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GRAVITY OF ATMOSPHERIC AIR.

The weight of the air compresses all fluids and resists theirdilatation; it opposes the evaporation of fluids, &c.

The gravity of the air resists the dilatation of the animal fluids;for the. blood often bursts through the skin, or from the lungs,on the top of high mountains, occasioning hemorrhages, &c.because the air is there considerably lighter than near the levelof the sea.

By the pressure of the air the sucking of animals, the art ofcupping, and many other phenomena are explained, too nume-rous to be noticed here.

Thus the doctrine of the gravity and pressure of the air beingestablished, by the most unexceptionable evidence; we are en-titled to assume, a priori, all its legitimate consequences.

SPECIFIC WEIGHT OF ATMOSPHERIC AIR.

A bulk of atmospheric air that is equal to an English quart,when taken near the level of the sea, and at about 50° Fahr.weighs at a medium about 16 grains, and the same bulk of rain-water at the same temperature, weighs about 14621 grains.Hence rain-water is about 914 times specifically heavier than air.

The pressure of the air is equal to about 15 pounds troy onevery square inch, and this pressure is the same in every' direc-tion. A square mile contains 27,8/8,400 square feet. Theearth’s surface in round numbers is 200,000,000 square miles,and therefore the pressure of the atmosphere circumfused aboutthe whole surface of the earth is 12 ,,/ ,043,468",800,000',000,000pounds.

The surface of a middle sized man is about 14 square feet,he sustains therefore a weight of air, that is equal at a mediumto about 30,240 pounds troy, or 24,882| pounds avoirdupoisnearly, or 1777 stone and 4 pounds, or It tons, 2 cwt. 18£ lbs.

Though our body supports such an enormous weight we neednot wonder at the freedom with which we traverse this atmos-phere. We see fish move with as great ease in a much more