tion to rain water, as 12 to 1006,66 and to oxygen gas, as 5 to 6; and to hydrogen, as 15 to 1; a cubit foot of it weighing 525 grains; or one ounce and a quarter, nearly. 594. The atmosphere is found to be very elastic; and, in consequence, to press on every side, equal to a weight of 33 feet of water, or 204 inches of mercury; and this elasticity is found to decrease, as we ascend higher and higher, so as to render the barometer a means of ascertaining heights. Ops. This elasticity is equally powerful in a cubic inch of the atmosphere, as in the whole mass; and an inch will raise the mercury in the barometer, as much as the whole atmosphere. One cannot, therefore, but wonder at the quackery, or inconsiderateness of authors, who copy, one after another, the idle nonsense about the at mosphere pressing a man with a weight of 30,000 lbs.j when, in fact, he is not pressed to the amount of an onnes i all the vesicles of his body being filled with air, which presses outward, at least as much as the atmosphere presses inward, and also upward as well as downward. In fact, in regard to animal and vegetable bodies, the slight gravity of the air is destroyed by its elasticity. 595. Comparing the atmosphere to fleeces of wool laid upon one another, it will be lighter or rarer as we ascend in it; or, in other words, its elasticity will be diminished. Were it all of uniform density, like water, it would be about five miles high; but the reflec tion of the sun's rays appears to be affected by it at the height of 44 miles; where it is calcu lated to be 4,000 times less elastic than on the surface of the earth. The blue colour of the atmosphere is its na tural colour. Its power of reflection produces the universal diffusion of light. Obs-On the elasticity of air, is founded the invention of the DIVING BELL; by means of which, an operator descends to any depth in water, and remains there for hours together. Weights are placed at the bottom to prevent it from turning; and a forcing pipe sends in fresh air, to supply the waste of air from the respiration of the operator. 596. By means of the air-pump, all the air may be drawn out of a large glass-vessel, and a vacuity or vacuum produced; in which a great number of curious experiments may be performed, shewing at once the properties and usefulness of air. The figure represents an air-pump on the best modern construction. The glass-receiver, as it is called, is placed at top; where there is a hole, to let out the air by the action of the pistons seen below. 597. In the aerial vacuum, a feather and a guinea will fall with equal velocity, owing to there being no resistance of air. If a bladder, apparently empty, be tied at the neck and left in it, the small quantity of air in the bladder will swell it, and presently burst it. A bell will cease to sound in vacuo. The smoke of a candle, having no air to float in, will fall to the bottom by its own weight. No animal will live, or any light burn, in vacuo. Obs. A bladder, tied in the same manner, will swell and burst, if laid before a fire; thereby proving the power of heat to rarefy air. 598. Common air may also be compressed, by cold or by mechanical means, into forty thousand times its ordinary space, and still maintain its elasticity; and on this principle is founded the invention of the air-gun. Air has a constant disposition to maintain its equilibrium, level, or equal diffusion, like water. Hence, if a bladder, filled with rarefied air, burst, an explosion takes place, from the rushing of the surrounding air to fill the space. The same principle is the cause of all wind, which may be traced to some local expansion or compression of air by heat and cold; thus, also, smoke is carried up chimneys. Obs. Is is evident, that the density of bodies must be diminished by expansion, and in the case of fluids and gases, the parts of which are mobile, many important phenomena depend upon this circumstance. If heat be applied to fluids or to gases, the heated parts change their places and rise; and the colder parts descend and occupy their places. Currents are constantly produced in the ocean and in great bodies of water, in consequence of this effect. The heated water rises to the surface in the tropical climates, and flows towards colder ones thus the warmth of the Gulf-stream is felt a thousand miles from its source, and deep currents pass from the colder to the warmer parts of the sea: the general tendency of these changes, is to equalize the temperature of the globe. 599. One of the principal foreign bodies mixed with, or dissolved in the atmosphere, is the vapour of water which is constantly rising at every degree of heat, provided the force of the vapour already in the atmosphere is not greater than that of vapour at the existing tem perature. By this perspiration of the globe, 36 inches of water per annum are raised from the surface of all seas or rivers; and, at least, 30 inches from all land. In December and January, it is 18 inches per month; and in July and August, more than 5 inches. 600. By this constant process of evaporation, 100,000 cubic miles of water are, every year, raised into the atmosphere; the greater part of which, at a certain height, parts with its beut, and is condensed into clouds. These are carried by the winds over the land, broken and precipitated by the action of moun tains and trees; and thus rendered the means of watering the soil. It then returns to the sea in the currents of rivers; so that there is a constant circuit of the waters! They are chiefly raised from the sea, are carried by the winds over the land; fall in rain; and then return again to the sea in rivers! The streams, their beds forsaking, upward move, |