Read Ebook: Every-day Science: Volume 7. The Conquest of Time and Space by Williams Edward Huntington Williams Henry Smith

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In order to visualize the contrast between the practical civilization of to-day and that of our grandparents, it suffices to recall that the first steam locomotive that carried passengers over a railway was put in operation in the year 1829; and that the first ship propelled by steam power alone did not cross the ocean until 1838. Not until well towards the middle of the nineteenth century, then, were the conditions of transportation altered materially from what they had been since the very dawn of civilization,--conditions under which one hundred miles constituted about the maximum extent of a hard day's land journey.

The elaboration of railway and steamship lines through which nearly all portions of the habitable globe have been made accessible, has constituted one of the most remarkable examples of economic development that man has ever achieved. It requires but the slightest use of the imagination to realize with some measure of vividness the extent to which the entire structure of present-day civilization is based upon this elaboration of means of transportation. To point but a single illustration, the entire central and western portion of the United States must have remained a wilderness for decades or centuries had not the steam locomotive made communication easy between these regions and the seaboard.

Contrariwise no such development of city life as that which we see throughout Christendom would have been possible but for the increased facilities, due primarily to locomotives and steamships, for bringing all essential food-stuffs from distant regions.

What this all means when applied on a larger scale may be suggested by the reflection that the entire character of the occupation of the average resident of England has been changed within a century. A century ago England was a self-supporting nation, in the sense that it produced its own food-stuffs. To-day the population of England as a whole is dependent upon food shipped to it from across the oceans. Obviously such a transformation could never have been effected had not the application of steam revolutionized the entire character of transportation.

Far-reaching as are the economic aspects of the problem of transportation, this extraordinary revolution, the effects of which are visible on every side, has been brought about by the application of only a few types of mechanisms. The steam engine, the dynamo, and the gas engine are substantially responsible for the entire development in question. In the succeeding pages, which deal with the development of steamships, locomotives, automobiles, and flying machines, we have to do with the application of principles with which our previous studies have made us familiar; and in particular with the mechanisms that have engaged our attention in the preceding volume. Yet the application of these principles and the utilization of these mechanisms gave full opportunity for the exercise of inventive ingenuity, and the story of the development of steamships, locomotives, electric vehicles, automobiles, gyro cars, and flying machines, will be found to have elements of interest commensurate with the importance of these mechanisms themselves. Before we take up these stories in detail, however, we shall briefly review the story of geographical discovery and exploration in its scientific aspects.

THE CONQUEST OF THE ZONES

The contrast between modern and ancient times is strikingly suggested by reflection on the limited range of geographical knowledge of those Oriental and Classical nations who dominated the scene at that remote period which we are accustomed to characterize as the dawn of history. The Egyptians, peopling the narrow valley of the Nile, scarcely had direct dealings with any people more remote than the Babylonians and Assyrians occupying the valley of the Euphrates. Babylonians and Assyrians in turn were in touch with no Eastern civilization more remote than that of Persia and India, and knew nothing of any Western world beyond the borders of Greece. Greeks and Romans, when in succession they came to dominate the world stage,--developing a civilization which even as viewed from our modern vantage-ground seems marvelous,--were still confined to narrow strips of territory about the shores of the Mediterranean, and had but the vaguest notions as to any other regions of the earth.

In the later classical period, to be sure, the globe was subjected, as we have seen, to wonderful measurements by Eratosthenes and by Posidonius, and the fact that man's abiding place is a great ball utterly different from the world as conceived by the Oriental mind, was definitely grasped and became more or less a matter of common knowledge. It was even conceived that there might be a second habitable zone on the opposite side of the equator from the region in which the Greeks and Romans found themselves, but as to just what this hypothetical region might be like, and as to what manner of beings might people it, even the most daring speculator made no attempt to decide. The more general view, indeed, precluded all thought of habitable regions lying beyond the confines of the Mediterranean civilization; conceiving rather that the world beyond was a mere waste of waters.

