"How's that?" morsed the wireless, as the operator of the "Antipas" sought advice and guidance from the Blimp.
"Missed!" replied the airship's wireless laconically. "If you can't do better than that, push off. You're in our light."
Ronald Tressidar, lieutenant-commander of the "Antipas," was nothing if not a sportsman. First upon the scene he had done his level best to send the U-boat to Davy Jones; failing at the first attempt, and not knowing the direction taken by the submerged pirate, he was not one to fail to recognise that the Blimp was better adapted to the task than the destroyer.
"Good luck!" flashed the aerial message from the "Antipas," as she steadied her helm and dashed away from the scene of her futile efforts.
The dark shadow was twisting and turning. The U-boat had dived so deeply that, viewed from the airship, she could hardly be distinguished from the water. It was enough for Barcroft: once on the trail it was a rare occurrence for him to be put off the scent when it came to Fritz hunting.
"Set to twenty fathoms!" he ordered.
"Twenty fathoms, sir!" replied Anderson, as he manipulated the fuse-timing that would allow the aerial torpedo to sink to the stated depth before detonating.
In his former seaplane career Barcroft had bombed his various objectives with uncanny precision. Good luck and sound judgment combined to make him a past master in the art of "getting there." But in aerial torpedo work against a submerged object a new factor had arisen—the effect of refraction. Unless a bomb-dropping machine—be it airship or seaplane—is directly over its objective, due allowance must be made for the deceptive qualities of air and water in conjunction. A simple experiment will easily show this. Take a bowl of water and place in it an object heavier than water—for example, a penny. Stand immediately over the bowl, and with a long rod attempt to "spear" the coin. Unless one's hand be wobbly the task will be easy enough. Next, take up a position so that an imaginary line from the eye to the penny forms an angle of about forty-five degrees with the horizontal. Repeat the thrusting operation and the coin will be missed handsomely, while the rod will appear to be sharply bent from the point where it enters the water.
Down to two hundred feet dropped the Blimp. The loss of altitude diminished the visibility of the presence of her prey, but there was just enough indication of the presence of the submerged submarine to enable Barcroft to risk a shot.
The motor was throttled down. Flying slowly and almost dead in the eye of the wind the airship was keeping pace with her blinded antagonist. It was like a keen-eyed hawk hovering over a stream and waiting to pounce upon an all unsuspecting fish.