“Sounding-lead,” Jack corrected him.

“Well, it sounded like a sledge-hammer, anyway,” grinned young Somers.  “While I was down below I found that the temperature rose four degrees.”

“Part of that was likely due to the warmth of your body, and the heat of the breath you gave off,” hinted Benson.

“You could have gotten it up to eighty or ninety degrees by turning on the electric heater far enough,” suggested Hal.

“I wanted to see whether it would be warmer in the depths; wanted to find out how low I could go and be able to do without heat in winter,” Somers retorted.

“I could have told you that, from my reading, without any experiment,” retorted Skipper Jack.  “Close your conning tower and go down a little way, and the temperature would gradually rise a few degrees.  That’s because of the absence of wind and draft.  But, if you could go down very, very deep without smashing the boat under the water pressure, you’d find the temperature falling quite a bit.”

“Where did you read all that?” inquired Eph, looking both astonished and sheepish.

“Here,” replied Jack, going to a small wall book-case, taking down a book and turning several pages before he stopped.

“Just my luck,” muttered Eph, disconsolately.  “Here I’ve been dull as ditch-water for an hour, trying to find out something new, and it’s all stated in a book printed—­ten years ago,” he finished, after rapidly consulting the title-page.

Jacob Farnum had been no listener to this conversation.  Taking the marine glasses from the conning tower, the shipbuilder was now well forward on the platform deck, scanning what was visible of the steam craft to the southward.  At last the yard’s owner turned around to say: 

“I don’t believe you young men can have things ship-shape a second too soon.  The craft heading this way has a military mast forward.  She must be the ‘Hudson.’  If there’s anything to be done, hustle!”

Jack and Hal sprang below, to scan their respective departments.  Five minutes later Grant Andrews hailed from the “Pollard,” and Eph rowed over in the shore boat to ferry over the machinists.

Half an hour later Andrews and his men had put in the few needed touches aboard the newer submarine boat.  The sun, meanwhile, had gone down, showing the hull of a naval vessel some four miles off the harbor.

Darkness came on quickly, with a clouded sky.  As young Benson stepped on deck Grant Andrews followed him.

“All finished here, Grant?” queried the yard’s owner.

“Yes, sir.  There’s mighty little chance to do anything where Hal Hastings has charge of the machinery.”

“That’s our gunboat out there, I think,” went on Mr. Farnum, pointing to where a white masthead light and a red port light were visible, about a mile away.

“Dunhaven must be on the map, all right, if a strange navigating officer knows how to come so straight to the place,” laughed Jack Benson.