Cutting Out a Disk.]

[Illustration: Fig. 81. The Sighting Disk]

[Illustration: Fig. 82 Nut Fastened in Block.]

We completed our outfit by making a surveyor's rod out of a straight
stick of wood about 6 feet long. A target or sighting disk was mounted
on the stick. This disk was 6 inches in diameter, and was sawed out of a
6-inch square board by making straight cuts across the corners and then
smoothing off the edge to a perfect circle with a draw-knife. The
thickness of the disk was only 1/2 inch. At the back of the disk we
fastened a block of wood with a slot cut in it to receive the rod, as
shown in Fig. 81. To hold the disk at different heights on the rod a
small bolt was used. The nut on this bolt was slipped into a hole on the
block at the bottom of the slot and held in place by driving in nails
about it, as illustrated in Fig. 82. The bolt was then passed through
the hole and threaded through the nut, with its inner end bearing
against the rod. The disk could thus be held at any desired position by
tightening up the bolt. A piece of white paper was now pasted over the
disk. The paper was marked off into quarters, and opposite quarters were
painted black so that it would be easy to sight, from a distance, the
exact center of the target.

A SIMPLE METHOD OF SURVEYING.

Of course, none of us had studied trigonometry, but Uncle Ed devised a
very simple method by which we could determine distances quite
accurately without much figuring.

"If you will tell me the length of one side of a triangle and the angles
it makes with the other two sides," said Uncle Ed, "I'll tell you the
length of the other two sides and the size of the third angle. This is
how I will do it:

[Illustration: Fig. 83. Diagram of Our First Lesson in Surveying.]

"Say the line is 6 inches long and one angle is 35 degrees, while the
other is 117 degrees. Let us draw a 6-inch straight line. This we will
call our base line. Now we will place the base edge of our protractor on
the base line with its center at the right hand end of the line. At the
37 degree mark we will make a dot on the paper so, and draw a line from
the right hand end of the base line through this dot. Now we will do the
same thing at the opposite end, making a dot at 107 degrees from the
line, and draw a line from the left hand end of the base line through
this dot.

"If we extend these lines until they intersect, we will have the
required triangle, and can measure t