[64] As the earth’s surface contains, observes Mr. Ferguson, in round numbers, 200,000,000 square miles, must contain no less than 5,575,680,000,000,000 square feet; which being multiplied by 2,160, the numbers of pounds on each square foot, amounts to 12,043,468,800,000,000,000 pounds, for the weight of the whole atmosphere. Mr. Coates computed that the weight of the air which pressed upon the whole surface of the earth, is equal to that of a globe of lead sixty miles in diameter. The following simple experiments within the reach of every one’s observation, show clearly the weight or gravitating power of the air. Let any one lay his hand on the top of a long perpendicular pipe, such as a pump filled to the brim with water, which is at first prevented from running out by the valve below: then let the valve be opened, so that the water may descend, and he will find his hand so hard pressed to the top of the pipe that he cannot draw it away. The prop is now gone; he has no pressure under his hand; a column of air, 45 miles high forces it down by its weight; and he must let in the air under it before the hand can be withdrawn.--If we shut the nozzle and valve-hole of a pair of bellows after having squeezed the air out of them, we shall find that a very great force, even some hundred pounds, is necessary for separating the boards; they being kept together by the pressure of the air which surrounds them.--If any one will apply the open end of a syringe to his hand, and then draw up the piston, he will find his hand sucked into the syringe with great force, and it will give pain, and the soft part of the hand will swell into it, being pressed in by the neighboring parts, which are subject to the action of the external air. [65] A heavy air is more favorable to health than a light one, because it promotes the circulation of the blood, and insensible perspiration. When the air is heavy, it is generally clear; whereas a light air is generally accompanied with clouds, rain, or snow, which render it damp. Too great a dryness of the air is very injurious to the human body; but this seldom happens for any length of time, except in sandy countries. A damp air is very unwholesome, because it relaxes the fibres, obstructs insensible perspiration, and if heat accompany the dampness, it disposes the humors to putrefy. An air too hot dilates all the fluids of the body, and occasions sweatings, which bring on weakness and oppression. On the other hand, when the air is to cold, the solid parts contract excessively, and the fluids are condensed; hence result obstructions and inflammations. The best air is that which is neither too heavy nor too light, too moist nor too dry, and which is not impregnated with noxious vapors. Sturm’s Reflections, vol. iv, p. 50.