Then he walked away twenty or thirty paces and drove another stake, sighting from one to the other, and taking pains to get them in line with a tree which stood at a little distance from the first stake.
"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.