the air-pump, at a little distance from the hole, I place lightly this small receiver x, and pour a spoonful or two of water round the edge of it (Plate v. Fig. 9.) I now cover it with a larger receiver A B, and exhaust the air.

Emma. I see by the bubbles round the edge of the small receiver that the air is making its way from under it.

Father. I have pretty well exhausted all the air; can you move the large receiver? Charles. No: but by shaking the pump, I see the little one is loose.

Father. The large one is rendered immoveable by the pressure of the external air. But the air being taken from the inside of both glasses, there is nothing to fasten down the smaller receiver.

Emma. But if suction had any thing to do with this business, the little receiver would be fast, as well as the other.

Father. Turn the cock v of the air-pump quickly. You hear the air rushing in with violence.

Charles.

ened again.

And the large receiver is loos

Father. Take away the smaller one, Emma.

Emma. I cannot move it with all my strength.

Father. Nor could you lift it up if you were a hundred times stronger than you are. For by admitting the air very speedily into the large receiver, it pressed down the little one before any air could get underneath it. Charles. Besides, I imagine you put the water round the edge of the glass to prevent the air from rushing between it and the leather.

Father. You are right; for air being the lighter fluid could not ascend through the layer of water in order to descend into the receiver. Could suction produce the effect in this experiment?

Charles.

I think not; because the little receiver was not fixed till after what might be thought suction had ceased to act. Father. Right: and to impress this fact

strongly on your mind, I will repeat the experiment. You observe that the air being taken from under both receivers, the large one must be fixed by the pressure of the atmosphere, and the smaller one must be Loose, because there is no pressure on its Outside to fasten it. But by admitting the air, the inner one becomes fixed by the very means that the outer one is loosened.

Emma. How will you get the small one away?

Father. As I cannot raise it, I must slide it over the hole in the brass plate; and now the air gets under it, there is not the mallest difficulty.

CONVERSATION XXVII,

Of the Pressure of the Air.

CHARLES. Although suction has nothing to do in the experiments which you made yesterday, yet I think I can show you an instance in which it has. This experiment, if such it may be called, I have made a hundred times. I fasten a string in the centre of a round piece of leather, and having thoroughly soaked it in water, I press it on a flat stone, and by pulling at the string the leather draws up the stone, although it be not more than two or three inches in diameter, and the stone weighs several pounds. Surely this is suction.

Father. I should say so too, if I could not account for it by the pressure of the atmosphere. By pressing the wet leather on the stone you displace the air, then by pulling the string a vacuum is left at the centre, and the pressure of the air about the edges of the leather is so great, that it requires a greater power than the gravity of the stone to separate them.

I have seen you drink water from a spring by means of a hollow straw.

Emma. Yes, that is another instance of what we have been accustomed to call suction.

Father. But now you know, that in this operation you made a syringe with the straw and your lips, and by drawing in your breath you cause a vacuum in the hollow straw tube, and the pressure of the air on the water in the spring forces it up through the straw into the mouth,

Charles. I cannot, however, help thinking that this looks like suction, for the moVOL. II.

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