Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/254

226 azimuth compass. The discovery of the variation of declination was made by Stephen Burrowes when voyaging between the north cape of Finmark and Vaigatch (Vay-gates), and was afterwards by Gillebrand, professor of geometry at Gresham College. In 1683, in a communication to the Royal Society (Phil. Trans., June 16, p. 214), Dr E. Halley shows that the irregularity observed in the variations of the compass at sea is not due the attraction to of the land, and comes to the conclusion that the whole globe of the earth is one great magnet, having four magnetical poles or points of direction. The declination for any place is subject to secular variations: thus, at Paris in 1681, it was 2° 30ʹ to the W., in 1865 it was 18° 44ʹ W. Halley, in a paper entitled “Account of the Cause of the Change of the Variation of the Magnetical Needle” (Phil. Trans., Oct. 19, 1692, pp. 563–578), points out, with other instances of secular variation, between 1580 and 1692 the direction of the needle at London changed from 11° 15ʹ E. to 6° W., or more than 17°, and demonstrates that the direction is in no place fixed or constant, though in some places it changes faster than in others. Besides the secular, there are annual and diurnal variations of small amount. The existence of the latter was discovered by Mr Graham about 1719. The deviation of the compass is the departure of the north and south line from the magnetic meridian, owing to the magnetism of the ship itself, or that induced in it by the earth’s magnetic force. It was first observed during 1772–74 by Mr Wales, the astronomer of Captain Cook. When surveying along the coast of New Holland in 1801 and 1802, Captain Matthew Flinders made the discovery that there was a difference in the direction of the magnetic needle, according as his ship’s head pointed to the E. or W.—westerly in the former, easterly in the latter case. When the ship’s head was N. or S. the needle took the same direction, or nearly so, that it would on shore, and showed a variation from the true meridian which was about a medium between that given by it when east and when west. He found, also, that the error in variation was nearly proportionate to the number of points which the ships head was from the north or south. (Phil,. Trans., 1805, p. 186.) The deviation in wooden ships can be practically obviated, but in iron ships it has to be partly allowed for, partly compensated. Barlow used a correcting plate of iron to overcome the directive action on the compass due to the magnetism of wooden vessels. On Professor Airy’s method, the permanent magnetism of ships is compensated by a steel magnet placed at a given distance below the compass; it is, however, liable to changes of intensity, occasioned by shocks, vibration, unequal heating, and other causes, a fact which led the late Dr Scoresby to propose the employment of a compass aloft, out of the region of the ship’s influence. The induced magnetism of ships can be only imperfectly compensated, since it varies according to the ship’s bearing, and as she rolls and pitches; but corrections can be made for the heeling error. The discovery of the dip of the magnetic needle is ascribed by Gilbert to Robert Norman, a nautical instrument maker at Wapping, who, about 1590, introduced the employment of a sliding weight on the needle for the correction of the dip at different points of the earth s surface. {{ti|1em|The earliest references to the use of the compass are to be found in Chinese history, from which we learn how, in the sixty-fourth year of the reign of Ho-ang-ti (2634 b.c.), the emperor Hiuan-yuan, or Ho-ang-ti, attacked one Tchi-yeou, on the plains of Tchou-lou, and finding his army embarrassed by a thick fog raised by the enemy, constructed a chariot (Tchi-nan) for indicating the south, so as to distinguish the four cardinal points, and was thus enabled to pursue Tchi-yeou, and take him prisoner. (Klaproth, Lettre à M. le Baron Humboldt sur l’invention de la Boussole, Paris, 1834. See also Mailla, Histoire générale de la Chine, tom. i. p. 316, Paris, 1777.) Several other allusions to the compass are contained in early Chinese records. The power of the loadstone to communicate polarity to iron is said to be for the first time explicitly mentioned in a Chinese dictionary, finished in 121 a.d., where the loadstone is defined as “a stone with which an attraction can be given to the needle.” The first mention of the use of the compass for the purpose of navigation—an art that has apparently retrograded rather than advanced among the Chinese—occurs in the Chinese encyclopædia, Poci-wen-yun-fou, in which it is stated that under the Tsin dynasty, or between 265 and 419 a.d., “there were ships directed to the south by the needle.” The Chinese, Mr Davis informs us, once navigated as far as India, but their most distant voyages at present extend not further than