"By jinks!" he exclaimed, with a gesture of vexation. "I forgot the old woman."
He hastened back to Obernai's house. The old woman had disappeared.
On returning to the house some time before, Kenneth and Harry found the officers, all in their night attire, examining the signalling apparatus in the upper room.
"They are all safely locked up, sir," Kenneth reported.
"That's well. How did you catch them?"
Kenneth gave an account of the night's work.
"You did very well, Amory," said the captain. "The battalion is lucky in having the Three Musketeers. And the whole brigade is indebted to you. This is a fiendishly ingenious arrangement."
He explained the working of the apparatus. The acetylene lamp faced one end of a long tube, which pierced the outer wall of the house. By means of a delicate mechanism the position of the tube could be altered by millimetres. The length of the tube prevented the rays from converging like the rays of a searchlight, so that the light, directed eastward, was not likely to be seen except by a person at an equal height.
"I have no doubt at all," said the captain, "that some miles away in the German lines there is an operator with a similar lamp, at the same height and in the same straight line with this. We have kept a look-out but seen nothing; no doubt the cessation of the flashing gave them warning. To them the light would appear like a star on the horizon, and the alternate exposure and dousing of it by means of the disc made the signals. No wonder we've got it unexpectedly hot sometimes."
Here Ginger came in.