"My surmise was correct, you see," said the professor some ten minutes afterwards, as he pointed to another gauge on the wall of the pilot- house. "We are now running steadily at a speed of one hundred and fifty miles per hour; and we have already travelled twelve miles from our starting-point. The gauge is, as you see, self-registering, and shows on that piece of paper the exact distance run through or along the surface of the water (but not through the _air_) between any two given points. When the ship's course is altered, or you desire for any other reason to commence the register afresh, all you have to do is, press that ivory knob, and the instrument will draw a line across the paper and, at the same moment, spring back to zero." The water, at the depth at which they were travelling, proved to be almost as transparent as crystal, of a dark olive-green tint beneath them, merging by imperceptible gradations to a faint greenish-blue above; the surface being discernible by the shifting lace work of gold incessantly playing over it where the sun's beams caught the ridges of the faint rippling wavelets raised by the languid summer breeze. Even small objects, such as medusae, and fragments of weed floating in mid- sea, were distinguishable at a considerable distance; and fishing-boats could be clearly made out at the distance of a mile. A very novel and curious effect was witnessed when objects floating on the surface (such as ships, fishing-boats, or aquatic birds) came into view, the submerged portions of them being as clearly defined as though they were floating in air, whilst the parts _above_ the surface were wavering and indistinct. A flock of diving gulls, for instance, which they passed at no great distance, presented the curious spectacle of little more than dark dots furnished with pairs of quickly-moving webbed feet whilst they floated on the placid surface; but directly a bird dived its whole body became distinctly visible, with a long stream of air-bubbles trailing behind it. At length it became apparent that they were approaching a large fleet of ships making their way up channel. A smile passed over the professor's features as he gazed out at them, and turning to his companions he remarked: "I feel mischievously inclined this morning. I think we will give the crews of those ships a little surprise, and furnish them with a new topic for conversation." "Ah, indeed!" said the baronet. "How do you propose to do it?"