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it The annexed figure represents the different degrees of velocity with which a liquid flows from a vessel furnished with three stop-cocks, at different heights. Since the lateral pressure is entirely owing to the pressure down

wards, it is not affected by the horizontal dimensions of the vessel which contains the liquid, but merely by its depth; for as every particle acts independently of the rest, it is only the column of particles immediately above the orifice that can weigh upon and press out the liquid.

The pressure of liquids upwards, though it seems in direct opposition to gravity, is also a consequence of their pressure downwards. When, for example, water is poured into a tea-pot, the water rises in the spout to a level with that in the pot. The particles of water at the bottom of the pot are pressed upon by the particles above them; to this pressure they will yield, if there is any mode of making way for the superior particles, and as they cannot descend, they will change their direction and rise in the spout.

Suppose the tea-pot to be filled with columns of particles of water, similar to those described in the figure

annexed, the particle 1 at the bottom will be pressed laterally by the particle 2, and by this pressure be forced into the spout, where, meeting with the particle 3, it presses it upwards, Lesson XV.

and this pressure will be continued from 3 to 4, from 4 to 5, and so on, till the water in the spout has risen to a level with that in the

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pot.

ON THE BAROMETER. A BAROMETER is an intrument 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, about three feet in length 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, 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 great accuracy. The weight of the atmosphere sustains the mercury at the height of about 297 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, is generally 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 and fevers.

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 atmosphere 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.

LESSON XVI.

THE COMMON PUMP. 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 the 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

Var

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 the 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 weight of the air is sometimes not sufficient to raise a column of mercury more than twenty-eight inches, or a column of water much more than thirty-two feet; but when once it has passed that opening it is no longer the pressure of air on the reservoir which makes it ascend; it is raised by

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