Page:Popular Science Monthly Volume 89.djvu/711

Rh A Simple Instrument Which Measures the Height of a Tree

XPERIKNCED foresters become

expert in judging oft-hand the height of average timber. By merely looking at a tree of medium height they are able to make a guess with a degree of accuracy that is sufficient for roughly estimating the number of board feet of lumber it contains. But in the case o timber two hundred feet or more in height even the most expert woodsman is apt to make an error of from ten to twenty feet in his calculations. Errors of this kind greatly impair the value of an estimate of the amount of timber in a tract that perhaps is measured by hundreds of acres.

By the use of the hypsometer, however, the height of the tallest of timber can be ascertained with a remarkable degree of accuracy. There are a number of types of hypsometers, but all of them operate on about the same principle.

One that has been adopted by the Forest Service consists of a round instrument about four or five inches in diameter and an inch in thickness. To one side is attached a convenient handle. Within the instrument there is a device which operates on the principle of a pendulum. Attached to this is a celluloid scale. On the outer edge of the instru- ment is a small peep-hole on one side and opposite it a square window-like opening somewhat larger in size. There is a convenient device for securing the pendulum at a fixed position.

To ascertain the height of a tree by means of the hypsometer, the operator takes a sight at the proper point on its " trunk by looking through the peep- hole and out through the open- ing on the opposite side of the instru- ment. Three differ- ent heights figure in timber calcula- tions. One is calk'd the merchantable jhe dog is harnessed length. In making structed and equipped

certain estimates the "clear" length is ascertained, while in other instances consideration is given only to the height from the ground to the first limb of appreciable size.

The height of the tree governs the angle at which the operator holds the hypsometer in taking the sight.

In His Merry "Dogmobile." How a Cripple Gets Along in the World

LTHOUGH he is a cripple, R. A. Burdick of Los Angeles, has traveled nearly five thousand miles in a single year with the aid of his dog Trix.

The dog is harnessed to a specialy built tricycle equipped with handle-bars, headlight, tail-light and a seat similar to that used on wheel-chairs.

He sells newspapers and chewing gum. When he wishes to stop he presses a leather pad to the wheel, and the dog stops immediately.