"What are you doing, Captain Sam?" asked Bob Sharp, unable to restrain
his curiosity.

"I am getting the points of the compass," replied Sam.

"Yes, but how are you a doin' it?" asked Sid Russell.

"Well," replied Sam, "I'll show you. Just before sunset yesterday I
wanted to mark my map, and I sat down right here," pointing to a spot
near the first stake, "because it was shady here. The trunk of that
big tree threw its shadow here. Now the sun does not set exactly in
the west in this latitude, but a little south of west at this time of
year. The line of a tree's shadow, therefore, at sunset must be from
the tree a trifle north of east. Now I have driven this stake"
(pointing to the first one) "just a little to the right of the middle
of the shadow, as I remember it, so that a line from the stake to the
middle of the tree-trunk must be very nearly an east and west line.
The other stake I drove merely to aid me in tracing this line. Now I
will go on with my work, explaining as I go."

Taking his pocket-rule he measured off twenty feet east and west from
his first stake, and drove a stake at each point.

"Now," he said, "I have an east and west line, forty feet long, with a
stake at each end and a stake in the middle."

This is what he had:

[Illustration]