“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 and see.”

We bent over the little machine, straining our eyes and ears to catch the visual and audible signals which it gave.

Gradually the light faded, as the thermopile adjusted itself to the change in temperature.

Suddenly, without warning, a low-toned bell rang before us and a bright-green light flashed up.

“That can have only one meaning,” cried Craig excitedly.  “Some one is down there in that inferno—­perhaps the bomb-maker himself.”

The bell continued to ring and the light to glow, showing that whoever was there had actually started the electric furnace.  What was he preparing to do?  I felt that, even though we knew there was some one there, it did us little good.  I, for one, had no relish for the job of bearding such a lion in his den.

We looked at Kennedy, wondering what he would do next.  From the package in which he had brought the two registering machines he quietly took another package, wrapped up, about eighteen inches long and apparently very heavy.  As he did so he kept his attention fixed on the telethermometer.  Was he going to wait until the bomb-maker had finished what he had come to accomplish?

It was perhaps fifteen minutes after our first alarm that the signals began to weaken.

“Does that mean that he has gone—­escaped?” inquired Carton anxiously.

“No.  It means that his furnace is going at full power and that he has forgotten it.  It is what I am waiting for.  Come on.”

Seizing the package as he hurried from the room, Kennedy dashed out on the street and down the outside cellar stairs, followed by us.

He paused at the thick door and listened.  Apparently there was not a sound from the other side, except a whir of a motor and a roar which might have been from the furnace.  Softly he tried the door.  It was locked on the inside.

Was the bomb-maker there still?  He must be.  Suppose he heard us.  Would he hesitate a moment to send us all to perdition along with himself?

How were we to get past that door?  Really, the deathlike stillness on the other side was more mysterious than would have been the detonation of some of the criminal’s explosive.