_Height of the Atmosphere._--The weight of a given volume of mercury is 10,517 times that of an equal volume of air at the earth's surface; and since the mean height of the mercurial column in the barometer is about thirty inches, if the atmosphere were of uniform density its altitude would be about 26,300 feet, or nearly five miles. The density rapidly diminishes, however, as we ascend above the earth's surface. Calling it unity at the sea level, the rate of variation is approximately expressed as follows: Altitude in Miles. Density. 0 1 7 1/4 14 1/16 21 1/64 28 1/256 35 1/1024 70 1/1000000 105 1/1000000000 140 1/1000000000000 etc. etc. From this table it will be seen that at the height of 35 miles the air is one thousand times rarer than at the surface of the earth; and that, supposing the same rate of decrease to continue, at the height of 140 miles the rarity would be one trillion times greater. The atmosphere, however, is not unlimited. When it becomes so rare that the force of repulsion between its particles is counterbalanced by the earth's attraction, no further expansion is possible. To determine the altitude of its superior surface is a problem at once difficult and interesting. Not many years since about 45 or 50 miles were generally regarded as a probable limit. Considerable light, however, has been thrown upon the question by recent observations in meteoric astronomy. Several hundred detonating meteors have been observed, and their average height at the instant of their first appearance has been found to exceed 90 miles. The great meteor of February 3d, 1856, seen at Brussels, Geneva, Paris, and elsewhere, was 150 miles high when first seen, and a few apparently well-authenticated instances are known of a still greater elevation. We conclude, therefore, from the evidence afforded by meteoric phenomena, that the height of the atmosphere is certainly not _less_ than 200 miles.