submerged periscopes before, and they had never behaved in that erratic fashion.

This one was stationary as regards direction. No feather-like spray denoted its passage through the water. It certainly was not forging ahead. It was, however, rolling erratically, its centre of semi-rotation being but a few inches beneath the surface. The periscope of a submarine, if it were inclining in a vertical plane at all, would have a very different movement, protruding as it was from the comparatively huge hull of the vessel.

"It's a dummy periscope," he announced.

"Sure of it, Mr. Webb?" asked Captain M'Bride.

"Positive, sir."

The skipper of the Portchester Castle did not hesitate. A warning blast from the armed merchant-cruiser's syren was followed by the peremptory signal, "Go astern instantly", while the white ensign hoisted aft imparted the necessary authority to the Two-Step.

An exchange of signals followed, with the result that the tramp forged ahead once more, and, altering her course slightly, passed quite a couple of cables' lengths from the sinister spar that bobbed lazily above the sea.

"And there are half a dozen destroyers leaving Gib. to-day," remarked Captain M'Bride. "If they had sighted this decoy one of them would have gone at it like a bull at a gate. We must risk it, I suppose. Away first cutter's and whaler's crews!"

The Portchester Castle had to slow down to enable the boats to be lowered. This in itself was a risky operation, since it was quite possible that a real hostile submarine might be lurking in the vicinity, awaiting the opportunity to discharge a torpedo at the almost stationary target afforded by the armed merchantman. Nevertheless the risk had to be undertaken. It fell within the scope of the duties of the Royal Navy in its gigantic task of rendering the maritime highways as safe as possible for the sea-borne commerce of Britain, her Allies, and of neutral nations.

Tom Webb was in charge of the cutter, his brother Sub-lieutenant, Dicky Haynes, having command of the whaler. The moment the two boats cast off, the Portchester Castle pelted off at full speed, maintaining an erratic course to minimize possible danger until the two Sub-lieutenants had carried out their hazardous investigations.

Each boat had two hundred yards of grass rope trailing astern, the other ends being made fast to the bight of 

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