in the ground. With a transit he took the
bearing of the path and laid it out on a large scale map.

"We'll stop him between Marr and Ritchie," he announced. "Jones, I am
going back and set up my apparatus. Keep track of his movements. If
he changes direction, let me know at once."

* * * * *

The doctor's car tore off to the west. Near Upper Marlboro, he met the
convoy of trucks and led them to the selected spot. The trucks were
unloaded and the apparatus laid out. Attached to a huge transformer
were a dozen strange-looking projectors. What puzzled Carnes most was
a huge built-up steel bar wound about with heavy cable. Dr. Bird had
this bar erected on a truck and located it with great exactness. The
projectors were set up in a battery just east of the bar.

"How about power?" asked the doctor.

"We'll have it in five minutes," replied one of the men. "A power
transmission line carrying twenty-two thousand passes within two
hundred yards of here. We are phoning now to have the power cut off.
As soon as the line is dead we'll cut it and bring the ends here."

The electrician was good at his word. In five minutes the power line
had been cut and cables spliced to the ends. The cables were brought
to the doctor's apparatus and the main lines were rigged to the ends
of the cable wound around the bar. In parallel on taps, the projectors
were connected. Huge oil-switches were placed in both lines.

"All ready, Doctor," reported the electrician.

"Good work, Avent. He'll be here soon, I fancy."

A car whirled up and a man leaped out with a surveyor's rod. He set it
up on the ground while a companion watched through binoculars. He
moved it a hundred yards to the north and then back twenty. When he
was satisfied he turned to Dr. Bird.

"The direction of movement has not changed," he said. "The path will
pass under this stake."

Under the doctor's supervision, the truck carrying the bar moved
forward until it stood over the surveyor's stake. The battery of
projectors moved to a new location a few feet east of the rod. Other
cars came racing up.

"He's less than half a mile away, Doctor!" cried Jones.

"Get your electroscopes out and spot him a hundred yards from this
truck."

"Very well, Doctor."

* * * * *

The men with the instruments spread out along the path of the borer.
Briskly they rubbed their sleeves with the rubber rods and charged
their i