Now, I'll set the surveying instrument with the plumb-bob right over the
end of this line, and sight through the two sight holes until I bring the,
two vertical hairs in line with each other and the tree. Look at the
compass needle. It points to the 173 degree mark on the cardboard ring.
Now, Bill, you hold the rod at the other end of our base line while I
swing this instrument around and sight it. There, the needle points to 92
degrees, and subtracting this from 173 the difference, 81 degrees, is the
angle at the right end of our base line. We'll do the same thing at the
other end of our line. See, the compass needle points to 189 degrees, and
now sighting to the pole at the other end of the line we find that the
needle points to 268. The difference, 79 degrees, is therefore the size of
the angle at the left end of our base line. Now we will draw this out on
paper, as we did our first triangle, using quarter-inches to represent
feet. Our base line was 10 feet long, and we will therefore draw a line 10
quarter-inches, or 2-1/2 inches long, on our drawing board. On this line
we will construct the triangle, using the angles 81 and 79 degrees. There,
that's how our triangle looks, and the right hand side measures 7-1/4
inches, while the left hand side measures 7-5/16 inches. That is, 29
quarter-inches for one side and 29-1/4 quarter inches for the other. As
each quarter-inch represents a foot, you will find that the tree is about
29 feet from the right end of our base line and 29 feet 3 inches from the
left hand end. Of course, our instrument is not perfect, neither is our
drawing; but if you measure it off with the chain you will see that I am
not very far from correct."

[Illustration: Fig. 84. Determining the Distance to the Tree.]



Mapping the Island.