is
steady, is about four hours--more or less--with perhaps a breath of ten
minutes once or twice at the surface when they're working deep."

"But why a breath at the surface?" asked Edgar. "Isn't the air sent
down fresh enough?"

"Quite fresh enough, Mister Edgar, but the pressure when we go deep--say
ten or fifteen fathoms--is severe on a man if long continued, so that he
needs a little relief now and then. Some need more and some less
relief, accordin' to their strength. Maxwell has only gone down fifteen
feet, so that he wouldn't need to come up at all durin' a spell of work.
We're goin' to blast a big rock that has bin' troublesome to us at low
water. The hole was driven in it last week. We moored a raft over it
and kep' men at work with a long iron jumper that reached from the rock
to the surface of the sea. It was finished last night, and now he's
gone to fix the charge."

"But I don't understand about the pressure, sur, at all at all," said
Machowl, with a complicated look of puzzlement; "sure whin I putt my
hand in wather I don't feel no pressure whatsomediver."

"Of course not," responded Baldwin, "because you don't put it deep
enough. You must know that our atmosphere presses on our bodies with a
weight of about 20,000 pounds. Well, if you go thirty-two feet deep in
the sea you get the pressure of exactly another atmosphere, which means
that you've got to stand a pressure all over your body of 40,000 when
you've got down as deep as thirty-two feet."

"But," objected Rooney, "I don't fed no pressure of the atmosphere on me
body at all."

"That's because you're squeezed by the air inside of you, man, as well
as by the atmosphere outside, which takes off the _feelin'_ of it, an',
moreover, you're used to it. If the weight of our atmosphere was took
off your outside and not took off your inside--your lungs an' the
like,--you'd come to feel it pretty strong, for you'd swell like a
balloon an' bu'st a'most, if not altogether."

Baldwin paused a moment and regarded the puzzled countenance of his
pupil with an air of pity.

"Contrairywise," he continued, "if the air was all took out of your
inside an' allowed to remain on your outside, you'd go squash together
like a collapsed indyrubber ball. Well then, if that be so with one
atmosphere, what must it be with a pressure equal to two, which you have
when you go down to thirty-two feet deep in the sea? An' if you go down
to twenty-five fathoms, or