as about to leap. For he realized that the second workman
neither saw nor heard him.

Yet some subconscious impression of danger must have reached his mind,
for the workman stopped too, instinctively assuming an attitude of
defense. Dick gathered a dozen links of his wrist-chain in his right
hand, leaped and struck.

The workman crumpled to the floor, a little thread of blood creeping
from his right temple.

It was the thing upon which Dick looked back afterward with less
satisfaction than any other, leaving the two unconscious men in that
room of death. Yet there was nothing else he could have done. He ran
to the trap, and saw a ladder leading down. In a moment he had swung
himself through and closed the trap behind him.

The material that lined the walls below must have had almost perfect
insulating qualities, for the temperature here was no hotter than in
the Bahamas on a hot summer day. Dick scrambled down the ladder and
found himself in a machine-shop. Nobody was there, and tools of all
sorts were lying about, as well as machinery whose purpose he did not
understand. A pair of heavy pliers and a vise were sufficient to rid
Dick of his wrist and ankle chains in a minute or two. With a knife he
slashed the cords of invisible stuff that bound him. He stood up,
cramped, but free.

He picked up an iron bar that was lying loose on a table beside a
machine, and advanced to the staircase in one corner of the shop. As
he approached it, another workman came running up.

* * * * *

Dick stood aside in an embrasure in the wall partly occupied by a
machine. The man passed within two feet of him and never saw him. Only
then did Dick quite realize that he was actually invisible.

The moment the man had passed him, Dick ran to the staircase. He
descended one flight; he was half way down another when a yell of pain
and imprecation came to his ears. He knew that voice: it was Luke
Evans's!

With three bounds Dick reached the bottom of the stairs. He saw a
large room in front of him. No mistaking the nature of this room; it
was an ordinary laboratory, fitted out with the greatest elaboration,
and divided into two parts by paneling. And sight and sound were on.

In the part nearer Dick three men were grouped about a large dynamo,
which was sending out a high, musical note as it spun. Levers and
dials were all about it, and above it was the base of the glass tube
that Dick had seen above. In