nd
then, probably, you can guess the reason why I have given this name to
the following pages.

If you will take a common glass tumbler, and plunge it into water,
with the mouth downwards, you will find that very little water will
rise into the tumbler. You can satisfy yourself better about this
matter, if, in the first place, you lay a cork upon the surface of the
water, and then put the tumbler over it.

Did you ever try the experiment? Try it now, if you never have done
so, and if you have any doubt on the subject.

You might suppose, that the cork would be carried down far below the
surface of the water. But it is not so. The upper side of the cork,
after you have pressed the tumbler down so low that the upper end of
it is even below the surface of the water--the upper side of the cork
is not wet at all.

"And what is the reason of this, Uncle Frank?"

I will tell you. There is air in the tumbler, when you plunge it into
the water. The air stays in the vessel, so that there is no room for
the water.

"Oh, yes, sir; I see how that is. But I see that a little water finds
its way into the tumbler, every time I try the experiment. How is
that?"

You can press air, the same as you can press wood, or paper, or cloth,
so that it will go into a smaller space than it occupied before you
pressed it. Did you ever make a pop-gun?

"Oh, yes, sir, a hundred times."

Well, when you send the wad out of the pop-gun, you do it by pressing
the air inside the tube. Now if your tumbler was a hundred or a
thousand times as large, the air would prevent the water from coming
in, just as it does in this instance. Suppose I had dropped a purse
full of gold into a very deep river, and it had sunk to the bottom.
Suppose I could not get it in any other way but by going down to the
bottom after it. I could go down to that depth, and live there for
some time, by means of a diving bell made large enough to hold me,
precisely in the same way that a bird might go down to the bottom of a
tub of water, in a tumbler, and stand there with the water hardly over
his feet. There is a good deal of machinery about a diving bell, it
is true. But I need not take up much time in describing it. It is
necessary for the man to breathe, of course, while he is in the diving
bell; and as the air it contains is soon rendered impure by breathing,
fresh air must be introduced into the bell by means of a pump, or in
some other way. I am not very familiar wit