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

as different from most others. In common echoes the repetition is not heard till some time after hearing the words spoken or notes sung; in this the person who speaks or sings is scarcely heard, but the repetition is perceived very clearly, and in surprising varieties: the echo in some cases appears to be approaching, in others receding; sometimes it is heard distinctly, at others scarcely at all: one person hears only one voice, while another hears several. And, to mention but one more instance, in Italy, near Milan, the sound of a pistol is returned 56 times.

Emma. This is indeed

To fetch shrill echoes from their hollow earth.

Father. The ingenious Mr. Derham applied the echo to measuring inaccessible distances.

Charles. How did he do this?

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 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 silver, 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 nearly 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 into 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 Why do you make use of horse-hair 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 as water; and its substance is of such a nature as not to imbibe moisture.

CONVERSATION XIII.

Of the Methods of finding the Specific Gravities of Bodies.

CHARLES. I have endeavoured to find out the specific gravity of this piece of beech-wood; but, as it will not sink in the water, I know not how to proceed with it.

Father. It is true, that we have hitherto only given rules for finding the specific gravity of bodies that are heavier than water; a little consideration, however, will show you how to obtain the specific gravity of