“You want to find the man with a weapon that bullet fits, and then make it warm for him,” advised one man in the front rank of the crowd.

“Why?” queried Captain Jack, coolly, examining the missile, then dropping it carelessly into his pocket.  “Some fellow fired an accidental shot, very likely, and is at this moment the most scared man at Spruce Beach.  What’s the use of jumping on anyone just because he had a moment of carelessness?”

“That’s right, young level-head!” nodded another man, approvingly.

Messrs. Farnum and Pollard hung back somewhat.  They were near enough to hear and see, and they had their instant suspicions.  But the crowd knew nothing of the spy outrages, and it was not necessary to inform strangers.

So, within a few minutes the crowd broke up, straying off in quest of something more interesting.  The submarine party kept on up to the hotel porch.

“That was a revengeful move, pure and simple,” declared Jacob Farnum, in a low voice.

“Of course,” assented Jack.  “It’s going to be something of a task though, to find out, for certain, just who fired that shot.”

Even as the four stood there on the veranda a door opened, and M. Lemaire, faultlessly attired for an afternoon stroll, stepped out.

“Ah, good afternoon, gentlemen,” was his unconcerned greeting, as he recognized the quartette.

This French spy had evidently dressed himself with a good deal of care.  He carried himself with much precision and lightly twirled a natty cane.

“Pardon me, monsieur,” spoke Jack, stepping forward, and looking past the Frenchman; “is that one of your friends down the road?”

As the Frenchman turned to look, young Benson swiftly and adroitly took his cane from him.

Like a flash, his eyes full of fire, Lemaire heeled about, then leaped at the young submarine captain.

But Hal Hastings stepped between them so neatly that the Frenchman collided with him instead.

“Hold this fellow a moment, please,” requested Captain Jack.  “I’ve found something interesting.”

Hal Hastings grabbed Lemaire’s right arm.  Jacob Farnum instantly possessed himself of the other.  David Pollard sprang forward so that he could take a hand, if need be.

Captain Jack stood holding the spy’s walking stick, ferule end upward.  It was a rather long, slender-looking ferrule of steel.  But what interested young Benson most was that he had found that the ferrule was hollow.

Quickly the submarine boy examined the rest of the cane.

“Release me!  Hand that stick back to me!” hissed the Frenchman.  “Oh, some one shall pay for this unpardonable outrage!”

But Hal and Mr. Farnum only gripped the spy the more tightly.

“I believe I’ve found out something,” announced Jack, in a low voice.  “Wait a second or two.”

He had come upon a concealed spring near the head of the cane.  Stepping to the edge of the porch, the submarine boy pointed the ferrule end at the ground, then pressed upon the spring.