Imagens das páginas
PDF
ePub

CONVERSATION XXIV.

Of the Air-Pump.

EMMA. You have told us, Papa, of taking away the air from vessels; will you show us how that is performed?

Father. I will; and I believe it will be the most convincing method of proving to you that the air is such a body as I have described.

This instrument (Plate v. Fig. 1.) is called an air-pump, and its use is to exhaust or draw away the air from any vessel, as the glass receiver L K.

Charles. Does it act like the common pump?

A A are two

Father. So much so, that if you comprehend the nature and structure of the one, you will find but little difficulty in understanding the other. I will, however, describe the different parts. strong brass barrels, within each of which, at the bottom, is fixed a valve, opening upwards; these valves communicate with a concealed pipe that leads to K. The barrels include also moveable pistons, with valves opening upwards.*

Emma.

How are they moved?

Father. To the upper parts of the pistons is attached rack-work, part of which you see at c c: these racks are moved up and down by means of a little cog-wheel, turned round by the handle R.

Charles. You turn the handle but half way round.

The reader is supposed to have attended to the structure of the common pump, described in Hy. drostatics. Conversation XXI.

Father. And by so doing, you perceive that one of the racks rises, and the other descends..

Emma. What is the use of the screw v?

Father. It serves to re-admit air into the receiver when it is in a state of exhaustion; for without such a contrivance, the receiver could never be moved out of its place, after the air was once taken from beneath it. But you shall try for yourselves. I first place a slip of wet leather under the edge of the receiver, because the brass plate is liable to be scratched, and the smallest unevenness between the receiver and plate would prevent the success of our experiment.-I have turned the handle but a few times try to take away the receiver.

Charles. I cannot move it.

Father. I dare say not: for now the greater part of the air is taken from under the receiver, consequently it is pressed down with the weight of the atmosphere on the outside.

Emma. Pray explain how the air was

taken away.

Father. By turning the winch R half way round, I raise one of the pistons, and thereby leave a vacuum in the lower part of the barrel, and a portion of the air in the receiver rushes through the pipe into the empty barrel. I then turned the winch the other way, which raised the other piston, and a vacuum would be left in that barrel, did not another portion of air rush from the receiver into it.

Charles. When the first piston descended, did the air in the barrel open the little valve, and escape by the rack c?

Father. It did

and by the alternate working of the pistons, so much of the air is taken away, that the quantity left has not force enough to raise the valve.

Charles. Cannot you take all the air from the receiver?

Father. Not by means of the air-pump. Emma. What is the reason that a mist comes on the inside of the glass receiver while the air is exhausting?

Father. It is explained by the sudden expansion of the air that is left in the receiver, which we shall notice more particularly in our conversations on Chemistry. The fact is described, as well as the general operations of the air-pump, by Dr. Darwin.

-How, as in brazen pumps the pistons move, The membrane valve sustains the weight above, Stroke follows stroke, the gelid vapour falls, And misty dew-drops dim the crystal walls; Rare and more rare expands the fluid thin, And silence dwells with vacancy within.

BOTANIC GARDEN.

The last line alludes to a fact hereafter to be explained,* namely, that where there is no air, there can be no sound.

Charles. You have not told us the use

* See Conversation XXXII.

« AnteriorContinuar »