the smoke can ascend no higher than where it meets with air of a density equal to itself, and there it will spread about like a cloud.

This figure (Plate 11, Fig. 20) is usually called the lungs-glass: a bladder is tied close about the little pipe a, which is screwed into the bottle A. I introduce it under the receiver AB, and begin to exhaust the air of the receiver, and that in the bladder communicating with it, will also be withdrawn; the elastic force of the air in the bottle A will now press the bladder to the shrivelled state represented in the figure: I will admit the air which expands the bladder; and thus, by alternately exhausting and re-admitting the air, I show the action of the lungs in breathing. But perhaps the following experi

ment will give a better idea of the subject (Plate 11, Figs. 21 and 22). A represents the lungs, в the windpipe leading to them, which is closely fixed in the neck of the bottle, from which the air cannot escape: D is a bladder.tied to the bottom, and in its distended state (Fig. 21) will, with the internal cavity of the bottle, represent the cavity of the body which surrounds the lungs, at the moment you have taken in breath: I force up D (as in Fig. 22), and now the bladder is shrivelled by the pressure of the external air in the bottle, and represents the lungs just at the moment of expiration.

Emma. Does Fig. 21 show the state of the lungs after I have drawn in my breath, and Fig. 22 when I have thrown it out forcibly?

Father. That is what the figures are intended to represent, and they are well adapted to show the elevation and compression of the lungs, although I do not mean to assert, that the action of the lungs in breathing depends upon air in the same manner as that in the bladder does upon the air which is contained in the cavity of the bottle.

I have exactly balanced on this scale-beam a piece of lead and a piece of cork in this state I will introduce them under the receiver, and exhaust the air.

Charles. The cork now seems to be heavier than the lead.

Father. In air each body lost a weight proportional to its bulk, but when the air is taken away, the weight lost will be restored; but as VOL. IV.

I

the lead lost least, it will now regain the least, consequently the cork will preponderate with the difference of the weights restored by taking away the air.

Thus you see that, in vacuo, a pound of cork, or feathers, would be heavier than a pound of lead.

Emma. Why do bodies, when weighed in air, lose weights proportional to their bulks?

Father.

Because the air, being a

fluid substance, tends to lift up a body immersed in it, and the larger the body, the more effect it will have it of course, it has more effect

upon

on an ounce of cork, than on an ounce of lead.

CONVERSATION .

Of the Air-gun, and Sound.

FATHER. The air-gun is an instrument, the effects of which depend on the elasticity and compression of air.

Emma. Is it used for the same purposes as common guns ?

Father. Air-guns will answer all the purposes of a musket or fowlingpiece: bullets discharged from them will kill animals at the distance of 50 or 60 yards. They make no report, and on account of the great