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8 THE MARINE RECORD. DECEMBER 20, I900, nn ———————— THE VARIATION OF THE COMPASS. By G. W. LiIrtLEHALES, HyDROGRAPHIC JJNGINEER. In seeking the truth concerning the pointing of the com- pass needle, upon which trans-oceanic navigation has rested throughout the history of the world, first cast aside the old and familiar saying, ‘‘True as the needle to the pole,’’ for it only holds good within a few shiftiug areas whose combined extent does not amount to1 per cent. of the area of the globe. There are portions of the earth’s surface where the compass needle points due east and west, and still others where the north end actually points south. In the Atlantic the average departure of the direction of the compass needle from the direction of the pole star is 20° west of north, while in the Pacific it is 10° east of north. At the entrance to Hudson Bay the compass points 55° west, and in the West Indies 5° east of the true meridian; hence, in a voyage along the eastern coast of the American continent, from Hudson Bay to the mouth of the Mississippi river, a change of 60° is experienced in the direction of the compass needle, and similarly in a trans-Pacific voyage, from Juan de Fuca to Yokohama, a change of 27° is experienced. And over so comparatively small a stretch of sea as lies between New- foundland and New York the pointing of the compass needle varies 25°. We are thus made acquainted with the variation of the compass which may be strictly defined as the angle between the true north and south line and the magnetic north and south line as pointed out by a compass needle or other magnetized needle so mounted as to swing freely about a vertical axis or pivot. For more than a-century it has been customary among chart makers to represent the change in the variation of the compass from place to place by lines conceived to be drawn upon the surface of the globe and passing through all places where there is the same difference between the direction of the true north and the direction of the needle. These are called lines of equal variation of the compass. They issue from one magnetic pole and pass by curved paths to the other and through the geographical poles of the earth, but since the earth’s magnetism, which is the cause of the vari- tion of the compass, is not fixed, either as to time or local- ity, they shift their positions hourly, daily, monthly, yearly, and through centuries. These changes are all believed to be periodic, and, with the exception of the last mentioned, which is generally called the secular change because it goes on and on throughout centuries, are of such small extent that they do not affect the use of the compass on the seas where commerce is carriedon. So that for purposes of navi- gation the lines of equal magnetic variation may be drawn so as to hold good for several years from a given time. It was several centuries after the introduction of the com- pass into Europe before this difference in direction between the magnetic meridian and the true meridian was discov- ered, and the discoverer was no less a person than Christo- pher Columbus. Thehistory of his first voyage of discovery tells us of the consternation that was caused on board ship when it was observed that the compass had shifted its direc- tion from east of north to west of north. Columbus had, in fact, crossed the line on September 13, 1492, along which the needle pointed true north—that is, the line of no varia- tion of the compass. To the east of this line the needle pointed east, and to the west the needle bore west. In those days the line passed a little to the west of Fayal in the Azores and figured quite prominently for many years in political geography as the boundary between the kingdoms of Por- tugal and Castile. Before the middle of the sixteenth century it was thought that the needle’s misdirection from the true north was due to mechanical imperfections, and the compasses of this period were frequently corrected so that they would point true; but when the fact of the variation of the compass came _ to be generally accepted, the necessity that mariners should take account of it in the navigation of their ships at once be- came apparent, and observations began to multiply in all those seas with which navigators were then acquainted. At - the beginning of the eighteenth century sufficient informa- tion had accumulated in England to warrant Edmund Hal- ley to publish a world chart showing the lines of equal var- iation of the compass for the year 1700 in all seas excepting the Pacific ocean. In describing his chart the author wrote: . “What is here properly new, is the curve lines drawn over the several seas, to show the degrees of the variation of the magnetical needle, or sea-compass; which are designed ac- cording to what I myself found, in the western and southern oceans, in a voyage I purposely made at the public charge, in the year of our Lord 1700; or have collected from the comparison of several journals of voyages lately made in the Indian Seas, adapted to the same year. ‘That this may be better understood, the curious mariner is desired to observe, that in this chart the double line pass- ing near Bermudas, the Cape de Verde Islands, and St. Helena, everywhere divides the east and west variation in this ocean and that on the whole coast of Europe and Africa the variation is westerly, as on the more northerly coast of America, but on the more southerly parts of America ’tis easterly. The degrees of variation, of how much the com- pass declines from the true north on either side, is reckoned by the number of the lines on each side of the double curve, which I call the line of no variation; and each fifth and tenth is distinguished in its stroke, and numbered accord- ingly. So that in what place soever your ship is, you find the variation by inspection. The like is to be observed in the Indian Ocean, where the variation is altogether westerly, increasing until you come about the meridian of the eastern part of St. Lawrence or Madagascar (where it is about two points) and thence it decreases till you arrive on the eastern coast of China, or at the Philippine Isles. Here another double line on which there is no variation, divides again the west from the east variation, that in all probability is to be met with almost all over that immense ocean we commonly call the South sea, but I have not attempted to describ the curves therein, wanting accounts and journals to ascertain the same. “As to the uses of this chart, they will be easily under- stood, especially by such as are acquainted with the azi- muth compass, to be to correct the courses of ships at sea. For if the variation of the compass be not allowed, all reckonings must be so far erroneous: And in continued cloudy weather where the mariner is not provided to ob- serve this variation duly, the chart will readily show him what allowances he must make for the default of his com- pass, and thereby rectify his Journal.”’ So that here was a chart, made nearly two centuries ago, which indicated the amount of the variation of the compass to the east or to the west to the true north in every locality in the Atlantic and Indian oceans. This chart would hold good to-day if it were not for the fact that the variation not only differs in different localities, but also suffers a perpet- ual, though slow, change in its amount at any particular locality. In 1634 Henry Gellibrand, of London, eatablished the fact of the secular change or variation in the variation of the compass and announced his conclusicn as follows: “Hitherto, according to the tenets of our magnetic philoso- phers, we have supposed the variation of all particular places to continue one and the same.* * * But most diligent ‘magnetical observations have plainly offered violence to the same, and proved the contrary, namely, that the variation is accompanied with a variation.’’ From this time reliable observations were recorded for the great populous centers of Europe, and soon observations of the variation of the compass had been made by navigators in most of the known parts of the world, although the older observations, hav- ing been made without the means of precise measurement, are subject to a probable error of as much as 1°, they can be accepted as serviceable in the discussion of a long series, and serve to reveal satisfactorily the secular change of the variation of the compass. Through the results of the observa- tions of navigators of successive generations, series of obser- vations of the variation extending over two or three centuries are available for most of the important maritime stations of the world. On plotting the observations at a given station with reference to rectangular coordinates, using values of the variation of the compass as ordinates and intervals of time as abscissas, sinnous curves are developed which suggest the periodic character of the secular change. By the study of such curves the rate of movement of the com- pass needle is found for any time within the range of the observations, the times when the needle is stationary are determined, and the values of the variation of the compass are predicted for ten or fifteen years beyond the limits of the series of observations. But the observations which have been taken at any particular place up tothe present time are manifestly insufficient to warrant a conclusion that after a certain period has elapsed the variation of the compass will be the same there as it is now and will then repeat its changes and again assume the same value after the lapse of the sameinterval of time. Sothat the natural law which regulates the secular change is not yet known, and, in the absence of complete knowledge of this law, predicted values can not be relied upon beyond the term of years that has been mentioned. It is, therefore, at once clear that the closeness to the truth with which the variation of the compass may be rep- resented by lines of equal magnetic variation, such as are shown on the Pilot Chart of the North Atlantic Ocean and the Pilot Chart of the North Pacific Ocean, must depend upon the remotene:s of the period when the observations for the variation of the compass were taken. As far as the open ocean is concerned, but few reliable observations for the variation have been taken since the epoch when nav- igation in iron built vessels became general. ’ So that the variation charts of the present time show very nearly the same lines of equal variation of the compass as would have resulted if a chart maker in the days of wood built ships had predicted the changes that would occur between that period and the year 1900 in the values of the variation of the com- pass in different parts of the oceans of the globe, and, ap- plying them to the variation lines of that period, had brought out a chart for the present time. Such a chart of variation lines would have had involved init uncertainties to the extent of a degree or two, due to predicting values beyond the limit of time during which predictions can be safely made in the present state of our knowledge, and this is precisely the amount of uncertainty which existsin the variation charts of the present time, for they have been brought forward from the time when observations for the variation of the compass were made in wood built ships, and with but few advantages over what a chart maker of that period could have employed in carrying the lines forward into their positions for the present time. This uncertainty of a degree or two in the values of the variation of the com- pass, asindicated by the variation lines on ocean charts, must not be forgotten in laying down magnetic courses, nor must the gradual change in the variation escape attention in laying down positions by bearings on charts. The magnetic compass diagrams placed on charts for the purpose of mak- ing plotting easier become in time slightly in error, and in some cases, such as with small scales, or when the lines are long, the error in the plotted position from neglect of this change may be of importance. The chart of the North At- lantic Ocean shows the yearly rates of the secular change of the variation of the compass in the localities where the num- bers in red are placed. They express, in minutes and deci mals of a minute, the angular change in the direction of the compass needle that takes place inone year, and when marked plus (+) indicate that the movement of the north end of the needle is to the westward, and when (—) that it is to the eastward. The region where the compass needle is stationary is indicated by the shaded band. Inthe At- lantic, to the eastward of this band, the westerly variation is decreasing, and to the westward the westerly variation is increasing (or the easterly variation decreasing asthe case may be). Since the beginning of the present century this sinuous band, marking out the region of no yearly change in the variation of the compass, has been moving slowly across the Atlantic like the crest of a huge wave. In 1880 . it passed over London and Paris; in 1852 over the Azores; in 1875 over Newfoundland and in 1895 ovér Halifax and the region to the southeast as shown on the chart. It will pass over the positions of Washington City, Bermuda, and the Cape Verde Islands about 1939 and will probably reach Havanna, the Windward Islands and eastern extremety of South America before the close of the twentieth century. At all points along its western edge the needle has reached its most westerly direction and after a short stationary period will commence a backward movement tothe eastward— slowly at first, but gradually increasing for nearly a century and then gradually decreasing until the next wave of no yearly change shall have overtaken their positions. Ol Consul GREENE reports from Antofagasta, August 17, 1900: The Pacific Steam Navigation Co., a British firm, and the South American, a Chilean company, are contemplating an extension of their lines from Central America to San Francisco. This service, now being rendered by the Kos- mos steamers, appears to be asuccess. The boats leave Valparaiso, call at the principal coast ports to Guayaquil, and go thence to Central American and North Pacific ports, not calling at Panama. They take and drop cargo on the trip north and generally return with full loads south. Formerly, Chilean wheat and flour had the coast trade as faras Guayaquil, but the Kosmos steamships, with their reasonable rates, now bring considerable lots, not only for coast supply, but for Bolivia, via Mollendo and this port. This last traffic has been favored by the suspension of Chil- ean railway service, owing to damage during the winter.