Swiftly, yet deliberately, Gerald set the pointers. There was no answering spark. He tried another and yet another square; still no response. The Z-rays apparatus had been tampered with.
Realising the uselessness of investigation at that critical moment, Gerald tore aft and reported the occurrence.
Captain Brookes looked in the direction of the retreating aviator.
Quickly the gun was manned and a shining copper and steel cylinder thrust into the breach.
"Four thousand yards," sang out Lieutenant Sinclair, who, without waiting for instructions, had taken the distance by means of his pocket range-finder.
"Good shot!" yelled several of the officers, carried away by their excitement, as, by the aid of their glasses the missile was seen to burst just above the fugitive.
For a full ten seconds the monoplane held on its course, then, lurching like a wounded bird, it swooped swiftly downwards, striking the water with a terrific splash.
"Full speed ahead. A leadsman in the chains," ordered Captain Brookes as he made his way to the bridge.
A few minutes were sufficient to bring the _Olive Branch_, with very little way on, immediately over the spot where the aeroplane had disappeared, the airman's cap and one of the inflated floats still marking the scene of the disaster.
The leadsman made a cast, then as he heaved the buoyed log-line overboard he shouted, "Eighty fathoms."
[Illustration: chapter XXII] CHAPTER XXII
"CLEAR away the cutter; pipe the creeping party. Three grapnels and the necessary line," ordered Captain Brookes; then turning to his officers he continued: "Before we take steps to sweep for the lost gear an investigation ought to be made. Will you, Mr. Slade, inspect the wireless room and report to me? Mr. White, you will please attend to Mr. Palmer, who evidently needs your services. Pull him together as far as you are able, so that he can give some account of this mystery. Now, Mr. Tregarthen, we'll see what's wrong with the Z-rays apparatus."
The fault in the conning-tower was quickly discovered. A small hole had been drilled in the steel column conveying the wires from the batteries to the indicator-board, and a metal rod had been inserted, thus causing a short circuiting of the current.