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 with the necessary machinery,
to tell the truth. I never explored the bottom of a river in this way,
and I think it will be a long time before I make such a voyage.

The diving bell has been used for a good many useful purposes--to lay
the foundations of docks and the piers of bridges; to collect pearls
at Ceylon, and coral at other places.