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.