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 looked over the ship before turning in for the night. The craft was made immensely strong to withstand powerful pressure at the bottom of the ocean. The submarine could penetrate to a depth of about three miles. Below that it was dangerous to go, as the awful force would crush the plates, powerful as they were.
"Well, we'll rush things to-morrow and the next day," observed Tom as he prepared to leave the building. "Then we'll soon see if it works."
For the next week there were busy times in the shop near the ocean. Great secrecy was maintained, and though curiosity seekers did stroll along now and then, they received little satis-
