Mechanically Gilbert looked round and saw that all was safe, then passed into the rooms of Dennis and Ross, both of whom were sleeping soundly; in the reflecting tubes he examined every berth and nook of the whole ship, to find all safe and the occupants sleeping calmly in their cabins. He then, in the dead silence, passed out of the _sanctum sanctorum_ to examine the air apparatus, which proved to be working satisfactorily, and then forward into the laboratory where, after a general glance round to see what experiments were in progress, he commenced some further analyses of the composition, weight, and nature of the atmosphere in which the ship rested. What was his surprise to find that the air outside was in motion, so slight that only the most delicate instrument recorded the faintest trace of sound, but sound it was, undoubtedly. Rushing back, he examined the switches and dials, to find the ship poised and absolutely still. The air must have been in motion, therefore, the night before, but so slightly that, the ship’s motion being present also, the instrument was unaffected. When the ship was still, the vibration of eighteen moving people, imperceptible as that seemed on so large and rigid a vessel, had, nevertheless, proved sufficient to annul the instrument’s record of sound. It was evidently an illustration of a law of physics—that if two sound-waves not in unison meet, and the swell of one encounters the opposite phase of the other, silence will result, for both will be neutralised; just as in the well-known experiment of the tumbler placed on a table and a second tumbler held at right-angles over it. A tuning-fork in rapid vibration is held in the centre of the angle formed by the two tumblers, and though its vibrations continue, no sound results; but the sound is made to become audible or cease as one of the tumblers is removed or replaced. In the one phase, by the upper tumbler being removed there is no check on the vibrations, which are free to produce sound, but when the upper tumbler is held as described, the sound-waves strike one another at opposite phases, and the plus of one is absorbed in the minus of the other. It was a most interesting point, for the sound-waves set in motion by the moving air and those disturbed by the moving people chanced to be at right-angles, and produced silent vibrations. For several hours Gilbert continued his observations and experiments, hearing first one and then another of his companions moving about, and at last he awoke Dennis and Ross, asking them not to start the ship for the present. Very soon all were in their accustomed places, refreshed and alert after their long sleep. Hearing that Gilbert had found out something important, everybody trooped into the laboratory and he explained his discovery. “What do you infer from that?” asked Godfrey. “I only see in it a most interesting physical experiment naturally conducted.”