“Guess not,” replied Jack.  “Look at the boats.  You wouldn’t think we were at war.”

“Not if it wasn’t for the warships,” agreed Frank.  “And there are enough of them to make it hot for any hostile fleet.  But it’s a wonder to me some of these German submarines haven’t taken a little trip up the Thames.”

“Mines,” replied Jack briefly.

“True,” said Frank, “but you will remember we took a pretty long jaunt up the Dardanelles, and passed through the Kiel canal.”

“And when you stop to think of it, we’re pretty lucky to be here right now,” returned Jack dryly.

“Well, so we are, if that’s the way you look at it.  However, I wouldn’t mind having another such chance.”

“You’ll probably get it.”

Conversation lagged as the boys took in the scenes about them; and there was little more talk during the trip.  They stopped more than once, and, loitering along, it was dark when they neared their destination.

As they would have drawn up to the wharf there was a sudden flash of light—­gone in a moment—­followed by a dark body that swished by them like a flash.

Frank uttered an exclamation of astonishment.

“See that?” he demanded.

“Yes.  What could it have been?”

“You’ve got me, but it’s heading toward the open sea.  Great Scott!  Maybe it’s an enemy.”

“An enemy?”

“Yes; you know how anxious the Germans are over this submarine business.  Maybe this fellow has been spying about.  May be going to report to a German submarine out there some place.”

“Think we had better follow and have a look?” asked Jack.

“Believe it would be a good idea.  Let’s go.”

Without another word, Jack brought the boat about and headed after the one that had so recently dashed by them.  In the darkness ahead there was nothing to be seen.

“Like looking for a needle in a haystack,” Jack called to Frank.

“That fellow can’t be up to any good,” declared Frank.  “He showed no light and was going in a terrible hurry.  There’s something up.”

“Does seem that way,” agreed Jack.

“Say!  Is this as fast as this thing can go?” demanded Frank.  “We won’t ever get any place this way.  Let her out a bit.”

Jack did so and the little boat seemed literally to fly over the dark water.  This terrific speed Jack kept up for some time and then slowed down.

“We’ll bump into something at this rate,” he said; “and that would settle the whole business.  We must be cautious.”

“Cautious!” repeated Frank.  “We won’t find that German being cautious.”

“If we weren’t cautious, it wouldn’t do us any good if we did find him,” argued Jack.  “First thing you know we would be at the bottom.”

Frank considered this point a moment.

“Guess you’re right,” he said at last.

“Swish!” went something at this moment, and, turning quickly, Frank saw a dark shape speeding away up the river.