reflectively when he had finished his
computations. "This darned instrument is still so sensitive that you may
have merely focused on a deep shadow or a cold spring or something of
that sort, but the magnet kicked clear around and it may mean that we
have located what we are looking for. It should be about two miles away
and almost due west of here."

"There is no spring that I know of, Doc, and I think I know of every
water hole in this country," remarked Bill.

"There could hardly be a spring at this elevation, anyway," replied the
doctor. "Maybe it is what we are seeking. We'll start out in that
direction, anyway. Bill, you had better take the lead, for you know the
country. Spread out a little so that we won't be too bunched if anything
happens."

* * * * *

For three-quarters of an hour the little group of men made their way
through the wilderness in the direction indicated by the doctor.
Presently Bill, who was in the lead, held up his hand with a warning
gesture. The other three closed up as rapidly as cautious progress would
allow.

"What is it, Bill?" asked the doctor in an undertone.

"Slip up ahead and look over that crest."

The doctor obeyed instructions. As he glanced over he gave vent to a low
whistle of surprise and motioned for Carnes to join him. The operative
crawled up and glanced over the crest. In a hollow before them was a
crude one-storied house, and erected on an open space before it was a
massive piece of apparatus. It consisted of a number of huge metallic
cylinders, from which lines ran to a silvery concave mirror mounted on
an elaborate frame which would allow it to be rotated so as to point in
any direction.

"What is it?" whispered Carnes.

"Some kind of a projector," muttered the doctor. "I never saw one quite
like it, but it is meant to project something. I can't make out the
curve of that mirror. It isn't a parabola and it isn't an ellipse. It
must be a high degree subcatenary or else built on a transcendental
function."

He raised himself to get a clearer view, and as he did so a puff of
smoke came from the house, to be followed in a moment by a sharp crack
as a bullet flattened itself a few inches from his head. The doctor
tumbled back over the crest out of sight of the house. Bill and Walter
hurried forward, their rifles held ready for action.

"Get out on the flanks, men," directed the doctor. "The man we want is
in a house in that hollo