y--he's a clever devil. I wonder if you wouldn't take
this compass and watch him yourself tonight, just on general principles?
Or let me do it. I'd be glad to. I say 'tonight' because if he did get
the stuff here he didn't deliver it anywhere last night. It's just a
chance, of course, but he may do it tonight."

After the compass had been explained to the detective he gladly
consented to the plan, declaring that he would willingly spend the time
just to watch such an unheard-of instrument work. After another hour of
fruitless discussion Prescott took his leave, saying that he would mount
an impregnable guard from that time on.

Late that evening Prescott joined the two men who were watching
DuQuesne's house. They reported that all was perfectly quiet, as usual.
The scientist was in his library, the instruments registering only the
usual occasional faint sounds of a man absorbed in study. But after an
hour of waiting, and while the microphones made a noise as of rustling
papers, the needle of the compass moved. It dipped slowly toward the
earth as though DuQuesne were descending into the cellar, but at the
same time the shadow of his unmistakable profile was thrown upon the
window shade as he apparently crossed the room.

"Can't you hear him walk?" demanded Prescott.

"No. He has heavy Turkish rugs all over the library, and he always walks
very lightly, besides."

* * * * *

Prescott watched the needle in amazement as it dipped deeper and deeper,
pointing down into the earth almost under his feet and then behind him,
as though DuQuesne had walked beneath him. He did not, could not,
believe it. He was certain that something had gone wrong with the
strange instrument in his hand, nevertheless he followed the pointing
needle. It led him beside Park Road, down the hill, straight toward the
long bridge which forms one entrance to Rock Creek Park. Though
skeptical, Prescott took no chances, and as he approached the bridge he
left the road and concealed himself behind a clump of trees, from which
point of vantage he could see the ground beneath the bridge as well as
the roadway. Soon the bridge trembled under the weight of a heavy
automobile going toward the city at a high rate of speed. He saw
DuQuesne, with a roll of papers under his arm, emerge from under the
bridge just in time to leap aboard the automobile, which slowed down
only enough to enable him to board it in safety. The detective