BAROMETER PRECAUTIONS.
If vacuum suspected, cause mercury to strike top of tube.
A clear metallic “click” indicates a good vacuum.
A dull “thud” indicates air or moisture.
In latter case return to optician, but if unable
Incline very gently until nearly inverted, when
Air if present will ascend in a bubble into the cistern.
Suspend barometer in good light out of sunshine.
Let no heat of fire or lamp affect it.
Let no sudden changes of temperature affect it.
It must hang absolutely vertically.
Note temperature of attached thermometer before reading barometer.
Then adjust mercury in cistern to touch ivory point.
Then adjust vernier and take reading quickly.
Ascertain height above sea-level according to direction.
The Storm Glass (Fig. 36) is a glass bottle, ten inches long, containing a mixture of camphor, nitre, sal-ammoniac, alcohol, and water. As “temperature affects the mixture much,” an arrangement has recently been designed in which the stem of a thermometer is immersed in the fluid, as shown at Fig. 37, thus imparting a higher value to its indications. The late Admiral Fitzroy says—
“Since 1825, we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, preferably, in the outer air, the chemical mixture in a so-called storm glass varies in character with the direction of the wind—not its force.”
The quarter from which the wind or storm is blowing is indicated by the substance adhering more closely to the bottom of the glass opposite to the point whence the wind or tempest arises.
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.
35. 36. 37.
Storm Glass, or Chemical Weather Glass. Scale about 1/5.
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.