Fahrenheit proposed to ascertain the height of mountains by this principle, and a simple apparatus was contrived for the purpose, which is now in successful use. The late Professor Forbes of Edinburgh, whose untimely death the friends of science have had so much reason to deplore, ascertained that the temperature of boiling water varied arithmetically with the height, and at the rate of one degree of the thermometric scale for every 549.05 feet. Multiplying the difference of the boiling-point by this number of feet, we have the elevation. The weight of the atmosphere, as indicated by the barometer, is also a means for ascertaining the height of mountains or of plains; but correction must be made for the effects of expansion or contraction, and for capillarity, or the attraction between the mercury and the glass tube, at least whenever great exactness is required. Tables for the convenience of calculation are given in several scientific works, and particularly in a paper of Professor Forbes, Ed. Trans. Vol. 15. Briefly, however, we may state, that between 0° and 32°, 34 thousandths of an inch must be allowed for depression or contraction, and between 32° and 52° 33 thousandths. The weight of the atmosphere is not only affected by rarefaction, but by currents of air, which give it a sudden density or rarity. Those who have ascended mountains have experienced both these changes.

A common experiment to prove the weight of air is that of the Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a merchant of that city. It is a part of every complete philosophical apparatus. It consists of brass caps, which, when joined together, fit tightly and become a globe. The air within being exhausted, it will be found difficult to separate them. If the superficies be 100 square inches and the height of the mercury be 30 inches, the atmosphere will press on these hemispheres with a weight of 1,475 lbs, requiring the efforts of seven or eight powerful men to tear them asunder. One of these instruments, of the diameter of a German ell, required the strength of 24 horses to separate it. The experiment was publicly made in 1650 at the Imperial Diet at Rendsborg, in the presence of the Emperor Ferdinand III. and a large number of princes and nobles, much to their astonishment.

As compared with water, the air (the barometer indicating 30°, and the thermometer 55°) is 833 times lighter.

It is this weight of the atmosphere which counterbalances that of a column of mercury 29 inches in height, and a column of water 32 to 34 feet in height.

The old quaint notion of Nature's abhorring a vacuum was found to be practically only an assertion that the air had weight. The ordinary pump, commonly called the suction-pump, is constructed on this principle. The weight of the atmosphere at the level of the sea is found to be the same all over the world.

We find the atmosphere with another characteristic,–Elasticity.

However it may be compressed, air returns, on liberation, to its original volume, and while thus perfectly elastic it is also the most compressible of bodies. This elasticity arises from the repulsive force of its particles, and is always equal to the compressive force which it balances. A glass vessel full of air, placed under a receiver and then exhausted by the air-pump, will burst into atoms. Water, on the other hand, is almost the reverse. Twenty cubic inches, introduced into a cannon whose sides are three inches thick, cannot be compressed into nineteen inches without bursting it. This non-elastic property of water, with another, that of communicating, when under the action of any force, an equal pressure in all directions, led to the invention of the hydraulic press.

The elasticity of the air enables fishes to rise and sink in water, through the action of the air-bladder.

The sudden compression of air liberates its latent heat, and produces fire. On this principle the pneumatic tinder-box is constructed.

Brockhaus says that air has as yet been compressed only into one-eighth of its original bulk.

For every degree of heat between the freezing-point and the boiling-point, 32° and 212°, the expansion of air is about 1/490th part, so that any invention which seeks to use rarefied air as a motive power must employ a very intense degree of heat, enough to fuse many kinds of metals.

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.

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.

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.

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