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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 1014 and about 1036. 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.

FOOTNOTES:

[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.

[5] Or exhaustion.

[6] A vacuum.

[7] See pages 24 and 25.

[8] Thirty-two degrees is the point at which water begins to freeze, or ice to thaw.

[9] Evaporation.

[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 2994 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.

[13] Southerly in South latitude.

[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.

[17] Aneroids, metallic barometers, and oil sympiesometers, seem to be much more affected than mercurial barometers by electrical changes.

[18] Southerly, in North latitude; the reverse in the Southern hemisphere.

[19] A "high dawn" is when the first indications of daylight are seen above a bank of clouds. A "low dawn" is when the day breaks on or near the horizon. The first streaks of light being very low.

[20] Indications of weather, afforded by colours, seem to deserve more critical study than has been often given to the subject. Why a rosy hue at sunset, or a grey neutral tint at that time, should presage the reverse or their indications at sunrise;--why bright yellow should foretell wind at either time, and pale yellow, wet;--why clouds seem soft, like water colour; or hard edged, like oil paint, or Indian ink on an oily plate;--and why such appearances are infallible signs--are yet to be shown satisfactorily to practical men.

[21] In the trade winds of the tropics there is usually a counter current of air, with light clouds,--which does not indicate any approaching change. In middle latitudes such upper currents are not so evident, except before a change of weather.

[22] _Much_ refraction is a sign of Easterly wind. _Remarkable_ clearness is a bad sign.

[23] The "young moon with the old moon in her arms" (Burns, Herschel, and others) is a sign of bad weather in the temperate zones or middle latitudes, because (probably) the air is then exceedingly clear and transparent.

[24] Even in ordinary changes of weather it is interesting, as well as useful, to mark the formation or disappearance of clouds, caused by colder and warmer currents of air mixing: or intermingling.

[25] Depending on pressure and temperature.

[26] Sir James Ross--M. Daussy.

[27] Williwaw (Whirl-awa?) of the old sealers and whalers.

[28] Seamen call the light sails, used only in very fine weather, "flying kites."

[29] Herschel.

[30] Dove.

[31] For a barometer of this kind, Admiral Milne has invented self-registering mechanism, that answers well.

[32] A small turnscrew being applied gently to the screw head at the back. This is often necessary, on receiving or first using an aneroid that has long been lying by, or that has been shaken by travelling.

[33] It is a good weather glass--to be suspended on or near the upper deck, for easy reference;--and is unlikely to be injured by mere concussion of air, or vibration of wood, when guns are fired.

[34] Allowing 0,0011 of an inch for each foot.

[35] The manufacture of these useful auxiliary instruments (all French originally) has increased much latterly: and now the patent has expired.

They might be so improved so to be worth more than double their present value.

[36] Like the sun's edge or limb, touching the sea horizon, as seen inverted when using a sextant.

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