Front Page Titles (by Subject) CHAP. V.: Of the Air and Atmosphere. - The Works, vol. 2 An Essay concerning Human Understanding Part 2 and Other Writings
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CHAP. V.: Of the Air and Atmosphere. - John Locke, The Works, vol. 2 An Essay concerning Human Understanding Part 2 and Other Writings 
The Works of John Locke in Nine Volumes, (London: Rivington, 1824 12th ed.). Vol. 2.
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Of the Air and Atmosphere.
We have already considered the earth as a planet, or one of the great masses of matter moving about the sun; we shall now consider it as it is made up of its several parts, abstractedly from its diurnal and annual motions.
The exterior part of this our habitable world is the air or atmosphere; a light, thin fluid, or springy body, that encompasses the solid earth on all sides.
The height of the atmosphere, above the surface of the solid earth, is not certainly known; but that it doth reach but to a very small part of the distance betwixt the earth and the moon, may be concluded from the refraction of the rays coming from the sun, moon, and other luminous bodies.
Though considering that the air we are in, being near 1000 times lighter than water; and that the higher it is, the less it is compressed by the superior incumbent air, and so consequently being a springy body the thinner it is; and considering also that a pillar of air of any diameter is equal in weight to a pillar of quicksilver of the same diameter of between 29 and 30 inches height; we may infer that the top of the atmosphere is not very near the surface of the solid earth.
It may be concluded, that the utmost extent of the atmosphere reaches upwards, from the surface of the solid earth that we walk on, to a good distance above us; first, if we consider that a column of air of any given diameter is equiponderant to a column of quicksilver of between 29 and 30 inches height. Now quicksilver being near 14 times heavier than water, if air was as heavy as water, the atmosphere would be about 14 times higher than the column of quicksilver, i. e. about 35 feet.
Secondly, if we consider that air is 1000 times lighter than water, then a pillar of air equal in weight to a pillar of quicksilver of 30 inches high will be 35000 feet; whereby we come to know that the air or atmosphere is 35000 feet, i. e. near seven miles high.
Thirdly, if we consider that the air is a springy body, and that that, which is nearest the earth, is compressed by the weight of all the atmosphere that is above it, and rests perpendicularly upon it; we shall find that the air here, near the surface of the earth, is much denser and thicker than it is in the upper parts. For example, if upon a fleece of wool you lay another; the under one will be a little compressed by the weight of that which lies upon it; and so both of them by a third, and so on; so that, if 10000 were piled one upon another, the under one would by the weight of all the rest be very much compressed, and all the parts of it be brought abundantly closer together, than when there was no other upon it; and the next to that a little less compressed, the third a little less than the second, and so on till it came to the uppermost, which would be in its full expansion, and not compressed at all. Just so it is in the air; the higher you go in it, the less it is compressed, and consequently the less dense it is; and so the upper part being exceedingly thinner than the lower part, which we breathe in (which is that that is 1000 times lighter than water); the top of the atmosphere is probably much higher than the distance above assigned.
That the air near the surface of the earth will mightily expand itself, when the pressure of the incumbent atmosphere is taken off, may be abundantly seen in the experiments made by Mr. Boyle in his pneumatic engine. In his “Physico-mechanical Experiments,” concerning the air, he declares* it probable that the atmosphere may be several hundred miles high; which is easy to be admitted, when we consider what he proves in another part of the same treatise, viz. that the air here about the surface of the earth, when the pressure is taken from it, will dilate itself about 152 times.
The atmosphere is the scene of the meteors; and therein is collected the matter of rain, hail, snow, thunder, and lightning; and a great many other things observable in the air.
[* ]New experiments Physico-mechanical, touching the spring of the air, and its effects; (made for the most part in a new pneumatical engine) written ..... by the honourable Robert Boyle, Esq.; experiment xxxvi. p. 155. Oxford, 1662, in 4to.