"WHAT'S that noise?" asked Mr. Hardley.

Mr. Hardley, Tom Swift, Mr. Damon, Ned Newton, Koku, and one or
two navigating officers of the craft, were gathered in the
operating cabin of the M. N. 1.

"That's water being pumped into the tanks," explained Tom. "We
are now going down. If you'll watch the depth gauge you can note
our progress."

"Going down, are we?" remarked Mr. Hardley. "Well, it's
interesting to say the least," and he observed the gauge, which
showed them to be twenty feet under the surface.

"Bless my hydrometer, but he's got nerve for a first trip in a
submarine! He's all right, isn't he?" whispered Mr. Damon to Tom.

"Well, I'm glad to see he isn't nervous," remarked Tom, honest
enough to give his visitor credit for what was due him. And
indeed many a person is nervous going down in a submarine for the
first time. "Still we can't go more than thirty feet down in this
water," went on Tom. "A better test will be when we get about
five hundred feet below the surface. That's a real test, though
as far as knowing it is concerned, a person can't tell ten feet
from ten hundred in a submarine under water, unless he watches
the gauge."