y others in the party. He kept to himself,
and a day later he was landed, at night, at a dock, and if he said
"good-bye" or wished Tom and his friends a safe voyage, they did not
hear him.

They were steaming along on the surface the next day, and at noon the
submarine suddenly halted.

"What's on now, Tom?" asked Ned, as he saw his chum prepare to go up on
deck with some of the craft's officers.

"We're going to 'shoot the sun' again," was the answer. "I want to make
sure that we were right in our former calculations as to the position
of the Pandora. The least error would throw us off."

Using the sextant and other apparatus, some of which Tom had invented
himself, the exact position of the submarine was calculated. As the
last figure was set down and compared with their previous location, one
of the men who had been doing the computing gave an exclamation.

"What's the matter?" asked Tom.

"Look!" was the answer, and he pointed to the paper. "There's where a
mistake was made before. We were at least two miles off our course."

"You don't say so!" exclaimed Tom, and, taking the sheet, he went
rapidly over the results.

CHAPTER XIX

THE SERPENT WEED

All waited eagerly for Tom Swift to verify the statement of the other
mathematician, and the young inventor was not long in doing this, for
he had what is commonly known as a "good head for figures."

"Yes, I see the mistake," said Tom. "The wrong logarithm was taken, and
of course that threw out all the calculations. I should say we were
nearer three miles off our supposed location than two miles."

"Does that mean," asked Mr. Damon, "that we began a search for the
wreck of the Pandora three miles from the place Hardley told us she
was."

"That's about it," Tom said. "No wonder we couldn't find her."

"What are you going to do?" Ned wanted to know.

"Get to the right spot as soon as possible and begin the search there,"
Tom answered. "You see, before we submerged as nearly as possible at
the place where we thought the Pandora might be on the ocean bottom.
From there we began making circles under the sea, enlarging the
diameter each circuit.

"That didn't bring us anywhere, as you all know. Now we will start our
series of circles with a different point as the center. It will bring
us over an entirely different territory of the ocean floor."

"Just a moment," said Ned, as the conference was about to break up. "Is
it possible, Tom, that in our