and learn," Dave explained. "That's the way that all
progress in this war has been registered. We are fighting an ingenious
enemy. Destroying the submarine mine-carriers, as we are doing today,
won't end the planting of German mines. As soon as the enemy finds out
how we are checkmating him he'll invent another scheme, which we'll have
to discover before we can beat it."

Half an hour later the British aircraft located a third submarine.

"A big one, too," she signalled. "Following the same course."

"Mr. 'Blimp' might try a bomb himself," suggested Ensign Andrews. "I
believe he carries a few."

"Not as powerful ones as we carry," Darrin answered. "Besides, he has to
be at a greater altitude, when hunting submarines, than it's handy to
drop a bomb from. There is too much margin of chance that the enemy craft
will graze by when the bomb is dropped from the air. In our case, if we
drop when directly over the Hun, there can hardly be a miss, and it's the
dirigible's business to tell us when we are directly over the enemy."

In the meantime, on board the destroyer, all was made ready, and Dave
followed the same tactics as before. This time, too, there was a normal
explosion, though a solid hit was made and the submarine destroyed. Apart
from the "blimp's" report there could be no doubt as to the destruction.
The spread of oil on the surface of the sea told the story.

"If you and we hurry, we may bag another before dark," Dave sent by
wireless, as the aircraft started back again.

"We'll do our best, believe us!" came back the word.

In the late afternoon a slight haze came up, which gradually deepened.

Darrin followed for a few miles, keeping the "blimp" in sight. She was
some six miles away when a radio message came from her in code in these
words:

"Can you see steamship about four knots north-west of us?"

Dave challenged the lookout on the military mast, but that seaman
reported the weather a bit too thick to enable him to make out the
steamship. Darrin accordingly wirelessed back this information.

"Looks like a tramp steamer," came the next message, "but she acted
suspiciously when she sighted us. Her skipper appears perturbed, which he
would hardly be if his business is honest. Weather is thickening so we
may lose him in the haze. Better close in."

"Will do so," Dave replied.

Then followed explicit directions as to the course the destroyer must
follow.

The next code message from the airship was