it is the latter we are to examine. There is no attraction of cohesion between the particles of elastic fluids, so that the expansive power of heat has no adversary to contend with but gravity; any increase of temperature, therefore, expands elastic fluids prodigiously, and a diminution proportionally condenses them. The most essential point in which air differs from other fluids is by its spring or elasticity: that is to say, its power of increasing or diminishing in bulk, according as it is less or more compressed-a power of which liquids are almost wholly deprived.

The atmosphere is thought to extend to about the distance of 45 miles from the earth; and its gravity is such, that a man of middling stature is computed to sustain the weight of about 14 tons. Such a weight would crush him to atoms, were it not that air is also contained within our bodies, the spring or elasticity of which counterbalances the weight of the external air, and renders us insensible of its pressure. Besides this, the equality of pressure on every part of the body enables us more easily to support it; when thus diffused, we can bear even a much greater weight, without any considerable inconvenience. In bathing we support the weight and pressure of the water, in addition to that of the atmosphere; but this pressure being equally distributed over the body, we are scarcely sensible of it: whilst if the shoulders, the head, or any particular part of the frame were loaded with the additional weight of a hundred pounds, we should feel severe fatigue. On the other hand, if the air within a man met with no external pressure to restrain its elasticity, it would distend his body, and at length bursting the parts which confine it, put a period to his existence. The weight of the atmosphere, therefore, so far from being an evil, is essential to our existence. When a person is cupped, the swelling of the part under the cup is produced by taking away the pressure of the atmosphere; in consequence of which the internal air distends the part.

A column of air reaching to the top of the atmosphere, and whose base is a square inch, weighs 15 lbs.

when the air is heaviest. The rule that fluids press equally in all directions applies to elastic fluids as well as to liquids: therefore, every square inch of our bodies. sustains a pressure of 15 lbs. and the weight of the whole atmosphere may be computed by calculating the number of square inches on the surface of the earth, and multiply them by 15.

The weight of a small quantity of air may be ascertained by exhausting the air from a bottle, and weighing the bottle thus emptied. Suppose that a bottle six cubic inches in dimension, weighs two ounces; if the air be then introduced, and the bottle re-weighed, it will be found heavier by two grains, showing that six cubic inches of air (at a moderate temperature) weigh about two grains. In estimating the weight of air, the temperature must always be considered, because heat, by rarefying air, renders it lighter. The same principle indeed applies, almost without exception, to all bodies. In order to ascertain the specific gravity of air, the same bottle may be filled with water, and the weight of six cubic inches of water will be 1515 grains: so that the weight of water to that of air, is about 800 to 1.

A barometer is an instrument which indicates the state of the weather, by showing the weight of the atmosphere. It is extremely simple in its construction, and consists of a glass tube, A B, about three feet in length, and open only at one end. This tube must first be filled with mercury, then stopping the open end with the finger, it is immersed in a cup, c, which contains a little mercury. Part of the mercury which was in the tube now falls down into the cup, leaving a vacant space in the upper part of the tube, to which the air cannot gain

access. This space is therefore a perfect vacuum; and consequently the mercury in the tube is relieved from the pressure of the atmosphere, whilst that in the cup remains exposed to it; therefore the pressure of the air on the mercury in the cup supports that in the tube, and prevents it from falling; thus the

equilibrium of the mercury is destroyed only to preserve the general equilibrium of fluids. This simple apparatus is all that is essential to a barometer. The tube and the cup or vase are fixed on a board, for the convenience of suspending it; the board is graduated for the purpose of ascertaining the height at which the mercury stands in the tube; and the small moveable metal plate serves to show that height with greater accuracy. The weight of the atmosphere sustains the mercury at the height of about 29 inches; but the exact height depends upon the weight of the atmosphere, which varies much according to the state of the weather. The greater the pressure of the air on the mercury in the cup, the higher it will ascend in the tube. The air therefore generally is heaviest in dry weather, for then the mercury rises in the tube, and consequently that in the cup sustains the greatest pressure; and thus we estimate the dryness and fairness of the weather by the height of the mercury. We are apt to think the air feels heavy in bad weather, because it is less salubrious when impregnated with damp. The lungs, under these circumstances, do not play so freely, nor does the blood circulate so well: thus obstructions are frequently occasioned in the smaller vessels, from which arise colds, asthmas, agues, fevers, &c.

