Hydrometers are tested by careful immersion in pure distilled water; of which the specific gravity is taken as unity.
In water less pure, more salt, dense, and buoyant, the instrument floats higher, carrying more of the graduated scale out of the fluid.
The zero of the scale should be level with the surface of distilled water, and rise above it in proportion as increase of density causes less displacement.
The scale is graduated to thousandths--as far as ·040 only--because the sea water usually ranges between 1·014 and about 1·036. Only the last two figures need be marked.
LONDON: Printed by GEORGE E. EYRE and WILLIAM SPOTTISWOODE, Printers to the Queen's most Excellent Majesty. For Her Majesty's Stationery Office.
[1] In South latitude the South wind corresponds to our North wind in its nature and effects. The Easterly and Westerly winds retain their respective peculiarities in both hemispheres.
[2] Exclusive of local land and sea breezes of hot climates.
[3] Glass, barometer, column, mercury, quicksilver, or hand.
[4] Or atmosphere, or the atmospheric fluid which we breathe.
[8] Thirty-two degrees is the point at which water begins to freeze, or ice to thaw.
[10] The two thus combined making a hygrometer: for which some kinds of hair, grass, or seaweed may be a make-shift.
[11] It stands lower, about a tenth of an inch for each hundred feet of height directly upwards, or vertically, above the sea; where its average height, in England, is 29·94 inches (at 32°).
[12] In an Aneroid, a metallic, or a wheel barometer, the hand's motion should correspond to that of mercury in an independent instrument.
[14] In the best columns, those of standards for example, no concavity is seen, at any time: but it is otherwise with many barometers, which do show a concavity.
[15] In these cases there is usually a combination or a contest of currents in the atmosphere, horizontally, _or_ one _above_ the other, or diagonally.
[16] Thunder clouds sometimes rise and spread against the wind (lower-current). It is probable that there is a meeting, if not a contest of air currents, electrically different, whenever lightning is seen. Their concurrence, when the new one advances from _polar_ regions, does not depress the barometer, except in oscillations of the mercury, which are very remarkable at some such times.