A single big gun boomed in the darkness and a shell screamed over the U-16 toward the German submarines beyond.

“The fool!” exclaimed Lord Hastings angrily.  “Why could he not wait?  There is no reason for concealment longer.”

Immediately he gave the signal agreed upon with the German admiral, the darkening of the tail light—­the signal for the others to submerge—­and the U-16 dashed forward faster than before—­to reach a place of safety as quickly as possible and to get out of range of the British ships, which, a moment later, opened upon the enemy with every available gun.

Directly opposite the U-16 there was a terrible explosion, a blinding flash, illumining the night like day.  A German submarine had launched a torpedo and it had gone home.  The foremost British torpedo boat had been destroyed.

But the others rushed on.

The Germans had heeded Lord Hastings’ signal to submerge and were doing their best to disappear beneath the water before the British could reach them, the while firing an occasional torpedo, in the hope that it would go true, for they were unable to find the range and were working on mere guesswork.

Before the first of the foe could submerge a shell from one of the little motorboats struck her squarely upon the bridge, killing the captain and other officers, and shattering the conning tower.  The men below no longer had a means of guiding the vessel, which drifted toward her nearest neighbor and rammed her amidships.  This blow, while not necessarily fatal, threw the latter out of her stride, and being unable to tell for the moment what was wrong, the German commander gave the order to cease submerging; and the vessel remained where she was until a shell from a British torpedo boat put an end to her.

The two enemy under-sea boats last in line had turned about with the first warning and were making full speed down river again.  The rest had submerged before the British could come upon them and had sought to come about and make their way to safety beneath the water.

But the British vessels were too swift for them.  With mine nets stretched clear across the river, now that the U-16 had passed to safety, the torpedo boats dashed ahead; and an hour later four more German submarines, caught in these powerful nets, were helpless.  It only remained to drag them to the surface and make prisoners of the crews, which work was deferred until daylight.

Aboard the U-16 none knew what had occurred save the three upon the bridge—­Lord Hastings, Frank and Jack.  Below, all stood to their posts, awaiting they knew not what.

Jack descended.

“All right, men,” he said, “you may turn in, if you wish.  There’ll be no fighting to-night.”

There was an exclamation of disappointment from among the German sailors, who apparently had been banking on dealing a severe blow to the British.  There were several questions, which Jack answered by saying: