AT′MOSPHERE

(-fēre). Syn. Atmosphe′′ra , L.; Atmosphère , Fr.; Atmosphäre , Dunstkreis , Ger. Primarily, a ‘vapour-sphere,’ appr., the a emblage of respirable gas and aëriform vapours which surround the earth; fig., any surrounding medium or influence. Comp., Chem. prop., Pur., Uses, . 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 po e ing 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 po e es only one fourth that density. On this fact depends the application of the barometer to the determination of the elevation or depre ion 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 , .:—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 5 1 ⁄ 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 le 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 gro er 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.’ Pre ure :—The weight or pre ure of the atmosphere is shown by the rise of water in the barrel of the common ‘lifting pump’ and the suspension of the mercurial column in the tube of the barometer. The last affords a ready means of determining the actual pre ure of the air, the column of mercury, and the column of air by which it is suspended, resembling two weights in equilibrio, at the opposite extremities of the same balance. The mean height of the barometer at the level of the sea, in England, is 28·6 inches (= about 33 1 ⁄ 2 feet of water); and as a cubic inch of mercury weighs 3425·92 gr., or ·48956 lb. , it follows that the weight of a column of mercury whose base is a square inch is 14·6 lbs. avoirdupois. The pre ure of the atmosphere is not merely downwards, but is equally diffused in all directions, and exerts a most powerful effect in the economy of organic beings. On the surface of the body of an adult of ordinary size (say = 15 sq. feet, or 2160 inches), it amounts to the enormous weight of 31,536 lbs. , which is not sensible, only because it is balanced by the force of the elastic fluids in the interior of the body. Were this equilibrium to be suddenly destroyed, the consequence would be, either that the body would be instantly torn to pieces with explosive violence, or that it would be crushed under the overwhelming weight that would suddenly fall upon it. Even the comparatively slight variations of atmospheric pre ure which occur with changes of wind, weather, and season, exercise a perceptible effect on the functions of life. Mean pre ure of the Atmosphere at the level of the Sea, in different latitudes, at 32° Fahr. , expre ed in inches of mercury . | Lat. | Height (inches). | Lat. | Height (inches). | Lat. | Height (inches). | | 0° | 29·930 | 40° | 30·019 | 541⁄2° | 29·926 | | 10 | 29·975 | 45 | 30·000 | 60 | 29·803 | | 20 | 30·064 | 49 | 29·978 | 64 | 29·606 | | 30 | 30·108 | 511⁄2 | 29·551 | 67 | 29·673 | Temperature :—The temperature of the atmosphere, independently of changes arising from variations of latitude and season, diminishes, like its density, with its elevation. In general, every 100 yards of ascent causes the temperature to fall 1° Fahr. See Air (Atmospheric), Epidemics, Ventilation , .