on, to return to the scene with four
marines who were to be stationed along that section of the shore for the
balance of the night.

What they saw upon their arrival astounded them. Three additional
launches had arrived upon the scene, and the commandant of the navy yard
was himself directing matters.

He had in his hand a slight rope that ran down into the water, and close
beside it was a hose line attached to an apparatus in the boat. The boys
knew at once that a diver was at work down on the river bed.

From the side of another launch anchored parallel with the first, and
fifteen feet distant, four husky bluejackets were waiting expectantly to
divide their strength on two stout ropes that were being attached to
something down in the water. The third launch played its flashlight upon
the work, while the fourth steamed about, doing patrol duty.

Even as the boys watched, the commandant gave a signal and the sailors
began hauling upward on the two heavy ropes. In a moment an oblong box,
about two feet long, a foot wide and of the same depth, came dripping
from the water. As it was brought to the boat's side two other men
grasped it carefully and placed it in the bottom of the launch. Then the
ropes, which were attached to a guide line, were hauled down into the
river again.

"What does it mean?" Joe asked of Sergeant Martin, who had changed his
clothes and arrived back ahead of them.

"What does it mean?" repeated the big sergeant. "It means that you
three young men are due for several credits and early recognition, or
I'm much mistaken. The man you discovered has not yet been caught, but
he cannot escape for long. And when he is captured it will be a long
time before he is free again.

"You lads have frustrated a dangerous plot by an enemy government. The
river bottom seems to be paved with those cases. They've taken out a
dozen already. One of them was opened, and, just as expected, it proved
to be a water-tight container for smokeless powder!

"The government that had those boxes hidden there undoubtedly was
scheming to have plenty of ammunition ready for use if it ever managed
to land its men on American soil.

"But you boys appeared here just in time to blow up the whole plot. You
have been in your first real action in the service of your country, and
you have come off with flying colors."

CHAPTER IV

FAREWELL, UNITED STATES

When the boys arose the following morning, each somewhat stiff and