ferent bodies; and also as a standard to which they may be all referred.

Here are three pieces of different kinds of wood, which I will put into this vessel of water: one sinks to the bottom; a second remains in any position of the water in which it is placed; and the third swims on the water with more than half of the substance 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 a 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 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 understand 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 mercury 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 upwards 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 is less than its tendency downwards, and therefore it will sink with the difference of these two forces.

You will now be at no loss to un

derstand 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 their 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 descend, and the force that opposes