ET us now return to William and see how he is faring. At the same moment that the danger-signal appeared in the car, a microphone fastened to the wall began to work, and William, highly puzzled, rapidly made his way to the instrument. This microphone was so arranged that it received the sounds from outside, and transmitted them, greatly magnified, to the interior of the car.

Had there been a perfect vacuum in the tube, the instrument would, of course, have been useless, as sound cannot travel through empty space. But, fortunately, there was, as we have said, a certain amount of air left in the tunnel, so that sounds could be received from the metal walls of the carbonite tube. These sounds were, it is true, considerably weakened in intensity, but by means of