His first action answered the question. A glance into the gasoline tank showed that it was empty of all fluid. The source of power had been used up.
"There may have been other causes, but that was enough," remarked Alvin. "I'll look into things on our way back."
His first plan was to borrow enough gasoline from George to run the Deerfoot on the homeward trip, but George thought he had none to spare.
"I can tow you to town," he added.
"That will do as well."
The tow-line was passed over the bow and George made it fast to a cleat on the stern deck of the Shark. Then he resumed his moderate speed toward Boothbay Harbor. It was not a long run and Alvin spent much of the time in inspecting the mechanism of his motor boat. To repress so far as he could his profound grief, he kept up a running commentary or explanation to Chester, who really did not require it, for he knew a good deal about motor boats. But he listened as if it were all new to him, and asked questions. His purpose was the same as his friend's: they talked about everything else in the vain effort to keep their minds from the awful theme that bowed them both with a sorrow they had never known before.
"If when the ignition system is in good order, the carburettor properly adjusted and the compression cocks closed, there is a lack of power, it may be due to carbon or some foreign substance on the seats of the exhaust or inlet valves. Even so small a thing as a flake of metal or of emery left from a former grinding may lodge on the valve seat or under the valve stem and cause loss of power, or a crack in the head of the piston or cylinder or a broken or worn piston ring may give the same result."
"I have heard of those and other causes," said Chester, as the two sat side by side, "but what is the most common one?"
"The valves, when they need grinding. I have not had that trouble yet with the Deerfoot, and when I do I shall not try to do the grinding myself. The work is so delicate that it should be done only by an expert mechanic."
"What causes backfiring, through the carburettor, Alvin?"