To the celebrated Mr. Boyle and to Henry Cavendish, both of Great Britain, we are indebted for most of what we know of this particular property of the air.

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Density, or closeness, is another quality of the atmosphere.  It has been found to be 770 times less than that of water, and 770 cubic inches of air weigh as much as a cubic inch of water.  It is in direct ratio with its elasticity, and there are tables by which it may be determined at different altitudes.  At the surface of the earth, this density is indicated as 1; at 2-1/2 miles, as 1/2; at 5 miles, as 1/4; and so on, the difference being in a geometrical progression.

As we proceed in the consideration of our general subject, we shall find, under the appropriate heads, that density is not without material influence on reflection and refraction, on transparency and the transmission of light, the presence or absence of moisture, and the amount of heat at the earth’s surface,—­and we might add, on health, and the increase or diminution of the vital energies.

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Temperature is another branch of our subject, and one involving a series of subordinate topics on which volumes have been written, and to which are still devoted the labors of the most learned men of our day.  In this place, merely an out-line can be attempted.

Temperature is the degree of heat or cold in the particles of all bodies, which is perceptible by sensation, and is measurable by their expansion or contraction.  It is the key to the theory of the winds, of rain, of aerial and oceanic currents, of vegetation and climate with all their multifarious and important differences.  While the inclined position of the earth on its axis and its movement in its elliptical orbit influence the general amount of heat, it is rather to the consequences of these in detail that we are called when we speak of temperature.  If the sun shone on a uniformly level surface, everywhere of the same conducting and radiating power, there would be but little difficulty in tracing the monotonous effects of temperature.

The reformer Luther, as eccentric as he was learned and sincere, is reported to have said, that, if he had been consulted at the Creation, he would have placed the sun directly over the centre of the world and kept it there, to give unchanging and uniform light and heat!  It is certainly much better that he was not consulted.  In that case, every parallel of latitude would have been isothermal, or of equal mean annual temperature.  The seasons would have been invariable in character.  Some portions of the earth would have been scorched to crispness, others locked up in never-changing ice.

Vegetation, instead of being universal, would have been confined to a narrow zone; and the whole human race would have been driven together into one limited habitable space, to interfere with, incommode, and destroy each other.  The arrangement is best as it is.