There was one more device on board the _Flying Fish_ which should be described in order that her wonderful manoeuvering under water may be understood. Just in front of the steering-wheel in the conning-tower was a square glass box measuring a foot in the side, and in the centre of this, attached to top and bottom by slender films of asbestos, was a needle ten inches long, so hung that it could turn and dip in any direction. The forward half of this needle was made of highly magnetised steel, and the other of aluminium which exactly counter-balanced it. The glass case was completely insulated and therefore the extremely sensitive needle was unaffected by any of the steel parts used in the construction of the vessel. But let any other vessel, save of course a wooden ship, come within a thousand yards, the needle began to tremble and sway, and the nearer the _Flying Fish_ approached it, the steadier it became and the more directly it pointed towards the object. If the vessel was on the surface, it of course pointed upward: if it was a submarine, it pointed either level or downwards with unerring precision. This needle was, in fact, the eyes of the _Flying Fish_ when she was under water. Castellan swung her head round to the north-west and dropped gently on to the water about midway between Selsey Bill and the Isle of Wight. Then the _Flying Fish_ folded her wings and sank to a depth of twenty feet. Then, at a speed of ten knots, she worked her way in a zigzag course back and forth across the narrowing waters, up the channel towards Portsmouth. To his surprise, the needle remained steady, showing that there was neither submarine nor torpedo boat near. This meant, as far as he could see, that the main approach to the greatest naval fortress in England had been left unguarded, a fact so extraordinary as to be exceedingly suspicious. His water-ray apparatus, a recent development of the X-rays which enabled him to see under water for a distance of fifty yards, had detected no contact mines, and yet Spithead ought to be enstrewn with them, just as it ought to have been swarming with submarines and destroyers. There must be some deep meaning to such apparently incomprehensible neglect, but what was it? If his brother Denis had not happened to recognise Captain Count Karl von Eckstein and haled him so unceremoniously on board the _Ithuriel_, and if his portmanteau full of papers had been got on board a French warship, instead of being left for the inspection of the British Admiralty, that reason would have been made very plain to him.