Consequently the air

grows gradually thinner, till at a considerable height it may be conceived to degenerate to nothing. The different densities of the air may be illustrated by conceiving twenty or thirty equal packs of wool placed one upon another: the lowest will be forced into a less space, that is, its parts will be brought nearer together, and it will be more dense than the next, and that will be more dense than the third from the bottom, and so on till you come to the uppermost, which sustains no other pressure than that occasioned by the weight of the incumbent air.

Let us now see the effects of condensed air, by means of an artificial fountain. This vessel is made of strong copper (Plate 11, Fig. 19), and

[blocks in formation]

is about half full of water.

With a

syringe that screws to the pipe B A I force a considerable quantity of air into the vessel, so that it is very much condensed. By turning the stop-cock B while I take off the syringe, no water can escape: and, instead of the syringe, I put on a jet, or very small tube, after which the stop-cock is turned, and the pressure of the condensed air forces the water through the tube to a very great height.

Charles. Do you know how high it ascends?

Father. Not exactly: but as the natural pressure of the air will raise water 33 feet, so if by condensation its pressure be tripled, it will rise 66


Emma. Why tripled? Ought it

not to rise to this height by a double pressure?

Father. You forget that there is the common pressure always acting against, and preventing the ascent of the water; therefore, besides a force within to balance that without, there must be a double pressure.

Charles. You described a syringe to be like a common water squirt -how are you able, by an instrument of this kind, to force in so great a quantity of air? Will it not return by the same way it is forced in?

Father. The only difference between a condensing syringe and a squirt is, that, in the former, there is a valve that opens downwards, by which air may be forced through it; but the instant that the downward pressure ceases, the valve, by means

of a strong spring, shuts of itself, so that none can return.

Emma. Will not air escape back during the time you are forcing in more of the external air?

Father. That would be the case if the syringe pipe went no lower than that part of the vessel which contains the air; but it reaches to a considerable depth in the water; and, as it cannot find its way back up the pipe, it must ascend through the water, and cause that pressure upon it which has been described.

Charles. To what extent can air be compressed?

Father. If the apparatus be strong enough, and a sufficient power applied, it may be condensed several thousand times; that is, a vessel, which will contain a gallon of air in

its natural state, may be made to contain several thousand gallons.

By means of a fountain of this kind, young people, like yourselves, may receive much entertainment with only a few additional jets, which are made to screw on and off. One kind is so formed that it will throw up and sustain on the stream a little cork ball, scattering the water all round. Another is made in the form of a globe, pierced with a great number of holes, all tending to the centre, exhibiting a very pleasing sphere of water. One is contrived to show, in a neat manner, the composition and resolution of forces explained in our first volume*. Some will form cascades; and by others you may, when the sun shines at a * See Vol. I, of Mechanics, Conver. XIII,

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