82 ATMOSPHERE its reception during the day, and preventing too rapid a loss of it during the night ; to carry the waters of the ocean in the form of clouds or vapors over the land ; to serve as a mechani- cal force ; and last, but not least, to diffuse the element, oxygen, which sustains the life of all conscious beings. 1. Mechanical properties. The first property of the air is weight ; hence it is attracted by the earth, and therefore it exerts a pressure, not only downward, but, according to the law of fluids, sideways, up- ward, &c., as by the mobility of fluid particles any pressure is transmitted in all directions. The direct proof of the fact that the air has weight is, that when it is compressed in a strong flask, the flask is heavier than before. If this flask has a capacity of 100 cubic inches, and 100 more cubic inches of air are pressed in by means of a compression pump, the flask will be found to have gained 31 grains in weight. This is the result when the barometer stands at 30 inches, and the thermometer at 60 F. ; but as the air expands 3 V P al 't f r every inch of decrease in the barometer, and -ffa part for every degree of increase of the thermometer, the weight will be so much less if the barometer is lower or the thermometer higher, and vice vena. The atmosphere having weight, and being perfectly elastic, causes the lower strata to be denser than the upper. Con- sequently, if the experiment described be per- formed on the top of a high mountain, we shall find the weight of the 100 cubic inches of air considerably less than 31 grains; at a height of 14,282 feet the air will weigh only half as much ; at twice that height it will weigh only one quarter ; at three times, one eighth, &c. In general the law is, that while the height increases in an arithmetical ratio, 1, 2, 3, 4, 5, the weight, and consequently the pressure, de- crease in a geometrical ratio, , , ^, ^ &c. On this property is founded the system of estimating heights by determining the pressure of the air, either by weighing by the barometer, or by noticing the temperature at which water boils. Near the surface of the ocean water boils at 212 ; if we go 550 feet upward, it will boil at 211 ; 1,100 feet, at 210 ; 5,500 feet, at 202; 11,000 feet, at about 192. The cause of this difference is, that in order to boil water the heat must be great enough to cause the expansive force of the vapor or steam to over- come the atmospheric pressure, and that thus in ascending, this pressure becoming less, a less amount of heat is required. This method, however, is only a rough approximation, and is now abandoned for more delicate methods. The atmosphere, like all gaseous bodies, pos- sesses elasticity in a most remarkable degree. The effect of this elasticity is seen in the un- roofing of houses and bursting outward of windows in hurricanes. A partial vacuum being produced by the rotary motion of the hurricane, the air within expands and lifts off the roof, or bursts open the doors and win- dows. A similar effect is observed in the ex- pansion of air confined in a bladder, arid taken from a low level to a great height. The ex- ternal pressure being reduced, the air within tends to expand to the same degree of rarity as that without, and with such force us to burst the bladder. It is this property, pos- sessed in the greatest perfection by the gasc. u- bodies, that renders air so excellent a material for springs, air beds, &c. The impenetrability of air is its property of preventing another body occupying the space where it is. The diving bell is a good illustration of it, as also of its elasticity ; for when sunk to the depth of 34 feet, the water will be forced in, so as to half fill it; at the depth of 100 feet it will be three quarters filled ; on drawing it up the air will expand and drive out the water again. This also shows that air may be condensed and expanded by mechanical force. A remarkable law prevails, called after its discoverer the law of Mariotte, to the effect that the volume of the air is inversely proportional to the pressure employed, and therefore also to the reacting pressure exerted by the air on the vessels in which it is confined. This pressure, which in the ordinary condition of the atmosphere amounts near the surface of the ocean to about 15 pounds to the square inch, is thus doubled or tripled if we introduce double or triple the amount of air ill the same space, as in the experiment above referred to for weighing the air. Mariotte's law, however, does not hold for excessive pressures, say of 25 or 50 atmos- pheres, when the volume is not exactly inversely proportional to the pressure; our atmospheric air and most other gases are condensed more for a given pressure, while hydrogen gas forms an exception, and is condensed less than the amount required by Mariotte's law. The shape of the atmospheric envelope of our planet is of course spheroidal like the earth, only it is im >st likely that its upper surface is still more de- pressed at the poles than the earth itself, while the air is there colder, consequently more condensed and heavier, than at the equator. The attempts to determine the absolute height of the atmosphere have given different results. according to the different data taken as the basis of the calculation. The most trustworthy data are those founded on the time that on a clear evening the last twilight reaches the zenith, in connection with the laws of refraction and reflection of light; this has given as result a height of about 40 miles for the extreme traces of atmospheric air, in so far as these laws of refraction act in an appreciable manner. It is most likely, however, that the rarefaction expands much further, till at the utmost limit of some thousands of miles it mingles and be- comes identical with the interplanetary medium or so-called ether, which, according to some of the latest opinions, is only infinitely rarefied atmospheric air, or inversely, our atmospheric air is nothing but the interplanetary medium, condensed by gravitation on the surface of our planet. The pressure of the atmosphere is
