There was a slight click. Peter, looking in the direction where he imagined the apparatus was, could discern nothing, but on the opposite wall was a small circular patch of greenish-hued light.
"Seen anything like that before?" inquired his Uncle.
"Rather," replied Peter. "Anti-aircraft searchlights during the war. Couldn't see the beam in its passage through air; when it hit a solid substance it lit it up."
"This is somewhat similar, but very different," said Uncle Brian. "Sounds a rummy thing to say, but there you are. I'll demonstrate. On with the light, Peter."
Again the room was flooded with electric light. Uncle Brian pointed to a four-cylindered motor standing in one corner.
"Get to work on that," he continued. "Turn the engine over as fast as you can and see that the plugs are firing. They are already loose in the cylinders. You may as well remove the magneto dust-cap while you are about it."
Peter did as requested, placing the plugs on the tops of the cylinders, so that he could observe the sparks jumping the gaps between the points and the central rods. There was no mistaking the efficient state of that magneto. It was giving a miniature Brocks' firework display.
"Now!" exclaimed Uncle Brian.
His nephew continued to turn the geared starting-handle for another dozen revolutions.
Then he stood up and wiped the perspiration from his face.