=121. Concept.= This term is used in the ordinary sense to indicate the relation of the surface of a habitat to the horizon. Although it is a complex of factors, or rather influences several factors, these are readily determinable. The primary effect of slope is seen in the control of run-off and drainage, and consequently of water-content, although these are likewise affected by soil texture and by surface. Slope, moreover, as a concomitant of exposure, has an important bearing upon light and heat by virtue of determining the angle of incidence, and also upon wind, and, through it, upon the distribution of snow. At present, while it can be expressed definitely in degrees, it has not yet been connected quantitatively with more direct factors. This is, however, not a difficult task, and it is probable that we shall soon come to express slope principally in amount of run-off, and of incident heat. =122. The clinometer.= In the simplest form, this instrument is merely a semicircle of paper, with each half graduated from 1–90°. It is mounted on a board and placed base upward, upon a wooden strip, 2 feet long and 2 inches wide, which has a true edge. At the center of the circle is attached a line and plummet for reading the perpendicular. A more convenient form is shown in figure 28, which is both clinometer and compass. This also necessitates the use of a basing strip to eliminate the inequalities of the surface. The dial face is graduated to show inches of rise per yard, as well as the number of degrees, but the latter, as the simpler term, is preferable for ecological work. In making a reading, the basing strip is placed upon a representative area of the slope, and pressed down firmly to equalize slight irregularities. The clinometer is moved slightly along the upper edge, causing the marker to swing freely. After the latter comes to rest, the instrument is carefully turned upon its back, when the angle of the slope in degrees may be read directly. Two or three such readings in different areas will suffice for the entire habitat, unless it be extremely irregular. The clinometer with compass may be obtained from the Keuffel and Esser Company, 111 Madison St., Chicago, Illinois, for $5. [Illustration: Fig. 28. Combined clinometer and compass.]