signal with that sledgehammer--"

"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.