navigation. It
is a complicated and difficult science, but by calculating the distance
of the sun above the horizon, sometimes by views of stars, by knowing
the speed of the ship, and by having the exact astronomical time at
hand, shown on an accurate chronometer, the exact position of a ship at
any hour may be determined.

By this means, if a navigator wants to get to a place where two certain
lines cross, indicating an exact spot in the ocean, he is able to do
so. He can tell for instance when he has reached the place where the
seventy-second degree of longitude, west from Greenwich, meets and
crossed the twentieth parallel of latitude. This spot is just off the
northern coast of Haiti. Other positions are likewise determined.

It was after about an hour of rather slow progress on the surface of
the calm sea, no excess speed being used for fear of over-running the
mark, that Tom and his associates gathered on deck again to make
another calculation.

Long and carefully they worked out their position, and when, at last,
the figures had been checked and checked again, to obviate the chance
of another error, the young inventor exclaimed:

"Well, we're here!"

"Really?" cried Ned.

"No doubt of it," said his chum.

"Bless my doormat!" cried Mr. Damon. "And do you mean to say, Tom
Swift, that if we submerge now we'll be exactly where the Pandora lies,
a wreck on the floor of the ocean.

"I mean to say that we're at exactly the spot Where Hardley said she
went down," corrected Tom, "and we weren't there before--that is not so
that we actually knew it. Now we are, and we're going down. But that
doesn't guarantee that we'll find the wreck. She may have shifted, or
be covered with sand. All that I said before in reference to the
difficulty in locating something under the surface of the sea still
holds good."

Once more, to make very certain there was no error, the figures were
gone over, Then, as one result checked the other, Tom put away the
papers, the nautical almanac, and said:

"Let's go!"

Slowly the tanks of the M. N. 1 began to fill. It was decided to let
her sink straight down, instead of descending by means of the vertical
rudders. In that way it was hoped to land her as nearly as possible on
the exact spot where the Pandora was supposed to be.

"How deep will it be, Tom?" asked Ned, as he stood beside his chum in
the forward observation cabin and watched the needle of the gauge move
higher and hig