second figure,) and now the bladder is shrivelled by the pressure of the exter nal air in the bottle, and represents the lungs just at the moment of expi ration.

Emma. Does the first figure show the state of the lungs after I have drawn in my breath, and the second 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 intro

Father. It must be first divided into 36 equal parts; then, beginning from the surface, take 11 of those parts for the first hour, nine for the second, seven for the third, five for the fourth, three for the fifth, and one for the sixth, and you will find that the surface of the water will descend regularly through each of those divisions in an hour.

I believe both of you have seen the locks that are constructed on the river Lea?

Charles. Yes; and I have wondered why the flood-gates were made of such an enormous thickness.

Father. But after what you have heard respecting the pressure of fluids, you will see the necessity that there is for the great strength employed.

Charles. I do; for sometimes the height of the water is 20 or 30 times greater on one side of the gates than

the body, the more effect it will have upon it of course, it has more effect on an ounce of cork, than on an ounce of lead.

that sometimes beats down the banks

of rivers?

Father. It is: for if the banks of a river or canal do not increase in strength in the proportion of the square of the depth, they cannot stand. Sometimes the water in a river will insinuate itself through the bank near the bottom, and, if the weight of the bank be not equal to that of the water, it will assuredly be torn up, perhaps with great violence.

I will make the matter clear by a drawing. Suppose this figure be a

section of a river, and c a crevice or drain made by time under the bank

g; by what we have shown before, the upward pressure of the water in that drain is equal to the downward pressure of the water in the river; therefore, if that part of the bank be not as heavy as a column of water the same height and width, it must be torn up by the force of the pressure.

Charles. Is there no method of securing leaks that happen in the embankments of rivers?

Father. The only method is that called puddling. If n be the bank of a canal in which a leak is discovered, the water must be first drawn off below the leak, and a trench 18 or 20 inches wide dug length-wise along the side of the canal, and deeper than the bottom of the canal: this is filled, by a little at a time, with clay or loam reduced into a semi-fluid state by mixing it with water: when the