Thrice more the "blimp" passed over the submarine that was invisible to surface eyes, and each time let loose a smoke bomb.
"Now, you're directly in line," came the radio message from above. "Move dead ahead. Will tell you when you are passing over. We'll signal the word 'drop'."
The meaning of "drop" would be clear enough. It would mean that the "Grigsby" was instantly to release, over the stern, a depth bomb.
As the "Grigsby" neared the spot speed was considerably reduced. Overhead hovered the "blimp," ready for instant signalling of one word. The command had already been passed to the men stationed by the depth bomb to let go as soon as the messenger gave the word from the operator.
As Darrin glanced upward he saw the "blimp" nearly overhead.
Suddenly the messenger's startled voice roared out the message passed by the radio operator:
"Full speed astern!"
In the same instant Lieutenant Fernald repeated the order over the engine-room telegraph. There was a jolting jar as the "Grigsby" shivered, then glided back in her own wake.
"Jove! That was a narrow squeak!" came down from the sky. "That hornet laid an egg in your path. It came within an ace of bumping your keel."
"Never did speed pay a prompter profit, then," uttered Darrin, his cheeks paling slightly.