ATMOM′ETER. Syn. Atmidom′eter; Atmom′etrum, &c., L.; Atmomètre, &c., Fr. In chemistry and meteorology, an instrument for measuring the rate of evaporation from a humid surface. It is of very simple construction, and possesses some practical value. It consists of a long glass graduated tube divided into inches, having attached to the bottom a hollow ball made of porous earthenware, similar to that used in water bottles. When used, water is poured in at the top until it rises to the zero point of the scale. The outside of the porous ball being always covered with dew, the more rapidly the evaporation takes place, the more quickly will the water fall in the tube.
AT′MOSPHERE (-fēre). Syn. Atmosphe′′ra, L.; Atmosphère, Fr.; Atmosphäre, Dunstkreis, Ger. Primarily, a ‘vapour-sphere,’ appr., the assemblage of respirable gas and aëriform vapours which surround the earth; fig., any surrounding medium or influence.
Comp., Chem. prop., Pur., Uses, &c. See Air (Atmospheric).
Mechanical properties of the atmosphere:—
Colour:—The prevailing colour of the atmosphere is blue; at considerable elevations this blue tint is lost, and the sky appears deep black. The prevalence of blue is referred to the greater facility with which the blue and violet rays are reflected, whilst the glowing tints of morning and evening are conceived to arise from the red rays possessing greater momentum than the other rays of the spectrum.
Density:—The density of the atmosphere diminishes with the distance from the earth’s surface, and this is the duplicate ratio of the altitude. Thus, if at a given altitude the density of the air is only one half what it is at the level of the sea, at twice that elevation it possesses only one fourth that density. On this fact depends the application of the barometer to the determination of the elevation or depression of any point above or below the level of the sea, taken as a standard.
Density of the Atmosphere at Different Elevations. By Prof. Graham.
| Height above the level of the Sea in miles. | Volume of Air. | Height of the Barometer. |
| 0· | 1 | 30 |
| 2·705 | 2 | 15 |
| 5·41 | 4 | 7·5 |
| 8·115 | 8 | 3·75 |
| 10·82 | 16 | 1·875 |
| 13·525 | 32 | ·9375 |
| 16·23 | 64 | ·46875 |
Height, &c.:—If the density of the air were uniform throughout its whole extent, the height of the atmosphere, measured by a corresponding column of mercury, would be barely 51⁄4 miles. As, however, its density decreases with the distance from the earth’s surface, its real height must be considerably greater. Kepler found that the reflection and refraction of the sun’s rays by the atmosphere, producing twilight, ceases when that luminary descends 18 degrees below the horizon, whence it is calculated that the atmosphere cannot have a greater altitude than 45 miles. On the other hand, there is reason to believe that it cannot be much less than this sum. “With a good air-pump air may be rarefied 300 times; supposing this to be the utmost limit to which rarefaction can be carried, the atmosphere would still extend to an altitude of above 40 miles.” Whether, in a state of extreme tenuity in which its grosser properties are lost, it extends indefinitely into space, was formerly a subject of controversy. That its boundaries are limited, and that it belongs exclusively to our earth appears almost certain. “We are warranted in concluding that the atoms of air are not infinitely divisible, and consequently that the atmosphere has a limit; and the limit must be situated at that height above the earth where the gravitation of the atoms is just equal to the force of their repulsion.”[100] Under ordinary circumstances the mercury of the barometer falls about one inch for every 1000 feet of elevation.
[100] Brande’s ‘Dict. of Lit., Sci., & Art.’