Va-ri-a′tion-com′pass.
A compass designed for observing the magnetic bearings of celestial objects in order to determine the deviation of the magnetic needle from the true meridian. The angular difference between the magnetic bearing of a circumpolar star when at its greatest eastern or western elongation and its polar distance gives the deviation directly; but any celestial object may be employed, its true bearing being readily calculated when its declination and the latitude of the place are known. At sea the sun is the object commonly observed. See azimuth-compass. In a more restricted sense, a compass provided with a long needle, which oscillates in a graduated circle, and has appliances for measuring and reading the small diurnal changes in the magnetic variation. Variation or declination is the horizontal angle which a needle makes with the meridian. The changes of declination are of three kinds. 1. Secular. That which takes place in cycles. At London, in 1550, the variation was 11° 17 E; about 1660 it was 0°. It then began to deviate to the west till it attained its maximum in 1815, 24° 17′ 18″. In 1865 it was 20° 38. 2. Annual. This was first remarked at Paris by Cassini. 3. Diurnal. First remarked at London, by Graham, 1722. The changes are greatest in summer, least in winter. The needle declines toward the west from 8 A. M. till 2 P. M., and then to the east till 8 or 9 P. M. See magnetometer. The variation of the compass was known to the Chinese philosopher, Keon-tsoung-chi, in the twelfth century. He determined it to be from 2° to 2° 30 at Pekin. The French savans who formed part of the plundering host at Pekin a few years since, found time, after participating in the scramble for toot, to test the correctness of the former observation. They found it to be about the same. What their ancestors were about in the twelfth century may be seen by reading the lives of the Capets. The same century saw 25 popes ruling in Rome. Gerhert, Alhazen, and Roger Bacon relieve that and neighboring centuries of their dreariness. The Chinese mode of suspending the needle was by a silken string, the most delicate which has yet been devised, and the one adopted at the magnetic observatories which are acting in concert in so many parts of the world. See compass. The variation of the compass was again noticed by Columbus in 1492, and Sebastian Cabot in 1540. The western line of no variation was discovered by Columbus in 1492, about 100 miles west of the Azores. It has since shifted.Columbus has not only the incontestable merit of having first discovered a line without magnetic variation, marking a memorable era in nautical astronomy, but also of having, by his considerations on the progressive increase of westerly declination, in receding from that line, given the first impulse to the study of terrestrial magnetism in Europe.
We know positively from the Chinese ‘Penthsaayan,’ which was written under the dynasty of the Song between 1111 and 1117, that the manner of measuring the amount of westerly declination had been then long understood. That which belongs to Columbus is not the first observation of the existence of the variation (which, for example, is noted in the map of Andrea Bianco, in 1436), but the remark which he made on the 13th of September, 1492, that 2 1/2° east of the island of Corvo the magnetic variation changes, passing from N. E. to N. W.— Humboldt.
The first variation-compass was constructed before 1525, by an ingenious apothecary of Seville, Felipe Guillen. So earnest were the endeavors to learn more exactly the direction of the curves of magnetic declination, that in 1585 Juan Jayme sailed with Francisco Gali from Manila to Acapulco for the sole purpose of trying in the Pacific a declination-instrument which he had invented.
The cosmographer Alonso de Santa Cruz, one of the instructors of Charles V., undertook the drawing up of the first general “Variation chart,” although indeed from very imperfect observations, as early as 1530, or a century and a half before Halley. The “movement” of the magnetic lines, the first recognition of which is usually ascribed to Gassendi, was not even yet conjectured by William Gilbert; but at an earlier period Acosta, “from the information of Portuguese navigators,” assumed four lines of no declination upon the surface of the globe. Hardly had the inclinometer, or dipping-needle, been invented in England by Robert Norman, in 1576, than Gilbert boasted that by means of this instrument he could determine the position of a ship in a dark and starless night.Pope Alexander VI adopted the line of no variation discovered by Columbus 100 miles west of the Azores, as the easterly limit of the papal grant to the Spaniards, May, 1493. He was not aware, no blame to him, that the line was slowly moving east, and would soon be far removed from its first-observed position. It was reserved for a future age to show the incorrectness of the then received opinion that “magnetism is an effluvium issuing forth from the root of the tail of the Little Bear.” Halley, in 1683, sketched his theory of four magnetic poles or points of attraction, and of the periodical movement of the magnetic lines of no variation. In 1698-1702 he made several voyages of observation, and the result was a general “variationchart,” in which the points of equal variation were connected by curved lines. Hansteen's chart (1787) gives the western and eastern lines of no variation at the date of his writing. These are given under magnetometer (which see). [2692] Variation-charts are now in common use at sea. An excellent one, derived from the most recent observations, is published from time to time by the British Admiralty. Before chronometers had attained their present excellence, and lunar observations were commonly practiced, it was proposed to determine the longitude at sea from the variation of the compass; but owing to the difficulty of its being accurately determined from such an unstable observatory as a ship's deck, and from other causes, this method was found to have no practical value.
