ble when I heard a low
exclamation from Craig, of "Look, Walter!"

I did look in amazement. There were indeed little sparks, in fact a
small burst of them in all directions, where there were metal surfaces
in close proximity to one another.

Kennedy had brought along with him a strange instrument and he was now
looking attentively at it.

"What is that?" I asked.

"The bolometer," he replied, "invented by Professor Langley."

"And what does it do?"

"Detects waves," he replied, "rays that are invisible to the eye. For
instance, just now it tells me that shooting through the darkness are
invisible waves, perhaps infra-red rays."

He paused, and I looked at him inquiringly.

"You know," he explained, "the infra-red rays are closer to the heat
rays than those of the upper end of the spectrum and beyond, the
ultra-violet rays, with which we have already had some experience."

Kennedy continued to look at his bolometer. "Yes," he remarked
thoughtfully, half to himself, "somewhere around here there is a
generator of infra-red rays and a projector of those rays. It reminds me
of those so-called F-rays of Ulivi--or at least of a very powerful
wireless."

I was startled at the speculations that his words conjured up in my
mind. Was the "evil eye" of superstition a scientific fact? Was there a
baneful beam that could be directed at will--one that could not be seen
or felt until it worked its havoc? Was there a power that steel walls
could not hold, which, in fact, was the more surely transmitted by them?

Somehow, the fact of the strange disappearance of Petzka, the wireless
operator, kept bobbing up in my mind. I could not help wondering
whether, perhaps, he had found this strange power and was using it for
some nefarious purpose. Could it have been Petzka who was responsible
for the fires? But, why? I could not figure it out.

Early the next morning we called at the Gaskell town house again.
Kennedy had brought with him a small piece of apparatus which seemed to
consist of two sets of coils placed on ends of a magnet bar. To them was
attached a long flexible wire which he screwed into an electric light
bulb socket. Then he placed a peculiar telephone-like apparatus,
attached to the other end, to his ears. He adjusted the magnets and
carried the thing carefully about the room.

At one point he stopped and moved the thing vertically up along the
wall, from floor to ceiling.

"That's a gas pipe," he said simply.