Every-day Science: Volume 7. The Conquest of Time and Space

by Henry Smith Williams; Edward Huntington Williams

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 (between 1580 and 1692) 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 (1772–74), 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.

Early in the twentieth century an American ship, christened the Carnegie, in honor of the philanthropist who supplied funds for the enterprise, was constructed for the express purpose of making accurate charts of the lines of magnetic declination in various parts of the globe. This ship differs from every other vessel of considerable size ever hitherto constructed in that no magnetic material of any kind was used in connection with its structure or equipment. For the most part iron was substituted by copper or other non-magnetic metal. Pins of locust-wood largely took the place of nails; and wherever it was not feasible to do away with iron altogether it was used in the form of non-magnetic manganese steel. The purpose of the Carnegie is to provide accurate charts of magnetic declination for the use of navigators in general. The value of observations made with this non-magnetic ship will be clear when it is reflected that with an ordinary ship the observer can never be absolutely certain as to what precise share of the observed fluctuation of the compass is due at any given moment to the ship's influence. In other words—using technical terminology—he can never apportion with absolute accuracy the influence of declination and of deviation. Yet it is highly important that he should be able to do so, inasmuch as the declination of the compass is an all-important element in reckoning the exact location of the ship, and would be the same for every ship at that place, whereas deviation denotes a purely local disturbance which would never be the same for any two ships of different construction.

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.