Java and the Malay Islands to the south (The Chinese, vol. iii. p. 14, London, 1844). According to an Arabic manuscript, a translation of which was published by Eusebius Renaudot (Paris, 1718), they traded in ships to the Persian Gulf and Red Sea in the 9th century. Staunton, in vol. i. of his Embassy to China (London, 1797), after referring to the early acquaintance of the Chinese with the property of the magnet to point southwards, remarks (p. 445), “The nature and the cause of the qualities of the magnet have at all times been subjects of contemplation among the Chinese. The Chinese name for the compass is ting-nan-ching, or needle pointing to the south; and a distinguishing mark is fixed on the magnet’s southern pole, as in European compasses upon the northern one.” “The sphere of Chinese navigation,” he tells us (p. 447), “is too limited to have afforded experience and observation for forming any system of laws supposed to govern the variation of the needle…The Chinese had soon occasion to perceive how much more essential the perfection of the compass was to the superior navigators of Europe than to themselves, as the commanders of the ‘Lion’ and ‘Hindostan,’ trusting to that instrument, stood out directly from the land into the sea.” The number of points of the compass, according to the Chinese, is twenty-four, which are reckoned from the south pole; the form also of the instrument they employ is different from that familiar to Europeans. The needle is peculiarly poised, with its point of suspension a little below its centre of gravity, and is exceedingly sensitive; it is seldom more than an inch in length, and is less than a line in thickness. It appears thus sufficiently evident that the Chinese are not indebted to Western nations for their knowledge of the use of the compass. “It may be urged,” writes Mr T. S. Davies, “that the different manner of constructing the needle amongst the Chinese and European navigators shows the independence of the Chinese of us, as theirs is the worse method, and had they copied from us, they would have used the better one (Thomson’s British Annual, 1837, p. 291). On the other hand, it does not seem improbable that a knowledge of the mariner’s compass was communicated by them directly or indirectly to the early Arabs, and through the latter was introduced into Europe. Sismondi has remarked (Literature of Europe, vol i.) that it is peculiarly characteristic of all the pretended discoveries of the Middle Ages that when the historians mention them for the first time they treat them as things in general use. Gunpowder, the compass, the Arabic numerals, and paper, are nowhere spoken of as discoveries, and yet they must have wrought a total change in war, in navigation, in science, and in education. Tiraboschi (Storia della Letteratura Italiana, tom. iv. lib. ii. p. 204, et seq., ed. 2., 1788), in support of the conjecture that the compass was introduced into Europe by the Arabs, adduces their superiority in scientific learning, and their early skill in navigation. He quotes a passage on the polarity of the loadstone from a treatise translated by Albertus Magnus, attributed by the latter to Aristotle, but apparently only an Arabic compilation from the works of various philosophers. As the terms Zoron and Aphron, used there to signify the south and north poles, are neither Latin nor Greek, Tiraboschi suggests that they may be of Arabian origin, and that the whole passage concerning the loadstone may have been added to the original treatise by the Arabian translators. {{div end}} {{hii|1|-1}}Dr W. Robertson asserts (Historical Disquisition concerning Ancient India, p. 227) that the Arabs, Turks, and Persians have no original name for the compass, it being called by them Bossola, the Italian name, which shows that the thing signified is foreign to them as well as the word. The Rev. G. P. Badger has, however, pointed out (Travels of Ludovico di Varthema, trans., J. W. Jones, ed. G. P. Badger, Hakluyt Soc., 1863, note, pp. 31 and 32) that the name of Bushla or Busba, from the Italian Bussola, though common among Arab sailors in the Mediterranean, is very seldom used in the Eastern seas, Daïrah and Beit el-Ibrah (the Circle, or House of the Needle) being the ordinary appellatives in the Red Sea, whilst in the Persian Gulf Kiblah-nâmeh is in more general use. Robertson quotes Sir J, Chardin as boldly asserting “that the Asiatics are beholden to us for this wonderful instrument, which they had from Europe a long time before the Portuguese conquests. For, first, their compasses are exactly like ours, and they buy them of Europeans as much as they can, scarce daring to meddle with their needles themselves. Secondly, it is certain that the old navigators only coasted it along, which I impute to their want of this instrument to guide and instruct them in the middle of the ocean…I have nothing but argument to offer {{div end}}{{div col end}}