Doubtless the imaginative mind of the period must have chafed under these restrictions of geographical knowledge; and now and again a more daring navigator must have pressed out beyond the limits of safety, into the Unknown, never to return. Once at least, even in the old Egyptian days, a band of navigators surpassing in daring all their predecessors, and their successors of the ensuing centuries, made bold to continue their explorations along the coast of Africa till they had passed to a region where--as Herodotus relates with wonder--the sun appeared "on their right hand," ultimately passing about the southern extremity of the African continent and in due course completing the circumnavigation, returning with wonder tales to excite the envy, perhaps, but not the emulation of their fellows.

Then in due course some Phoenician or Greek navigators coasted along the northern shores beyond the "Pillars of Hercules" and discovered at the very confines of the world what we now term the British Isles. But this was the full extent of exploration throughout antiquity; and the spread of civilization about the borders of the known world was a slow and haphazard procedure during all those centuries that mark the Classical and Byzantine periods.

THE MARINER'S COMPASS

The change came with that revival of scientific learning which was to usher in the new era that we speak of as modern times. And here as always it was a practical mechanism that gave the stimulus to new endeavor. In this particular case the implement in question was the mariner's compass, which consists, in its essentials, as everyone is aware, of a magnetized needle floated or suspended in such a way that it is made under the influence of terrestrial magnetism to point to the north and south.

The mysterious property whereby the magnetized needle obeys this inscrutable impulse is, in the last analysis, inexplicable even to the science of our day. But the facts, in their cruder relations, had been familiar from time immemorial to a nation whose habitat lay beyond the ken of the classical world--namely, the Chinese. It seems to be fairly established that navigators of that nation had used the magnetized needle, so arranged as to constitute a crude compass, from a period possibly antedating the Christian Era. To Western nations, however, the properties of the magnetized needle seem to have been quite unknown--at least its possibilities of practical aid to the navigator were utterly unsuspected--until well into the Middle Ages. There is every reason to believe--though absolute proof is lacking--that a knowledge of the compass came to the Western world from the Far East through the medium of the Arabs. The exact channel of this communication will perhaps always remain unknown. Nor have we any clear knowledge as to the exact time when the all-important information was transmitted. We only know that manuscripts of the twelfth century mentioned the magnetic needle as an implement familiar to navigators, and from this time forward, we may feel sure, the new possibilities of exploration made possible by the compass must have suggested themselves to some at least of the more imaginative minds of each generation. Indeed there were explorers in each generation who pushed out a little into the unknown, as the discovery of various groups of Islands in the Atlantic shows, although the efforts of these pioneers have been eclipsed by the spectacular feat of Columbus.

The exact steps by which the crude compass of the Orientals was developed into the more elaborate and delicate instrument familiar to Western navigators cannot be traced by the modern historian. It is known that sundry experiments were made as to the best form of needle, and in particular as to the best way of adjusting it on approximately frictionless bearings. But a high degree of perfection in this regard had been attained before the modern period; and the compass had been further perfected by attaching the needle to a circumferential card on which the "points of the compass," thirty-two in number, were permanently marked. At all events the compass card had been so divided before the close of the fourteenth century, as is proved by a chance reference by Chaucer. The utility of the instrument thus perfected--indeed its entire indispensableness--was doubtless by this time clearly recognized by all navigators; and one risks nothing in suggesting that without the compass no such hazardous voyage into the unknown as that of Columbus would ever have been attempted.

No doubt the earliest observers of the needle believed that it pointed directly to the North. If such were indeed the fact the entire science of navigation would be vastly simpler than it is. But it required no very acute powers of observation to discover that the magnetized needle does not in reality point directly towards the earth's poles. There are indeed places on the earth where it does so point, but in general it is observed to deviate by a few degrees from the exact line of the meridian. Such deviation is technically known as magnetic declination. That this declination is not the same for all places was discovered by Columbus in the course of his first transatlantic voyage.

A century or so later, the accumulated records made it clear that declination is not a fixed quantity even at any given place. An Englishman, Stephen Burrows, is credited with making the discovery that the needle thus shifts its direction slightly with the lapse of time, and the matter was more clearly determined a little later by Gillebrand, Professor of Geometry at Graham College. Dr. Halley, the celebrated astronomer--whose achievements have been recalled to succeeding generations by the periodical return of the comet that bears his name--gave the matter attention, and in a paper before the Royal Society in 1692 he pointed out that the direction of the needle at London had changed in a little over a century from 11 degrees 15 minutes East to 6 degrees West, or more than 17 degrees.

