before, it drives over a quantity of the fluid equal in weight to itself. Put in two pennypieces, and you perceive the box sinks deeper into the water.

Charles. And they drive more water over: as much, I suppose, as is equal in weight to the copper coin.

- Father. Right: how long could you go on loading the box?

Charles. Till the weight of the copper and box, taken together, is something greater than the weight of as much water as is equal in bulk to the box.

Father. You understand, then, the reason why boats, barges, and other vessels, swim on water; and to what extent you may load them with safety.

Emma. They will swim so long as the weight of the vessel and its lading together,. is less than that of a quantity of water, equal in bulk to the vessel.

Father. Can you, Charles, devise any method to make iron or lead swim, which are so much heavier than water?

Charles. I think I can. If the metal be beat out very thin, and the edges turned up, I can easily conceive that a box or a boat of it may be made to swim. Of this kind is the copper ball which is contrived to turn off the water when the cistern is full.

Emma. I have often wondered how that

acts.

Father. If upon reflection you could not satisfy yourself about the mode of its acting, you should have asked; it is better to get information from another than to remain ignorant.

The ball, though made of copper, which is eight or nine times heavier than water, is beat out so thin, that its bulk is much lighter than an equal bulk of water. By means of a handle it is fastened to the cock, through which the water flows, and as it sinks or rises, it opens or shuts the cock.

If the cistern is empty, the ball hangs down, and the cock is open, to admit the water freely as the water rises in the cistern it reaches the ball, which, being lighter than the water, rises with it, and, by rising,

gradually shuts the cock, and, if it be properly placed, it is contrived to shut the cock just at the moment that the cistern is full.

In the same way that these balls are made, boats of iron are now constructed at the iron-works in Shropshire: they will last longer than wood, and cause less friction in passing through the water.

Can you, Emma, find the specific gravity of this piece of silver?

Emma. It weighs in air 318 grains: I now fasten it to the hook with the horsehair, and it weighs in water 288 grains, which, taken from 318, leave 30, the weight it lost in water. By dividing 318 by 30, the quotient is about 10, consequently the specific gravity of the silver is ten and a half times greater than that of water.

Father. What is the specific gravity of this piece of flint-glass? It weighs 12 pennyweights in air.

Charles. And in water it weighs only 8, and consequently loses 4 by immersion; and 12 divided by 4 gives 3, therefore the

specific gravity of flint-glass is 3 times greater than that of water.

Father. This is not the case with all flint-glass; it varies from 2 to almost 4.

Here is an ounce of quicksilver; let me know its specific gravity by the method now proposed.

Emma. How will you manage that? you cannot hang it upon the balance.

Father. But you may suspend this glass bucket (Plate 111. Fig. 21.) on the hook at the bottom of A; immerse it in the water, and then balance it exactly with weights in the opposite scale.

I will now put into the bucket the ounce, or 480 grains of quicksilver, and see how much it loses in water.

Charles. It weighs 445 grains, and consequently it lost 35 grains by immersion; and 480 divided by 35 give almost 14, so that mercury is almost 14 times heavier than water.

Father. In the same manner we obtain the specific gravity of all bodies that consist of small fragments. They must be put in

to the glass bucket and weighed; and then if from the weight of the bucket and body in the fluid, you subtract the weight of the bucket, there remains the weight of the body in the fluid.

Emma. Why do you make use of horsehair to suspend the substances with? would not silk or thread do as well?

Father. Horse-hair is by much the best, for it is very nearly of the same specific gravity of water; and its substance is of such a nature as not to imbibe moisture.