Part 10

The Sympiesometer is an instrument used chiefly at sea for purposes of comparison with the mercurial and aneroid barometers. Its indications result partly from the pressure and partly from the temperature of the atmosphere; it would, therefore, be more correctly named a Thermo-Barometer.

The height of the atmosphere has been variously estimated:—By Bravais, from the duration of twilight, at 66 to nearly 100 miles; by Dalton, in 1819, from observations of the auroral light, at 102 miles; by Sir John Herschel, from similar observations in 1861, at 83 miles; from observations of meteors, from 100 to 200 miles; by Liais, in 1859, from observations on the polarisation of the sky, at no less than 212 miles.

The density of the atmosphere diminishes with distance from the earth’s surface, in accordance with the following rule:—“At a height of seven miles the density of the atmosphere is reduced to one-fourth the density at the sea-level, and for every additional seven miles, the rarity of the air is similarly quadrupled.”

NOTE ON THE VERIFICATION OF INSTRUMENTS AT THE KEW OBSERVATORY.

The Kew Committee of the Royal Society receive, for verification and comparison with the standard instruments of the Kew Observatory, barometers, thermometers, and other instruments intended for meteorological observation or scientific investigations.

Any persons ordering instruments of opticians may direct them to be previously forwarded to the observatory for verification.

A scale of charges is issued by the Committee which is exclusive of packing and carriage, or of rail expenses, when a special messenger is sent out. The Meteorological Office, Victoria Street, London, also receives and forwards instruments for verification to the Kew Observatory.

The Committee wish it to be understood that they cannot undertake the verification of an inferior class of instruments (such as barometers mounted upon wooden frames, and thermometers not graduated on the stem), and that the superintendent of the observatory may at his discretion decline to receive such instruments as he may consider unfit for scientific observation.

BAROMETER WARNINGS.

May be Expected

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Increasing storm If mercury falls during a high wind from S.W., S.S.W., W. or S.

Violent but short If the fall be rapid.

Less violent but of longer If the fall be slow. continuance

A violent storm from the If the mercury falls suddenly while the N.W. or N. wind is due W.

N.W., N., or N.E. winds, or If the mercury having been at its usual less wind, or less rain, or height, 29·95, is steady or rising, less snow while the thermometer falls and the air becomes drier.

Wind and rain from S.E., S., If the mercury falls, while the and S.W. thermometer rises and the air becomes damp.

A violent storm from N.W., When the mercury falls suddenly with a N., or N.E. W. wind.

Snow If the mercury falls when the thermometer is low.

Less wind, or a change to When the mercury rises, after having N., or less wet been some time below its average height.

Strong wind or heavy squalls With the first rise of the mercury from N.W., N., or N.E. after it has been very low (say 29).

Improved weather When a gradual continuous rise of the mercury occurs with a falling thermometer.

Winds from S. or S.W. If the mercury suddenly rising, the thermometer also rises.

Heavy gales from N. Soon after the first rise of the mercury from a very low point.

Unsettled weather With a rapid rise of the mercury.

Settled weather With a slow rise of the mercury.

Very fine weather With a continued steadiness of the mercury with dry air.

Stormy weather with rain (or With a rapid and considerable fall of snow) the mercury.