Halley conclusively showed that similar variations occurred at all other places where records had been kept. He had already demonstrated, a few years earlier, that the deviations of the compass noted at sea are not due to the varying attractions of neighboring bodies of land, but to some influence having to do with the problem of terrestrial magnetism in its larger aspects. Halley advocated the doctrine, which had first been put forward by William Gilbert, that the earth itself is a gigantic magnet, and that the action of the compass is dependent upon this terrestrial source and not, as many navigators believed, on the influence of a magnetic star, or on localized deposits of lodestone somewhere in the unknown regions of the North.

Further observations of the records presently made it clear that there are also annual and even daily variations of the compass of slight degree. The fact of diurnal variations was first discovered by Mr. Graham about the year 1719. More than half a century later it was observed by an astronomer named Wales, who was accompanying Captain Cook on his famous voyage round the world , that there is yet another fluctuation of the compass due to the influence of the ship on which it is placed. Considerable quantities of iron were of course used in the construction of wooden ships, and it was made clear that the ship itself comes under the influence of the earth's magnetism and exerts in turn an appreciable influence on the compass. The fluctuation due to this source is known as deviation, in contradistinction to the larger fluctuation already referred to as declination.

Not only is the deviation due to the ship's influence a matter of importance, but it was discovered by Captain Matthew Flinders, in the course of his explorations along the coast of New Holland in the year 1801-02, that the influence of the ship over its compass varies with the direction of the ship's prow.

Needless to say, the problem of the deviation of the compass due to the influence of the ship is enormously complicated when the ship instead of being constructed chiefly of wood is made of iron or steel. It then becomes absolutely essential that the influence of vessels shall be reckoned with and so far as possible compensated. Such compensation may be effected by the adjustment of bodies of iron, as first suggested by Barlow, or by the use of permanent magnets, as first attempted by England's Astronomer Royal, Professor Airy. At the very best, however, it is never possible totally to overcome the ship's perverting influence, allowance for which must be made if an absolutely accurate conclusion is to be drawn from the record presented by the compass.

Not only does the magnetized needle thus tend to vary in the direction of its horizontal action, but it also tends when suspended at the middle to shift its vertical axis. In regions near the equator, indeed, the magnetized needle maintains a horizontal position, but if carried into northern or southern latitudes it progressively "dips," its polar end sinking lower and lower. This dipping of the needle seems to have been first observed by Robert Norman, an English nautical instrument maker, about the year 1590. It was brought to the attention of Gilbert and carefully tested by him in the course of his famous pioneer experiments. Gilbert was led to predicate the existence of magnetic poles, the exact location of which would be indicated by the dipping needle, which, sinking lower and lower as northern latitudes were attained, would ultimately at the magnetic pole itself assume a vertical direction.

That this is a correct expression of the facts was determined in the year 1831 by Sir James Ross, who in the course of his Arctic explorations observed the vertical dip and so located the northern magnetic pole at about 70 degrees 5 minutes north latitude and 96 degrees 43 minutes west longitude. It was thus proved that the magnetic pole is situated a long distance--more than 1,200 miles--from the geographical pole. The location of the south magnetic pole was most accurately determined in 1909 by Lieutenant Shackleton's expedition at about 73 degrees south latitude and 156 degrees east longitude. The two magnetic poles are thus not directly opposite each other on the earth's surface, and the magnetic axis of the earth does not coincide with the geographical center of the globe itself.

From the standpoint of practical navigation the dip of the needle is a matter of much less significance than its horizontal fluctuations. Robert Norman himself attempted to overcome the dip by a balancing apparatus applied to the needle; and the modern compass is suspended in such a way that the propensity to dip does not interfere with the lateral movements which supply the navigator with all important information. The modern compass in question is the invention of Lord Kelvin and was patented by him in 1876. It consists of a number of small magnets arranged in parallel and held in position by silk threads, each suspended, cobweb-like, from the circular rim of aluminum. The weight--which in the aggregate is relatively slight--being thus largely at the circumference, the instrument has a maximum period of oscillation and hence a high degree of stability. Its fluctuations due to the ship's influence are corrected by a carefully adjusted disposition of metal balls and magnets.

