w. He's armed, and he means business."

* * * * *

Bill and Walter crawled under the shelter of the rocks to a short
distance away and then, rifles ready, advanced to the attack. A report
came from the hollow and a bullet whined over Bill's head. Almost
instantly a crack came from Walter's rifle and splinters flew from the
building in the hollow a few inches from a loophole, through which
projected the barrel of a rifle.

The rifle barrel swung rapidly in a circle and barked in Walter's
direction; but as it did so, Bill's gun spoke and again splinters flew
from the building.

"Good work!" ejaculated Dr. Bird as he watched the slow advance of the
two guides. "If we just had rifles we could join in the party, but it's
a little far for effective pistol work. Let's go ahead, and we may get
close enough to do a little shooting."

Pistols in hand, Carnes and the doctor crawled over the crest and joined
the advance. Again and again the rifle spoke from the hollow and was
answered by the vicious barks of the rifles in the hands of the guides,
Carnes and the doctor resting their pistols on rocks and sending an
occasional bullet toward the loophole. The conditions of light and the
moving target were not conducive to good marksmanship on the part of the
besieged man, and none of the attackers were hit. Presently Walter
succeeded in sending a bullet through the loophole. The rifle barrel
suddenly disappeared. With a shout the four men rose from their cover
and advanced toward the building at a run.

As they did so an ominous whirring sound came from the apparatus in
front of the house and a sudden chill filled the air.

"Back!" shouted Dr. Bird. "Back below the hill if you value your
lives!"

He turned and raced at full speed toward the sheltering crest of the
hill, the others following him closely. The whirring sound continued,
and the concave reflector turned with a grating sound on its gears. As
the path of its rays struck the ground the rocks became white with frost
and one rock split with a sharp report, one fragment rolling down the
slope, carrying others in its trail.

* * * * *

With panic-stricken faces the four men raced toward the sheltering
crest, but remorselessly the reflector swung around in their direction.
The intense cold numbed the racing men, cutting off their breath and
impeding their efforts for speed.

"Stop!" cried the doctor suddenl