in the tube; but admit the air into the receiver, and its action upon the surface of the quicksilver in the cup will force it instantly into the tube.

This is called the Torricellian experiment, in honour of Torricelli, a learned Italian, and disciple of Galileo, who invented it, and who was the first person that discovered the pressure and weight of the air. : Charles. Was not the true nature of the atmosphere understood before the time of Torricelli ?

Father. No: he was the father of all the modern discoveries respecting the properties of the atmospheric air. He died at the age of 40, when great hopes were formed of his taJents and genius.

CONVERSATION IV.

Of the Pressure of the Air.

CHARLES. It seems very surprising that the air, which is invisible, should produce such effects as you have described.

Father. If you are not satisfied with the evidence which your eyes are capable of affording, you would perhaps have no objection to the information which your feelings may convey to your mind. Place this little glass AB, open at both ends (Plate 1, Fig. 6), over the hole of the pump plate, and

lay your hand close upon the top в, while I turn the handle of the pump a few times.

Charles. It hurts me very much: I cannot take my hand away.

Father. By letting in the air, I have released you. The pain was occasioned by the pressure of the air on the outside of your hand, that being taken away from under it, which served to counterbalance its weight.

This is a larger glass of the same kind (Plate 1, Fig. 7); over the large end, I tie a piece of wet bladder very tight, and will place it on the pump, and take the air from under it.

Emma. Is it the weight of air that bends the bladder so much? Father. Certainly and if I turn the handle a few more times it will burst.

:

Charles. It has made a report as

loud as a gun.

Father. A piece of thin flat glass may be broken in the same manner. Here is a glass bubble ▲, with a long neck (Plate 1, Fig. 8), which I put into a cup of water, and place them under a receiver on the plate of the air-pump, and, by turning the handle, the air is not only taken from the receiver, but that in the hollow glass ball will make its way through the water and escape.

Emma. Is it the air which occasions the bubbles at the surface of the water?

Father. It is. Now the bubbling is stopped, and therefore I know that as much of the air is taken away as can bet out by means of the pump. The hollow.

ball is still empty: but by turning the cock v of the pump (Fig. 1) the air rushes into the receiver and presses upon the water, thereby filling the ball with the fluid.

Charles. It is not quite full.

Father. That is because the air could not be perfectly exhausted, and the little bubble of air at the top is what, in its expanded state, filled the whole glass ball, and now, by the pressure of the external air, it is reduced into the size you see it.

Another very simple experiment will convince you that suction has nothing to do with these experiments. On the leather of the airpump, 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 1,