I saw no reason why it should not be used to measure heat as well as cold.
"You see, down there I placed the couples of the thermopile beneath the electric furnace on the table. Here I have the mechanism operated by the feeble current from the thermopile, opening and closing switches, and actuating bells and lights. Then, too, I have the recording instrument. The thing is fundamentally very simple and is based on well-known phenomena. It is not uncertain and can be tested at any time, just as I did then, when I showed a slight fall in temperature. Of course it is not the slight changes I am after, not the gradual but the sudden changes in temperature."
"I see," said Carton. "If there is a drop, the current goes one way and we see the red light; a rise and it goes the other, and we see a green light."
"Exactly," agreed Kennedy. "No one is going to approach that chamber down-stairs as long as he thinks any one is watching, and we do not know where they are watching. But the moment any sudden great change is registered, such as turning on that electric furnace, we shall know it here."
It must have been an hour that we sat there discussing the merits of the case and speculating on the strange actions of Loraine Keith.
Suddenly the red light flashed out brilliantly.
"What's that?" asked Carton quickly.
"I can't tell, yet," remarked Kennedy. "Perhaps it is nothing at all. Perhaps it is a draught of cold air from opening the door. We shall have to wait see."
