Charles. Is it not by the exertion of their fins?

Father. A fish swims by the help of his fins and tail; and fish in general are nearly of the same specific gravity with water. Take away

the water from the vessel, and the fish would still have the use of their fins and tail, at least for a short period.

Emma. And they would flounder about at the bottom.

Father. Now consider the case of birds, how they fly; the swallow, for instance, glides as smoothly along in the air, as fish do in the water but if I were to put a bird, or even a butterfly, under a glass receiver, however large, and take away the air, they would have no more use of their wings, than fish have of their fins when out of water. You shall see the experiment in a day or two:

-If this support

Were wanting, all the feather'd tribes must drop
The useless wing.

EUDOSIA.

Emma. And would they die in this situation, as fish die when taken from their natural element, the water?

Father. The cases are precisely similar: some fish, as the carp, the eel, and almost all kinds of shell-fish, will live a considerable time out of water; so some creatures, which depend upon air for existence, will live a long time in an ex

hausted receiver; a butterfly, for instance, will fall to the bottom apparently lifeless, but admit the air again into the receiver, and it will revive; whereas experiments have been made on mice, rats, birds, rabbits, &c. and it is found that they will live without air but a very few minutes.

Emma. These are very cruel experiments.

Father. And ought by no means to be indulged in; they can only be justified upon the presumption, that in the hands, and under the direction of able philosophers, they may lead to discoveries of importance to the health and happiness of the human race.

Charles. Can fish live in water from which the air is wholly excluded?

Father. The air is, in fact, as necessary to their existence as it is to ours. Besides their

fins, fish have the use of an air-vessel, which gives them full command of their various motions in all depths of water, which their fins without it would not be equal to.

Emma. What do you mean by an air-vessel? Father. It is a small bladder of air, so disposed within them, that, by the assistance of their muscles, they are able to contract and dilate it at pleasure. By contraction they become specifically heavier than the water, and sink; by dilatation they are lighter, and rise to the surface more readily.

Charles. Are these operations effected by the external air?

Father. Very much so; for if you take away the air from the water in which a fish is swimming, it will no longer have the power of contracting the air-vessel within, which will then become so expanded as to keep it necessarily on the surface of the water, evidently to its great inconvenience and pain.

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?

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. A A are two 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 к. 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 .cc: 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.

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

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

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?