which it is placed; and the third swims on the water with more than half of the sub stance above its surface.

Charles. The first, then, is heavier than the water, the second is of the same weight with an equal bulk of the fluid, and the third is lighter.

Father. Since fluids press in all directions, a solid that is immersed in water sustains à pressure on all sides, which is increased in proportion to the height of the fluid above the solid.

Emma. That seems natural, but an experiment would fix it better in the mind.

Father. Tie a leathern bag (Plate 1. Fig. 8.) to the end of a glass tube, and pour in some quicksilver. Dip the bag in water, and the upward pressure of the fluid will raise the quicksilver in the tube, the ascent of which will be higher or lower, in proportion to the height of the water, above the bag.

Emma. I now undertand that the upper part of the tube being empty, or, at least, only filled with air, the upward pressure of

the water against the bag must be greater than the downward pressure of the air: and that as the pressure increases according to the depth, therefore the murcury must keep rising in the tube.

What is the reason that a body heavier than water, as a stone, sinks to the bottom, if the pressure upward is always equal to that downwards?

Father. This is a very proper question. The stone endeavours to descend by the force of gravity: but it cannot descend without moving away as much of the water as is equal to the bulk of the stone; therefore it is resisted, or pressed upwards, by a force equal to the weight of as much water as is equal in magnitude to the bulk of the stone; but the weight of the water is less than that of the stone, consequently the force pressing against it upwards in less than its tendency downwards, and therefore it will sink with the difference of those two forces.

You will now be at no loss to understand the reason why bodies lighter than water swim:

As passing straws and buoyant leaves
The yielding surface but receives;
While pearls, that lure the searching eye,
Deep treasur❜d in its bosom lie.
May trifles such reception find,
Float merely transient on my mind,
While weightier thoughts admission win,
Sink its whole depths, and rest within.

BROWNE.

Charles. The water being heavier, the force upwards is greater than the natural gravity of the body, and it will be buoyed up by the difference of the forces.

Father. Bodies of this kind, then, will sink in water, till so much of them is below the surface, that a bulk of water equal to the bulk of the part of the body which is below the surface, is of a weight equal to the weight of the whole body.

Emma, Will you explain this more particularly?

Father. Suppose the body to be a piece of wood, part of which will be above and part below the surface of the water: in this state conceive the wood to be frozen into the water.

Charles. I understand you: if the wood be taken out of the ice, a vacuity will be left, and the quantity of water that is required to fill that vacuity will weigh as much as the whole substance of the wood. Father. That was what I meant to have

said.

There is one case remaining:-where equal bulks of the water and the wood are of the same weight,the force with which the wood endeavours to decend, and the force that opposes it, being equal to one another, and acting in contrary directions, the body will rest between them, so as neither to sink by its own weight, nor to ascend by the upward pressure of the water.

Emma. What is the meaning of this glass jar with the images in it? (Plate 111. Fig. 19.)

Father. I placed it on the table in order to illustrate our subject to-day. You observe that, by pressing the bladder with my hand, the three images all sink.

Emma. But not at the same moment.

Father. The images are made of glass

and about the same specific gravity with the water surrounding them, or perhaps rather less than it, and consequently they all float near the surface. They are hollow, with little holes in the feet. When the air, which lies between the bladder and the surface of the water, is pressed by my hand, there is a pressure on the water which is communicated through it, and that part of it which lies contiguous to the feet of the images will be forced into their bodies, by which their weight is so much increased as to render them heavier than the water, and they de scend.

Charles. Why do they not all descend to the same depths?

Father. Because the hollow part of the image E is larger than the hollow part of D, and that is larger than that of c; consequently the same pressure will force more water into E than into D, and more into D than into c.

Emma. Why do they begin to ascend now you have taken your hand away? Father. I said the hollow parts of the