in consequence of this, a much more violent and rapid discharge of the water takes place, than would occur by the pressure of that in the upright tube alone. The centrifugal force and the force of the discharge thus acting at the same time, and each increasing the force of the other, this machine revolves with great velocity and proportionate power. The friction which it has to overcome, when compared with that of other machines, is very slight, being chiefly at the point c, where the weight of the upright tube and its contents is sustained.

By fixing a cog wheel to the shaft at d, motion may be given to any kind of machinery required.

Where the quantity of water is small, but its height considerable, this machine may be employed to great advantage, it being under such circumstances one of the most powerful en. gines ever invented.

PNEUMATICS.

The term Pneumatics is derived from the Greek pneuma, which signifies breath, or air. It is that science which investigates the mechanical properties of air, and other elastic fluids.

Under the article hydraulics, it was stated that fluids were of two kinds, namely, elastic and non-elastic, and that air and the gases belonged to the first kind, while water and other liquids belonged to the second.

The atmosphere, which surrounds the earth, and in which we live, and a portion of which we take into our lungs at ́every breath, is called air, while the artificial products which possess the same mechanical properties, are called gases.

When therefore the word air is used, in what follows, it will be understood to mean the atmosphere which we breathe. Every hollow, crevice, or pore, in solid bodies not filled with a liquid, or some other substance, appears to be filled with air thus a tube of any length, the bore of which is as small as it can be made, if kept open'will be filled with air, `and hence when it is said that a vessel is filled with air, it is only meant that the vessel is in its ordinary state. Indeed, this fluid finds its way into the most minute pores of all substances, and cannot be expelled, and kept out of any vessel, without the assistance of the air pump, or some other mechanical means.

.

What is pneumatics? What is air? What is gas? What is meant, when it is said that a vessel is filled with air? Is there any difficulty in expelling the air from vessels?

By the elasticity of air, is meant its spring, or the force with which it re-acts when compressed in a close vessel. It is chiefly in respect to its elasticity, and lightness, that the mechanical properties of air differ from those of water, and other liquids.

Elastic fluids differ from each other, in respect to the permanency of the elastic property. Thus steam is elastic only while its heat is continued, and on cooling returns again to the form of water.

Some of the gases also, on being strongly compressed, lose their elasticity, and take the form of liquids. But air differs from these, in being permanently elastic; that is, if it be com pressed with ever so much force, and retained under com. pression for any length of time, it does not therefore lose its elasticity, or disposition to regain its former bulk, but always Fig. 98. re-acts with a force, in proportion to the power by which it is compressed.

Thus, if the strong tube, or barrel, fig. 98, be smooth, and equal on the inside, and there be fitted to it the solid piston, or plug a, so as to work up and down air tight, by the handle b, the air in the barrel may be pressed, without difficulty, into a hundred times less than its usual bulk. Indeed, if the vessel be of sufficient strength, and the force employed sufficiently great, its bulk may be lessened a thousand times, or in any proportion according to the force employed; and if kept in this state for years it will regain its former bulk the instant the pressure is removed. Thus it is a general principle in pneumatics, that air is compressible in proportion to the force employed. On the contrary, when the usual pressure of the atmosphere is removed from a portion of air, it expands and occupies a space larger than before; and it is found by experiment that this expansion is in a ratio, as the removal of the pressure is more or less complete. Air also expands, or increases in bulk when heated.

If the stop-cock, c, fig. 98, be opened, the piston a, may be pushed down with ease, because the air contained in the barrel will be forced out at the aperture. Suppose the piston to be

What is meant by the elasticity of air? How does air differ from steam, and some of the gases, in respect to its elasticity? Does air lose its elastic force by being long compressed? In what proportion to the force employed is the bulk of air lessened?

pushed down to within an inch of the bottom, and then the stop-cock closed, so that no air can enter below it. Now, on drawing the piston up to the top of the barrel, the inch of air will expand and fill the whole space, and were this space a thousand times as large, it would still be filled with the expanded air, because the piston removes the pressure of the external atmosphere from that within the barrel.

It follows therefore, that the space which a given portion of air occupies, depends entirely on circumstances. If it is under pressure, its bulk will be diminished in exact proportion; and as the pressure is removed, it will expand in proportion, so as to occupy a thousand, or even a million times as much space as before.

Another property which air possesses is weight, or gravity, This property, it is obvious, must be slight, when compared with the weight of other bodies. But that air has a certain

degree of gravity, in common with other ponderous substances, is proved by direct experiment. Thus if the air be pumped out of a close vessel, and then the vessel be exactly weighed, it will be found to weigh more when the air is again admitted.

It is however the weight of the atmosphere which presses on every part of the earth's surface, and in which we live and move, as in an ocean, that here particularly claims our attention.

