portable oxygen tanks. Then they entered a chamber into
which water was admitted until it was equal in pressure to
that outside. Then a steel door was opened, and they could
step out. To re-enter the ship the operation was reversed.
This was not a new feature. In fact, many submarines to-day
use it,

At certain places there were thick bull's-eye windows, by
means of which the under-water travelers could look out into
the ocean through which they were moving. As a defense
against the attacks of submarine monsters there was a steel,
pointed ram, like a big harpoon. There were also a bow and a
stern electrical gun, of which more will be told later.

In addition to ample sleeping accommodations, there were
many conveniences aboard the Advance. Plenty of fresh water
could be carried, and there was an apparatus for distilling
more from the sea water that surrounded the travelers.
Compressed air was carried in large tanks, and oxygen could
be made as needed. In short, nothing that could add to the
comfort or safety of the travelers had been omitted. There
was a powerful crane and windlass, which had been installed
when Mr. Swift thought his boat might be bought by the
Government. This was to be used for raising wrecks or
recovering objects from the bottom of the ocean. Ample
stores and provisions were to be carried and, once the
travelers were shut up in the Advance, they could exist for
a month below the surface, providing no accident occurred.

All these things Tom and Mr. Sharp thought of as they