excitement to the silent, breathless,
determined man in her rear, watching her as the tiger watches the
quarry, and with the same tiger spring latent in his eye. In a moment
her secret would be out, and then----

CHAPTER XIII

"_I am from headquarters_"

For just a minute Grace Lee paused before the blank door of the safe,
then she passed around to an unused speaking tube in the neighboring
wall. Halting before it, in low but distinct tones she began to sing the
famous aria from "The Magic Flute."

All agog, with eyes starting and ears alert, the man behind listened and
watched. Nothing happened.

Then came a change. Gradually her voice rose, sweet and piercing, till
it reached that famous F in alt so rarely attempted, so exciting to the
ear when fairly taken and fairly held. Grace Lee could take it, and as
it hung, sweet and deliciously thrilling in the air, Beau Johnson saw,
to his amazement, though he was in a way prepared for it, the heavy safe
door slip softly ajar. She had done it with her voice. How, he could
only vaguely guess. He was better educated than most of his class, or he
could not have understood it at all. As it was, he laid it to the
vibration caused by a certain definite note acting on some delicate
mechanism set in accord with that note, which mechanism starting another
and a stronger one gradually led up to that which drew the bolts and set
the door ajar. Whether his theory were true or not mattered little at
the moment. The event for which he waited had been accomplished and
accomplished before his eyes. To profit by it was his next thought, and
to this end he held himself ready for the spring which had laid latent
in his eyes since he first saw her advance toward the safe.

She was ignorant of his presence. This was evident from the jaunty way
she turned from the tube, still singing, but in a desultory way, which
showed that her thoughts were no longer on her music. But she was not so
engrossed that she did not see him. The moment that her face turned his
way, her eyes enlarged, her body stiffened, her whole personality took
on power and purpose and _she_ sprang more quickly than he did and shut
the safe door with one quick movement of her hand that fastened it as
securely as before. Then she drew herself up to meet his rush, a noble
figure of resolute womanhood which any other man would have hesitated to
assail. But he was proof to any appeal of this kind. She had been
quicker tha