In half an hour, as planned, the “Oakland,” after firing a warning gun, steamed away from her moorings.  Gradually the gunboat’s speed increased, until the full sixteen miles were being made—­miles, instead of knots, since gasoline boats, like these submarines, are usually rated by miles instead of by the longer “knot.”

It was a rattling rate of speed to exact from these little craft, when it was considered that the gait would have to be continued, without break, for at least twenty-four hours.

Eph was at the wheel, at the start, and Jack standing back by the conning tower.  Mr. Farnum had gone below, for a nap, as he intended to relieve Hal in the engine room after a few hours.

“Benson,” remarked Danvers, approaching the submarine boy, “I guess your remark of a few minutes ago exactly defines this trip.”

“What remark?” asked Jack.

“You spoke of it as a lightning cruise.  It is going to be one, indeed, for these little submarine craft.”

“Our boat can stand it, I think,” smiled the submarine skipper.

“And so can the Rhinds boat, probably.  But some of the others will find themselves sorely put to to keep up the speed for twenty-four hours.”

“And, if they don’t?” queried Jack.

Danvers shrugged his shoulders.

“Then I guess they’ll have to be satisfied with being left far behind, unless they signal that they’re in actual distress.”

“This speed,” mused Captain Jack, “must be part of the government’s plans for another test.  The Navy Department must have planned to see whether any of these boats could stand such a gait for twenty-four long hours.”

“I couldn’t tell you if I knew,” remarked Lieutenant Danvers, with a quizzical look, then turned and strolled away.

“And I guess,” muttered the submarine boy to himself, “that that’s about as near as a fellow can go to giving a tip, once he has had the Navy muzzle padlocked to his jaws.”

Some of the submarines in this long race—­for such it was—­were better equipped as to the number of the crew.  The Rhinds had this advantage, carrying a captain and four men, in addition to Rhinds himself and his secretary.  Yet Jack and Eph relieved each other regularly at the wheel, catching long naps between.  Hal and Mr. Farnum did the same thing with the engine room, and the “Hastings” kept well in the van through the day, and also through the long night that followed.

Two hours after daylight the “Oakland” signaled to the submarines to run up close to this “parent vessel,” the gunboat.

“Further orders, of course,” muttered Jack, who was at the wheel at the time.  “Well, we’re not such a very long run, now, from the reported location of that derelict.”

The fleet was wholly out of sight of land.  The wind was fresh and the sea lively with short, choppy waves, crested by white-caps.  Yet, for boats as staunch as these submarines, sea was not a difficult one for boat handling.