was their custom when a great chief died, to bury him in a ship, heaping earth over it to form a great mound. In most instances the wood of such boats, buried for a thousand years, has entirely disappeared; but in some mounds the boats have been preserved almost intact.
From the specimens so preserved it is known that the Norsemen knew how to shape the hulls of their boats almost as well as the modern boat-builder. This fact is interesting because the immediate successors of the Norsemen, either through ignorance or choice, reverted to most primitive types in building their boats. Thus it required centuries for them to develop a knowledgeof hull-construction that was familiar in ancient times to the northern rovers. Scandinavia itself never entirely forgot the art, and there are boats built in Norway to-day closely similar in all essentials to some of the boats constructed by the Norsemen.
The contrast in shape and construction between the trim ships of the Norsemen and the short, top-heavy vessels which were the approved European type during the early Middle Ages, is most striking. The Mediæval shipbuilders in striving to improve their craft, making them as seaworthy and as spacious as possible, first added decks, and then built towering superstructures at bow and stern. The result was a vessel which would have been so top-heavy that it would be likely to capsize had it not been so broad that "turning turtle" was out of the question.
It was in such ships that Columbus made his voyage of discovery in 1492, although the superstructures fore and aft on his boat were less exaggerated than in some later vessels. Nevertheless they were veritable "tubs"; and we know from the experience of the crew that sailed the replica of theSanta Mariaacross the ocean in 1893, that they were anything but comfortable craft for ocean traveling.
This replica of theSanta Mariawas reproduced with great fidelity by the Spanish shipbuilders, and, manned by a Spanish crew, crossed the ocean on a course exactly following that taken by Columbus onhis first voyage. Sir George Holmes' terse description of this voyage is sufficiently illuminating without elaboration. "The time occupied was thirty-six days," he says; "and the maximum speed attained was about 6-1/2 knots. The vessel pitched horribly!"
Two full centuries before the discovery of America the rudder had been invented. There is no record to show who was responsible for this innovation, although its superiority over the older steering appliances must have been appreciated fully. But after the beginning of the fourteenth century the rudder seems to have come into general use, entirely supplanting the older side-rudder, or clavus.
For a full century after the voyage of Columbus little progress was made in ship construction; short, stocky boats, with many decks high above the water-line at bow and stem continuing to be the most popular type. In the opening years of the seventeenth century, however, the English naval architect, Phineas Pett, departed from many of the accepted standards of his time, and produced ships not unlike modern full-rigged sailing vessels, except that the stern was still considerably elevated, and the bow of peculiar construction. One of Pett's ships,The Sovereign of the Seas, was a vessel 167 feet long, with 48 foot beam, and of 1,683 tons burthen. The introduction of this type of vessel was a distinct step forward toward modern shipbuilding.