as were Tom's, he could make out nothing but a black
smudge, now larger and darker.

"It might be a cloud for all I can tell," he said, as he handed the
binoculars back to Tom.

"Well, it's a steamer all right, and she's under forced draft, too, if
I'm any judge. We'll go below before she sights us."

"Perhaps she has already," suggested Ned, as the crew began clearing
the submarine's deck.

"No, we lie too low in the water for that. Well, now we can start our
underwater observations of current trends."

It did not take long, once she started, for the M. N. 1 to go down.
Just as the sun sank below the horizon, and while the smudge of smoke
was becoming more distinct, the waves closed over the steel deck of the
submarine. Half an hour later she was nearly a quarter of a mile below
the surface, resting on the bottom of the sea again.

On this trip Tom did not go to any such depths as he did on his former
voyage in the Advance. Not that the reconstructed submarine was not
capable of it, for she was even stronger than when first built. But the
wreck they were seeking did not lie in so great a depth of water, and
there was no need of running useless risks.

"Well," remarked Ned, when they came to a stop, "I don't believe any
one will find us here."

"Not an ordinary diver, at any rate," Tom agreed. "And after supper I'm
going to have another go at the currents."

The meal was served as usual, and a very good one it was, considering
the fact that not as many supplies could be carried in the rather
limited space of a submarine as may be transported in an ocean liner.
Then, as it was still early, Tom and Ned, with the help of some of the
officers, got ready for a new series of experiments.

The big searchlight was set aglow, and, going out on the ocean bed in
diving suits, Tom and his friends dropped on the sand various weighted
objects.

These were made in the shape of the hull of a steamer, and in
proportion. Once they were on the sand, an iron rod was thrust into the
ocean bed near each object.

"Now," remarked Tom, as they all went into the submarine again, "we'll
let them drift until morning. Then we'll make new calculations. I think
we'll arrive at some results, too."

"Just what are you aiming to do?" asked Mr. Damon.

"See how far each one of those weighted objects drifts," Tom replied.
"We have planted them in different spots on the ocean bed. Some will
drift farther than others. Some are large and so