Part 3

For ascertaining the dryness or moisture of air, the readiest, and surest method is the comparison of two thermometers; one dry, the other just moistened, and kept so. Cooled by evaporation as much as the state of the air admits--the moist (or wet) bulb thermometer shows a temperature nearly equal to that of the other one, when the atmosphere is extremely damp, or moist; but lower at other times,--in proportion to the dryness of air, and consequent evaporation,--as far as twelve or fifteen degrees in this climate; twenty or even more elsewhere. From four to eight degrees of difference is usual in England; and about seven is considered healthy for living rooms.

The thermometer fixed to a barometer intended to be used only as a weather-glass shows the temperature of air about it nearly--but does not show the temperature of mercury within exactly. It does so however near enough for ordinary practical purposes--provided that no sun, nor fire, nor lamp heat is allowed to act on the instrument partially.

The mercury in the cistern and tube being affected by cold or heat, makes it advisable to consider this when endeavouring to foretell coming weather by the length of the column.

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Briefly, the barometer shows weight or pressure of the air; the thermometer--heat and cold, or temperature; and the wet thermometer, compared with a dry one, the degree of moisture or dampness.

It should be remembered that the state of the air foretells, rather than shows present weather (an invaluable fact too often overlooked); that the longer the time between the signs and the change foretold by them, the longer such altered weather will last; and, on the contrary, the less the time between a warning and a change, the shorter will be the continuance of such foretold weather.

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If the barometer has been about its ordinary height, say near thirty inches, at the sea level, and is steady, or rising--while the thermometer falls, and dampness becomes less--North-westerly, Northerly, or North-easterly wind--or less wind--may be expected.

On the contrary--if a fall takes place, with a rising thermometer and increased dampness, wind and rain may be expected from the South-eastward, Southward, or South-westward.

A fall, with a low thermometer, foretells snow.

Exceptions to these rules occur when a North-easterly wind, with wet (rain or snow) is impending, before which the barometer often rises (on account of the direction of the coming wind alone), and deceives persons who, from that sign only, expect fair weather.

When the barometer is rather below its ordinary height, say, below twenty-nine inches and nine-tenths (at the sea level only), a rise foretells less wind, or a change in its direction towards the Northward,--or less wet; but when the mercury has been low, say near 29 inches--the first rising usually precedes, and foretells, strong wind--(at times heavy squalls)--from the North-westward--Northward--or North-eastward--after which violence a rising glass foretells improving weather--if the thermometer falls. But, if the warmth continue, probably the wind will back (shift against the sun's course), and more Southerly, or South-westerly wind will follow. "Backing" is a bad sign, with any wind.

The most dangerous shifts of wind, and the heaviest Northerly gales happen after the mercury first rises from a very low point.

Indications of approaching changes of weather, and the direction and force of winds are shown less by the height of mercury in the tube, than by its falling or rising. Nevertheless, a height of about 30 inches (at the level of the sea) with a continuance of it, is indicative of fine weather and moderate winds.

The barometer is said to be falling when the mercury in the tube is sinking, at which time its upper surface is sometimes concave or hollow. The barometer is rising when the mercurial column is lengthening; its upper surface being then, as in general, convex or rounded.

A rapid rise of the barometer indicates unsettled weather. A slow rise, or steadiness, with dryness, shows fair weather.

A considerable and rapid fall is a sign of stormy weather and rain. Alternate rising and sinking show very unsettled weather.

The greatest depressions of the barometer are with gales from the S.E., Southward, or S.W.; the greatest elevations, with winds from the N.W., Northward, or N.E., or when calm.

Although the barometer generally falls with a Southerly, and rises with a Northerly wind, the contrary sometimes occurs; in which cases the Southerly wind is dry and the weather fine; or the Northerly wind is wet and violent.

When the barometer sinks considerably, high wind, rain, or snow will follow: the wind will be from the Northward if the thermometer is low (for the season)--from the Southward if the thermometer is high.

Sudden falls of the barometer, with a Westerly wind, are sometimes followed by violent storms from N.W. or North.

If a gale sets in from the Eastward or S.E., and the wind veers by the South, the barometer will continue falling until the wind becomes S.W., when a comparative lull may occur; after which the gale will be renewed; and the shifting of the wind towards the N.W. will be indicated by a fall of the thermometer as well as a rise of the barometer.

Three things appear to affect the mercury in a barometer:--

1. The direction of the wind--the North-east wind tending to raise it most--the South-west to lower it the most, and wind from points of the compass between them proportionally as they are nearer one or the other extreme point.

N.E. and S.W. may therefore be called the wind's extreme bearings (rather than poles?)

The range, or difference of height, of the mercury, due to change of direction only, from one of these bearings to the other (supposing strength or force, and moisture, to remain the same) amounts in these latitudes to about half an inch (shown by the barometer as read off).

2. The amount, taken by itself, of vapour, moisture, wet, rain, hail, or snow, in the wind or current of air (direction and strength remaining the same) seems to cause a change amounting, in an extreme case, to about half an inch.

3. The strength or force alone of wind from any quarter (moisture and direction being unchanged) is preceded, or foretold, by a fall or rise, according as the strength will be greater or less, ranging, in an extreme case, to more than two inches.