y. "Fire at that reflector! It's our
only chance!"

He set the example by turning and emptying his pistol futilely at the
turning mirror. Bill, Walter and Carnes followed his example. Nearer
and nearer to them came the deadly ray. Bill was the nearest to its
path, and he suddenly stiffened and fell forward, his useless gun still
grasped in his hands. As his body struck the ground it rolled down hill
for a few feet, the deadly ray following it. His head struck a rock, and
Carnes gave a cry of horror as it broke into fragments.

Walter threw his rifle to his shoulder and fired again and again at the
rotating disc. The cold had became intense and he could not control the
actions of his muscles and his rifle wavered about. He threw himself
flat on the ground, and, with an almost superhuman effort, steadied
himself for a moment and fired. His aim was true, and with a terrific
crash the reflector split into a thousand fragments. Dr. Bird staggered
to his feet.

"It's out of order for a moment!" he cried. "To the house while we
can!"

As swiftly as his numbed feet would allow him, he stumbled toward the
house. The muzzle of the rifle again projected from the loophole and
with its crack the doctor staggered for a moment and then fell. Walter's
rifle spoke again and the rifle disappeared through the loophole with a
spasmodic jerk. Carnes stumbled over the doctor.

"Are you hit badly?" he gasped through chattering teeth.

"I'm not hit at all," muttered the doctor. "I stumbled and fell just as
he fired. Look out! He's going to shoot again!"

The rifle barrel came slowly into view through the loophole. Walter
fired, but his bullet went wild. Carnes threw himself behind a rock for
protection.

* * * * *

The rifle swung in Walter's direction and paused. As it did so, from the
house came a strangled cry and a sound as of a blow. The rifle barrel
disappeared, and the sounds of a struggle came from the building.

"Come on!" cried Carnes as he rose to his feet, and made his stumbling
way forward, the others following at the best speed which their numbed
limbs would allow.

As they reached the door they were aware of a struggle which was going
on inside. With an oath the doctor threw his massive frame against the
door. It creaked, but the solid oak of which it was composed was proof
against the attack, and he drew back for another onslaught. From the
house came a pistol shot, followed by a des