Fig. 99.

2

a

The pressure of the atmosphere may be easily shewn by the tube and piston, fig. 99.

Suppose there is an orifice, to be opened or closed by the valve b, as the piston a is moved up or down in its barrel. The valve being fastened by a hinge on one side, it is obvious that on pushing the piston down, it will open by the pressure of the air against it, and the air will make its escape. But when the piston is at the bottom of the barrel, on attempting to raise it again, towards the top, the valve is closed by the force of the external air acting upon it. If therefore the piston be drawn up in this state, it must be against the pressure of the atmosphere, the whole weight of which, of the size

In what proportion will a quantity of air increase in bulk as the pres sure is removed from it? How is this illustrated by fig. 98? On what circumstance, therefore, will the bulk of a given portion of air depend? How is it proved that air has weight? Explain in what manner the pressure of the atmosphere is shewn by fig. 99.

of the piston, must be lifted, while there will remain a vacuum or void space below it in the tube. If the piston be only three inches in diameter, it will require the full strength of a man to draw it to the top of the barrel, and when raised, if sudden. ly let go, it will be forced back again, by the weight of the air, and will strike the bottom with great violence.

The force thus pressing upon the piston is often called suction, and many persons believe that there is something within the barrel, which keeps the piston from rising, or which makes it move upwards with so much difficulty, never suspecting that the force is on the upper side of the piston, instead of being within the barrel.

Now, that it is the weight of the atmosphere which presses the piston down, is proved by the fact, that if its diameter be enlarged, a greater force, in exact proportion, will be requir ed to raise it. And further, if when the piston is drawn to the top of the tube, a stop-cock, as at e, fig. 98, be opened, and the air admitted under it, the piston will not then be forced down in the least, because then the air will press as much on the under, as on the upper side of the piston.

By accurate experiments, an account of which it is not necesssary here to detail, it is found that the weight of the atmosphere, on every inth square of the surface of the earth, is equal to fifteen pounds. If then a piston, working air tight, in a barrel, be drawn up from its bottom, the force employed, besides the friction, will be just equal to that required to lift the same piston, under ordinary circumstances, with a weight laid on it equal to fifteen pounds for every square inch of surface.

The number of square inches in the surface of a piston of a foot in diameter, is 113. This being multiplied by the weight of the air on each inch, which being 15 pounds, is equal to 1695 pounds. Thus the air constantly presses on every surface, which is equal to the dimensions of a circle one foot in diameter, with a weight of 1695 pounds.

Air Pump.

The air pump is an engine by which the air can be pump. ed out of a vessel, or withdrawn from it. The vessel so ex

What is the force pressing on the piston when drawn upward, sometimes called? How is it proved that it is the weight of the atmosphere, instead of suction, which makes the piston rise with difficulty? What is the pressure of the atmosphere on every square inch of surface on the earth? What is the number of square inches in a circle of one foot in diameter ? What is the weight of the atmosphere on a surface of a foot in diameter ?

hausted is called a receiver, and the space thus left in the vessel, after withdrawing the air, is called a vacuum.

The principles on which the air pump is constructed are readily understood, and are the same in all instruments of this kind, though the from of the instrument itself is often considerably modified.

The general principles of its construction will be comprehended by an explanation of fig. 100. In this figure let g, be

Fig. 100.

α

a glass vessel, or receiver, closed at the top, and open at the bottom, standing on a perfectly smooth surface, which is called the plate of the air pump. Through the plate is an aperture a, which communicates with the inside of the receiver, and the barrel of the pump. The piston rod P, works air tight through the stuffed collar C, and the piston also moves air tight through the barrel. At the extremity of the barrel there is a valve e, which opens outwards, and is closed with a spring.

Now suppose the piston to be drawn up to e, it will then leave a free communication between the receiver g, through the orifice a, to the pump barrel, in which the piston works. Then if the piston be forced down by its handle, it will compress the air in the barrel between d and e, and in consequence, the valve e will be opened, and the air so condensed will es. cape. On drawing the piston up again, the valve will be closed, and the external air not being permitted to enter, a vacuum will be formed in the barrel, from e to a little above d. When the piston comes again to c, the air contained in the glass vessel, together with that in the passage between the vessel and the pump barrel, will rush in to fill the vacuum. Thus there will be less air in the whole space, and consequently in the receiver, than at first, because all that contained in the barrel is forced out at every stroke of the piston.

What is the air pump? What is the receiver of an air pump? What is a vacuum? In fig. 100, which is the receiver of the air pump? When the piston is pressed down, what quantity of air is thrown out? When the piston is drawn up, what is formed in the barrel? How is this vacuum again filled with air?