"But what is the motive power?" asked Gerald.
"Petrol, paraffin, or, in fact, any inflammable oil capable of passing through the vaporisers. The motors, which can be attended to by a staff of ten engineers only, actuate five propellers. Thus we are not under the obligation of having to carry stokers. As a matter of fact, the personnel of the Olive Branch, thanks to mechanical appliances of the most modern type, amounts to 105 officers and men."
The conning-tower consisted of a circular armoured structure 25ft. in diameter, and barely 5ft. above the upper deck. Around the walls were electrical indicators and a maze of pipes painted in distinctive colours similar to those on board a British man-of-war. But the apparatus that riveted Gerald's attention was a board composed of copper and zinc squares resembling a draught board, with a pair of pointers at two adjacent corners. This device travelled on steel lines that formed an almost complete circle, a gap being left in the direction of the after end of the ship.
"Now, Mr. Tregarthen, what do you suppose this arrangement is for?"
"It appears to me to be a sort of position-finding instrument."
"It is more than that. But first I must request you to maintain a strict secrecy regarding what you see in the conning-tower, at least until I withdraw this restriction. Beyond that you are at perfect liberty to discuss with your companion any details concerning the ship," replied Captain Brookes. "Before you left Poole Harbour did you see a report in the papers concerning the German cruiser Zietan?"
Like a flash the truth swept across Tregarthen's mind. The ex-Brazilian vessel, Almirante Constant, the mysterious agency that had temporarily incapacitated the Zietan, and the ship on which he stood were one and the same. So astonishing was the revelation that even he—an iron-nerved naval officer—gasped with amazement. The captain, who was watching the effect of his question, kept silent, awaiting Gerald's reply.
"I did," he assented after a prolonged interval. "But how did you do it? And how were you aware?"
"One question at a time, please. I am on the point of showing you a somewhat ingenious device; what it has already done I will inform you of in due course. This chequered board is divided, as you see, into eighty-one squares, each division representing one square mile. Thus a 'field,' meaning nine miles in either direction, is at my command. Now, supposing a hostile ship is sighted. Her position is determined by the ordinary range-finding instruments. By placing these two pointers on the square representing the ship's position—the pointers, as you will observe, being capable of alteration of length—I release a wireless current, which I prefer to term the Z-ray—upon the enemy's ship. Instantly the whole of her electrical gear is completely disorganised, and, knowing as you do the vital importance of electricity on board a modern man-of-war, you can realise what it means."
"Then you have already committed a hostile action against the ship of a friendly nation?"