barometer

The Sailor's Word-Book

A glass tube of 36 inches in length, filled with the open end upwards with refined mercury thus boiled and suddenly inverted into a cistern, which is furnished with a leathern bag, on which the atmosphere, acting by its varying weight, presses the fluid metal up to corresponding heights in the tube, easily read off by an external scale attached thereto. By attentive observations on this simple prophet, practised seamen are enabled to foretell many approaching changes of wind or weather, and thus by shortening sail in time, save hull, spars, and lives. This instrument also affords the means of accurately determining the heights or depressions of mountains and valleys. This is the mercurial barometer; another, the aneroid barometer, invented by Monsr. Vidi, measures approximately, but not with the permanence of the mercurial. It is constructed to measure the weight of a column of air or pressure of the atmosphere, by pressure on a very delicate metallic box hermetically sealed. It is more sensible to passing changes, but not so reliable as the mercurial barometer. 29·60 is taken as the mean pressure in England; as it rises or falls below this mark, fine weather or strong winds may be looked for: 30·60 is very high, and 29·00 very low. The barometer is affected by the direction of the wind, thus N.N.E. is the highest, and S.S.W. the lowest therefore these matters govern the decision of men of science, who are not led astray by the change of reading alone. The seaman pilot notes the heavens; the direction of the wind and the pressure due to that direction not forgetting sudden changes of temperature. Attention is due to the surface, whether convex or concave.

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