PHYSICAL PROPERTIES OP THE EARTH.
The Adopting the conven ient terminology of geologists, the earth may be divided into four parts, namely: the atmosphere; the hydrosphere, or oceans; the lithosphere, or crust; and the centrosphere, or nucleus. These are considered in turn without encroaching on the fields of the meteorologist or of the geol ogist.
The atmosphere is a gaseous envelope cover ing the earth to an extent not yet fully deter mined. The pressure it exerts at the surface of the earth is easily measured and is definitely known to be about 14.7 pounds per square inch, or about 1,033 grams per square centimeter.
The pressure, density and temperature of the atmosphere are observed to decrease rapidly with increase of height above the earth's sur face, but the exact laws of decrease are not yet known. Hence it is impossible in the pres ent state of science to assign a height to the atmosphere. It appears certain, however, that it is more than 200 miles high, since it is dense enough to set meteorites on fire at that height.
Many investigations of the of the atmosphere have been made during the past century. These are all too technical for presen tation, even in abstract, here. But their results may be summarized as follows: (a) If the earth did not rotate and if the atmosphere were of the same density through out as at the earth's surface, its height would be 26,200 feet, or about five miles.
(b) If the earth did not rotate, and if the adiabatic law (pressure proportional to the 1.4 power of the density) held in the atmosphere, it would be about 17 miles high.
(c) But the earth rotates and the atmos phere must be supposed to rotate with the same angular velocity. From this fact, and from the assumption that the atmosphere be haves like a fluid, Laplace concluded that it must be limited by a lenticular-shaped envelope as indicated in Fig. 4. The revolution of this figure about the axis LM, which is supposed to be coincident with the axis of rotation of the earth, E, will generate the envelope and the earth in their true relative dimensions. The polar and equatorial axes LM and AB are, re spectively, 4.4 and 6.6 times the diameter of the
earth. According to this theory, therefore, the atmosphere may be about 17,000 miles high above the poles of the earth and about 26,000 miles high above the equator. The latter lim iting height is that at which the centrifugal force due to rotation is just balanced by the attraction of the earth. The volume of the envelope is 156 times the volume of the earth. Whether the atmosphere actually extends to the limits thus defined has not been determined either by La place or by subsequent investigators.
A question of great interest is that of the quantity of the atmosphere, since the oxygen thereof is essential to the existence of all higher forms of life on our planet. By quantity is meant the amount of mass of the atmosphere. If the law of distribution of this mass within the Laplacian envelope were known the total quantity could be computed. But the law is not known, and hence it is only possible to at tain an approximation, or to set limits, to the actual amount. A lower limit may be found from the adiabatic distribution of (b), above, for a non-rotating earth. This distribution does not differ widely from the actual distribution near the earth's surface; but since it is limited to a height of 17 miles it will give too small an amount. A computation on this basis makes the mass of the atmosphere 1-1170000th part of the total mass of the earth; or, in round num bers, one millionth part of the mass of the earth. This result is also reached, substantially, if it is assumed that the mass of the atmosphere is the same as the mass of a uniform layer of water, or mercury, which would produce the same pressure at the earth's surface. On the other hand, if the adiabatic distribution is sup posed to hold within the Laplacian envelope, of (c), above, an upper limit of about 1-1200th of the mass of the earth is obtained. The actual value of the mass of the atmosphere is undoubt edly much closer to the lower than to the upper limit; how much closer is still a matter for investigation.