A quart of air, at ordinary temperature, weighs about eight hundred times less than a quart of water, yet the aggregate pressure of the atmosphere is equal to fifteen pounds on every square inch. A person of average size presents a surface of about two thousand square inches. This would receive a pressure of fifteen tons, a weight more crushing than that of all the shields cast upon the traitorous Tarpeia[4] at the Roman gate. Herschel calculates that the total weight of the atmosphere is one twelve-hundred-thousandth of that of the earth. Why does not such enormous pressure destroy life? Because it is counterbalanced by the pressure of air, gases and blood within the body. That this is true may readily be seen in the process of dry-cupping. Bare the arm, take a bit of writing paper an inch and a half long, dip it in alcohol, light, and instantly place in a small wine glass, and at once apply the glass to the soft part of the arm. The flesh under the glass will rise like a pin-cushion, and become red from the pressure of the blood within. Persons going down in diving-bells suffer from the condensation of the air in the bell, while on a high mountain they experience a pressure in the opposite direction, on account of the rarefaction of the atmosphere, the blood often gushing from the nose and ears. We shall better understand the phenomena of the air by first considering some of its distinctive properties. [Illustration: MAGDEBURG HEMISPHERES.] AIR PRESSES EQUALLY IN ALL DIRECTIONS. This great principle, which applies to all gases as well as to fluids, has many illustrations in nature. The haliotis[5] is held to the rock with a tenacity which sometimes resists the strength of the collector, who would add its iridescent beauty to his cabinet of shells. Alas for the fisherman’s line whose bait has been swallowed by a skate![6] Quickly descending to the bottom, this broad, flat fish expels the air from beneath it, and defies all effort at capture. The most complete demonstration of this law is shown by the Magdeburg hemispheres,[7] invented by Otto von Güricke[8] and used by him before Charles V. and his brilliant court. They are still preserved in the ancient city which gave them their name, are twenty-four inches in diameter, and after the air in them had been removed, required twelve horses to separate them. The pressure of the air varies greatly at different altitudes. At the height of three and one-half miles the column of mercury in a barometer falls to fifteen inches, showing that below that elevation we have as much air as in all the space above. [Illustration: SHOWING TORRICELLI’S HISTORICAL EXPERIMENT AND THE PRINCIPLE OF THE BAROMETER.] The boiling point of liquids is materially influenced by the pressure of the atmosphere. On high mountains potatoes and even eggs can not be cooked by boiling, as the water will all evaporate before it is heated sufficiently to cook them.