Page:Handbook for Boys.djvu/85

64 Also, "The Jack of All Trades." By Dan C. Beard, Scribner's; and "Field and Forest Handy Book."

Measuring Distances

The height of a tree is easily measured when on a level, place by measuring the length of its shadow, then coming that with your own shadow, or that of a ten-foot pole.

Thus, the ten-foot pole is casting a fifteen-foot shadow, and the tree's shadow is one hundred and fifty foot long, apply

15 : 150 :: 10 : x = 100

But it is seldom so easy, and the good old rue of the triangle can be safeely counted on: Get a hundred or more feet from your tree on on open ground, as nearly as possible on the level of its base. Set up a ten-foot pole (A B, page 65). Then mark the spot where the exact line from the top of the tree over the top of the pole touches the ground (C).

Now measure the distance from that spot (C) to the foot of the ten-foot pole (B); suppose it is twenty feet. Measure also the distance from that spot (C) to the base of the tree (D); suppose it is one hundred and twenty feet, then your problem is:

20 : 10 :: 120 : x = 60

i.e., if at that angle twenty feet from the eye gives ten feet elevation, one hundred and twenty feet meet give sixty.

To make a right angle, make a triangle whose sides are exactly six, eight, and ten feet or inches each (or multiples of these). The angle opposite the ten must be a true right angle.

There are many ways of measuring distance across rivers, etc., without crossing. The simplest perhaps by the equilateral triangle. Cut three poles of exactly equal length; peg them together into a triangle. Lay