300

HISTORY OF MARINE ARCHITECTURE.

‘ As to the compass, it is, among the Chinese, in universal use. With them themagnetic needle is seldom made to exceed an inch in length, and is less than aline in thickness. It is poised with great nicety, and is remarkably sensible ; bywhich is meant, that it appears to move at the least change of position, towardsthe east or west of the box in which it is suspended ; though, in fact, the natureof the magnet, and the perfection of the machine containing it, consists in theneedle’s privation of all motion, or, its continuing to point always steadily towardthe same portion of the heavens, however rapidly may be whirled the compass-box, or other object surrounding. This steadiness in the Chinese compass is ac-complished by a particular contrivance, as observed by Mr. Barrow. tl A piece ofthin copper is strapped round the centre of the needle. The copper is riveted byits edges to the upper part of a small hemispherical cup, of the same metal, turneddownwards. The cup, so inverted, serves as a socket to receive a steel pivotrising from a cavity made in a round piece of light wood or cork, which thusforms the compass-box. The surfaces of the socket and pivot intended to meeteach other are perfectly polished, to avoid, as much as possible, all friction. Thecup has a proportionably broad margin, which, independent of adding to its weight,tends, from its horizontal position, to keep the centre of gravity in all situationsof the compass, nearly in coincidence with the centre of suspension. The cavityin which the needle is thus suspended, is, in form, circular, and is little morethan sufficient to receive the needle, cup, and pivot. Over this cavity is placeda thin piece of transparent talc, which prevents the needle from being affected byany motion Of the external air ; but permits the apparent motion of the former tobe easily observed. The small and short needle of the Chinese has a material ad-vantage over those of the usual size in Europe, with regard to the inclination, ordip, towards the horizon ; which, in the latter, requires that one extremity of theneedle should bo made so much heavier than the other, as will counteract themagnetic attraction. This being different in different parts of the world, can onlybe accurately true at the place for which it had been constructed ; but, in shortand light needles, suspended after the Chinese manner, the weight below thepoint of suspension is more than sufficient to overcome thq magnetic power of thedip, or inclination, in all situations of the globe; and therefore such needleswill never deviate fiom their horizontal position

* Authentic Account of an Embassy from the King of Great Brita’n to the Emperor of China;vol. I. p. 411.

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