SAILING BY DEAD RECKONING

While the compass gives indispensable information as to direction, and is constantly under the eye of the pilot, it of course can give no direct information as to the distance traversed by the ship, and hence does not by itself suffice to tell the navigator his whereabouts. In the early days there was indeed an expectation that the observed declination of the compass would reveal to the navigator his longitude and that the observation of the dip might enable him to determine his latitude. But more extended observation shows that this was asking altogether too much of the compass, and while it may be useful as an accessory it is by no means the navigator's chief reliance in determining his location. This is accomplished, as everyone is aware, in clear weather by the observation of the heavenly bodies. In cloudy weather, however, such observations obviously cannot be made, and the seaman must direct his ship and estimate his location--an all important matter when he is approaching the coast--by what is called dead reckoning. One element of this reckoning is furnished by the compass, inasmuch as that is his sole guide in determining the direction of the ship's progress. The other element is supplied by the log which furnishes him a clue as to the distance traversed hour by hour.

It is rather startling to reflect that the navigators of the middle ages had no means whatever of determining the rate of progress of a ship at sea, beyond the crudest guesses unaided by instrument of any kind. When Columbus made his voyage he had no means of knowing what distance he had actually sailed; nor was any method of measuring the ship's speed utilized throughout the course of the ensuing century. In the year 1570, however, one Humfray Cole suggested a theoretical means of measuring the ship's rate of progress by means of an object dropped back of the ship and allowed to drag through the water; and this suggestion led a generation later to the introduction of the log, which was first actually tested, so far as can be learned, in the year 1607.

The original log was so called because it consisted essentially of an actual log or piece of wood. To the center of this a string was attached, and in testing the ship's rate of progress this string was allowed to slip through the fingers of a sailor who counted the number of knots--placed, of course, at regular intervals on the string--that passed through his fingers in a given time. As the log itself would remain practically stationary in the water where it was dropped, the number of knots counted indicated the distance traversed by the ship in a given time. In practice the time was usually gauged by a half-minute sand glass, and the knots were arranged at such a distance on the cord that, in the course of the half minute, one knot would pass through the fingers for each nautical mile covered by the ship in an hour. The actual distance between the knots was therefore about fifty feet. The nautical or geographical mile represents one degree of the earth's circumference at the equator, amounting therefore to 6,008 feet, as against the 5,280 feet of the statute mile. It was the use of the log-line with its knots, as just explained, that led to the dubbing of the nautical mile by the name "knot," which is still familiarly employed, though the knotted log-line itself has been superseded in recent times, except on very old-fashioned sailing ships.

The log retains its place even in the most modern ship, though its form is materially altered, and its importance is somewhat lessened owing to the fact that the experienced skipper can test the speed of his ship very accurately by noting the number of revolutions per minute of the ship's propellers. It is indeed the ship's propeller that supplies the model for the modern log, in which the primitive piece of wood is replaced by a torpedo-like piece of metal with miniature propeller-like blades at its extremity. This apparatus is towed at the end of a long line, and its blades, whirling more or less rapidly according to the speed of the ship, communicate their motion to a recording apparatus, adjusted at the ship's stern, to which the line is attached and the face of which ordinarily presents a dial on which the speed of the ship may be observed as readily as one observes the time by the clock.

Some recent modifications of the log employ an electrical device to register the progress, but the principle of the revolving vanes, which owe their speed to the rate at which they are dragged through the water, is the fundamental one upon which the action of the log usually depends, though attempts have been made to substitute pressure-gauge systems.

While the modern log records the speed of the ship with a fair degree of accuracy, its register shows at best only an approximation of the facts. As already mentioned, the rate of revolution of the ship's propeller blades furnishes what most navigators regard as a rather more dependable test of speed. An apparatus for recording this is found on the bridge of the modern ship. But due allowance must of course be made for the effect of winds, waves, and ocean currents. These constantly variable factors obviously make the estimate as to the precise distance traversed by a ship in a given time a matter not altogether devoid of guess work; and no navigator who has been obliged to sail for several days by dead reckoning approaches a coast with quite the same degree of satisfaction that he may entertain if his log has been checked by observation of the sun or stars. In case, however, a navigator is able to check his reckoning by astronomical observations, aided by the chronometer, he determines his location with great accuracy.

