CHAPTER XXX.

638. What is the atmosphere?

The atmosphere is the transparent and elastic body of mixed gases and vapours which envelopes our globe, and which derives its name from Greek words, signifying sphere of vapour.

639. To what height does the atmosphere extend?

It is estimated to extend to from forty to fifty miles above the surface of the earth.

640. Why is it supposed that the atmosphere does not extend beyond that height?

Because it is found, by experiment and observation, that the air becomes less dense in proportion to its altitude from the earth's surface. The gradual decrease of atmospheric density observed in ascending a mountain, or in a balloon, supplies sufficient data to enable us to calculate the height at which the atmosphere would probably altogether cease.

At an altitude of 18,000 feet the air is indicated by the barometer to be only half as dense as at the surface of the earth. And as the densities of the atmosphere decrease in a geometrical progression, the density will be reduced to one-fourth at the height of 36,000 feet; and to one-eighth at 54,000 feet. The effects of the decreasing density of the atmosphere are, that the intensity of light and sound are diminished, and the temperature is lowered. Persons who have reached a very high elevation, state that the sky above them began to assume the appearance of darkness; and there can be no doubt that, if it were possible to reach an altitude of some fifty to sixty miles, there would be perfect blackness although the sun's rays might be pouring through the darkened space, to illuminate the atmosphere. Upon the summit of Mont Blanc, the report of a pistol at a short distance can scarcely be heard. When Gay Lussac reached the height of 23,000 feet, he breathed with great pain and difficulty, and felt distressing sensations in his ears, as though they were about to burst. Upon the high table-lands of Peru, the lips of Dr. Ischudi cracked and burst; and blood flowed from his eyelids.

"For he looketh to the ends of the earth, and seeth under the whole heaven; To make the weight for the winds."—Job xxviii.