Captain Cain could hear the squeaking of blocks as the destroyer's boat was being lowered. He was rather dubious about the step he proposed taking. He estimated, although he had not taken soundings, that the Alerte was in eleven fathoms, with a sandy bottom. In the absence of electrically propelled motors, the submarine had either to go up or go down. She could not maintain a midway depth, for although fitted with compensating tanks, these alone, without the assistance of the horizontal rudders—which were useless unless the submarine were making way—would fail to keep her at a constant depth. Should the soundings prove much greater than he expected, the Alerte's hull might be unable to withstand the enormous pressure of water. If, on the other hand, the depth were considerably less, then the Alerte's mastheads would show above the surface, since there was no time to lower them before submerging.
The creaking of oars announced that the destroyer's boat had pushed off and was heading for the supposed Memnon. Again the dazzling searchlight was unscreened. There was no time to be lost.
Descending the bridge at breakneck speed, Captain Cain ran to the after-end of the conning-tower. Here, stowed in an air-tight box, was the smoke-producing apparatus used in conjunction with the dummy funnel to give the effect of a vessel with steam, propelled engines. In the same compartment were several explosive rockets.
Disconnecting the pipe that conveyed the smoke to the base of the funnel the pirate captain laid the nozzle on the deck. Then, hastily securing one of the rockets to a stanchion, he ignited the touchpaper.
The moment the detonator exploded, Captain Cain released the smoke cloud, descended the hatchway, and closed the water-tight cover.
"Flood ballast tanks!" he shouted.
Three minutes later, the Alerte sank on practically an even keel to the bed of St. Ives Bay. The depth gauge registered eleven and a quarter fathoms, which meant that at high tide she would be lying in eighty-seven feet—sufficient to immerse the trucks of the masts to a depth of twenty-eight feet.
"That's done them!" exclaimed Captain Cain exultantly to his second in command.
"Unless they depth-charge us," added Pengelly gloomily.
"They won't—why should they?" rejoined the skipper. "They don't know but that we blew a hole in the old hooker and sank her for good and all."