"We'll hang on to it, never fear," replied Frank. "Come on, Jack."

"Wait a minute," ordered Lord Hastings. "You'll need this written order
to get the boat."

"I'd forgotten, sir," said Frank.

Lord Hastings scribbled rapidly on a piece of paper, which he passed to
Jack.

"This will fix you up," he said. "Now remember, 10 o'clock sharp."

The boys nodded their understanding of this order, saluted and left.
Getting their things together, they hurried to the river, where Lord
Hastings kept his motorboat; and an hour and a half later they were
proceeding slowly down the river.

"Guess none of the enemy will ever get in here," declared Frank, after a
careful survey of the river.

"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