Page:Popular Science Monthly Volume 10.djvu/731

Rh This is the instrument employed in the Geological Survey. It is carried in a leather case, or pocket, hung from the waist-belt on the side of the body, opposite to the hammer (Fig. 1). The directions of the dip and strike of rocks, the trend of dislocations and dikes, the line of boundaries, escarpments, and other geological features, are observed accurately, and noted on the spot at the time of observation, either on the map or in the note-book. A convenient instrument for light and rapid surveys, or reconnaissances, combines the compass and the next instrument I have to describe—the clinometer. I shall refer to it again.

The clinometer, or dip-measurer, is employed to find the angle at which strata are placed to the horizon—an important observation in the investigation of the geological structure of a country, and one having frequently a special economic value—as, for instance, when it points out the depth to which a well or mine must be sunk. Various patterns have been proposed and used for this instrument. Formerly a spirit-level was commonly employed. But, apart from the difficulty of rapid adjustment for the requirements of the field, the spirit-levels in the clinometers were apt to get broken. A much more portable and serviceable form of clinometer may be made by the geologist himself. It consists of two thin leaves of wood, each two inches broad and six inches long, neatly hinged together, so as to open out and form a foot-rule when required (Fig. 2). On the inside of one of these leaves a small brass pendulum is so fixed that when it swings freely and hangs vertically it forms an angle of 90° with the upper edge of the leaf to which



it is attached. An arc, graduated to 90° on each side of the vertical, is drawn on the wood, or on paper or brass fastened to the wood, so that when the leaf is moved on either side the exact number of degrees of inclination is shown by the pendulum on the graduated arc. The corresponding face of the opposite leaf is hollowed out just enough to let the two leaves fit closely, and keep the pendulum in its place when the instrument is not in use. This form of clinometer, made of boxwood and bound with brass, may be obtained of instrument-makers. It is light and strong, and its durability may be understood from the appearance of the instrument which I hold in my hand,