THE DEVELOPMENT OF THE SEXTANT

The instrument with which such astronomical observations are made is known as the sextant. Its purpose is to measure with great accuracy the angle between two objects, which in practice are the horizon line on one hand and some celestial body, usually the sun, on the other. The determination of the latitude of the ship, for example, is a matter of comparative ease, if the sun chances to be unobscured just at midday. The navigator has merely to measure the exact elevation of the sun as it crosses the meridian,--that is to say when it is at its highest point,--and, having made certain corrections for so-called dip and refraction, to which we shall refer more at length in a moment, a very simple calculation reveals the latitude--that is to say, the distance from the terrestrial equator.

That the latitude of a ship could thus be determined, with greater or less accuracy, has been familiar knowledge to seamen from a very early period. It was by the use of this principle that the earth was measured by Eratosthenes and Posidonius in classical times, and the sailors of antiquity probably carried with them a crude apparatus for measuring the height of sun and stars, as the mediaeval navigators are known to have done.

The simplest and crudest form of measurer of which the record has been preserved is known as the cross-staff. This consisted essentially of a stick about a yard in length, called the staff, on which a cross-piece was arranged at right angles, so adjusted at the center as to slide back and forth on the staff. An eye-piece at one end of the staff was utilized to sight along projections at either end of the cross-piece. If the apparatus is held so that one of the lines of sight is directed to the horizon, and then the cross-piece slid along the staff until the other line of sight is directed toward the sun or a given star, the angle between the two lines of sight will obviously represent the angle of altitude of the celestial body in question. But the difficulty of using an apparatus which requires two successive observations to be made without shift of position is obvious, and it is clear that the information derived from the cross-staff must have been at best very vague--by no means such as would satisfy the modern navigator.

Even the navigators of the fifteenth century were aware of the deficiencies of the cross-staff and sought to improve upon it. The physicians of Henry the Navigator of Portugal, Roderick and Joseph by name, and another of his advisers, Martin de Bohemia, are credited with inventing, or at least introducing, a much improved apparatus known as the astrolabe. This consists of a circle of metal, arranged to be suspended from a ring at the side, so that one of its diameters would maintain the horizontal position through the effect of gravity. A superior quadrant of the circle was marked with degrees and minutes. A straight piece of metal, with sights so that it could be accurately pointed, was adjusted to revolve on a pivot at the center of the circle. This sighting piece being aimed at the sun, for example, the elevation of that body could be read directly on the measuring arc of the circle. Here, then, was no new principle involved, but the instrument had obvious points of advantage over the cross-staff, in particular because only a single sight need be taken, the horizon line being determined, as already explained, through the action of gravitation.

The astrolabe did not gain immediate favor with practical navigators, and it was at best a rather clumsy instrument, subject to peculiar difficulties when used on a rolling ship. Many attempts were made to improve upon it, but for a long time none of these was altogether successful. The final suggestion as to means of overcoming the difficulties encountered in measuring the altitude of astronomical bodies was made by Sir Isaac Newton. But nothing practical came of his discovery, as it was not published until a long time after his death. Meantime independent discovery of the same principle was made by Thomas Godfrey of Philadelphia, in 1730, and by the English astronomer Hadley, who published his discovery before the Royal Society in 1731. The instrument which Hadley devised was called a quadrant. The principle on which it worked involved nothing more complex than the use of two mirrors, one of them fixed in the line of vision of a small telescope; the other movable with the arm of an indicator, which is so adjusted as to revolve about the axis of the quadrant. In operation these two mirrors enable the images of two objects, the distance between which is to be measured, to be superimposed. The telescope may be pointed at the horizon, for example, directly under the position of the sun, and the arm of the instrument, altering the position of the so-called index mirror, may be rotated until the limb of the sun seems just to touch the horizon--the latter being viewed through the unsilvered half of the horizon glass. The scale at the circumference of the instrument is marked in half-degrees, which, however, are registered as whole degrees, and which, so interpreted, give the direct measurement of the angular distance between the horizon and the sun; in other words the measurement of the sun's altitude or so-called declination.

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.

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 object lies 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.

PERFECTING THE CHRONOMETER

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 the efforts 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 was subsequently 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 the ship, 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 been caused 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.

FINDING THE TIME WITHOUT A CHRONOMETER

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; but on 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.

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