As the atmosphere diminishes in density in the upper regions, the air must be more rare upon a hill than in a plain; and this difference may be ascertained by the barometer. This instrument is so exact in its indications, that it is used for the purpose of measuring the height of mountains, and of estimating the elevation of balloons. Considerable inconvenience is often experienced from the thinness of the air in such elevated situations. It is sometimes oppressive, from being insufficient for respiration; and the expansion which takes place in the more dense air contained within the body is often painful it occasions distension, and sometimes causes the bursting of the smaller blood-vessels in the nose and ears. Besides, in such situations, the body is more exposed both to heat and cold; for though the atmos

phere is itself transparent, its lower regions abound with vapours and exhalations from the earth, which float in it, and act in some degree as a covering, which preserves us equally from the intensity of the sun's rays and from the severity of the cold.

Now, since the weight of the atmosphere supports mercury in the tube of a barometer, it will support a column of any other fluid in the same manner; but as mercury is the heaviest of all fluids, it will support a higher column of any other fluid; for two fluids are in equilibrium, when their heights vary inversely as their densities: : as, for instance, if a cubic foot of one fluid weighs twice as much as a cubic foot of the other, a column of the first ten feet in height will weigh as much as a column of the other twenty feet in height.Thus the pressure of the atmosphere, which will sustain a column of mercury of twenty-nine inches, is equal to sustaining a column of water of no less than thirty-four feet above its level. The weight of the atmosphere is therefore as great as that of a body of water surrounding the globe of the depth of thirty-four feet; for a column of air of the height of the atmosphere is equal to a column of water of thirty-four feet, or one of mercury of twenty-nine inches, each having the same base.

The common pump is constructed on this principle. By the act of pumping, the pressure of the atmosphere is taken off one part of the surface of the water: this part therefore rises, being forced up by the pressure communicated to it by that part of the water on the surface of which the weight of the atmosphere continues to act. The body of a pump consists of a large tube or pipe, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down it, by means of a metallic rod fastened to the centre of the piston.

The various parts of a pump are here delineated. A B is the pipe or body of the pump; P the piston; v

a valve, or little door in the piston, which, opening upwards, admits the water to rise through it, but prevents its returning; and y a similar valve in the body of the pump. When the pump is in a state of inaction, the two valves are closed by their own weight; but when, by drawing down the handle of the pump, the piston ascends, it raises a column of air which rested upon it, and produces a vacuum between the piston and the lower valve, y; the air beneath this valve, which is immediately over the surface of the

water consequently expands, and forces its way through it; the water then, relieved from the pressure of the air, ascends into the pump. A few strokes of the handle totally exclude the air from the body of the pump, and fill it with water, which, having passed through both the valves, flows out at the spout. Thus the air and the water successively rise in the pump on the same principle that the mercury rises in the barometer. Water is said to be drawn up into a pump by suction ; but the power of the suction is no other than that of producing a vacuum over one part of the liquid, into which vacuum the liquid is forced by the pressure of the atmosphere on another part. The action of sucking through a straw consists in drawing in and confining the breath, so as to produce a vacuum, or at least to lessen materially the quantity of air, in the mouth in consequence of which, the air within the straw rushes into the mouth, and is followed by the liquid, into which the lower end of the straw is immersed. The principle is the same; and the only difference consists in the mode of producing a vacuum. In suction, the muscular powers answer the purpose of the piston and valves. The distance from the level of the water in the well to the valve in the piston ought not to exceed thirty-two feet, otherwise the water would not be sure to rise through that valve, for the