PHYSIOGRAPHY 85 



cause considerable error. It is obvious that with the same slope or on a 

 level the trechometer may also be used to advantage to determine the ab- 

 sorptive power of soils of different texture. It serves well a similar pur- 

 pose when used in different habitats to measure the composite action of 

 slope, soil, and cover in dividing the rainfall into run-off and absorbed 

 water. 



Exposure 



124. Exposure refers primarily to the direction toward which a slope 

 faces, i. e., its exposition or insolation with respect to sun and wind. It is 

 not altogether separable from slope, however, inasmuch as the angle of the 

 slope has some eft'ect upon the degree of exposure. The chief influence of 

 exposure is exerted through temperature, since slopes longest exposed to 

 the sun's rays receive the most heat. This is supplemented in an important 

 degree by the fact that a group of rays i foot square will occupy this area 

 only on slopes upon which they fall at right angles. In all other cases the 

 rays are spread over a longer area, with a consequent reduction in the 

 amount of heat received. This effect is felt principally in evaporation from 

 the soil, and in soil temperatures. For the leaf, it is largely if not entirely 

 negligible, since the angle of incidence is determined by the position of 

 the leaf, which is the same for each species whether on the level or upon 

 a slope. On this account, exposure has little or no bearing upon light, 

 except that the total amount of light received by the aggregate vegetation 

 of a slope will be greater than for a level area of the same size. The 

 effect of wind varies with the exposure. It is naturally most pronounced 

 in those directions from which the prevailing dry or cold winds blow, and 

 it is greatly emphasized by the fact that the opposite exposure is corres- 

 pondingly protected. The influence of wind, especially in producing 

 evaporation from the plant and the soil, increases with the slope, since 

 the mutual protection of the plants, or that afforded the soil by the cover, 

 is much reduced. Finally, . the distribution of the snow by the wind, a 

 matter of considerable importance for early spring vegetation, is largely 

 determined by exposure. 



Exposure is expressed directly in terms of direction, to which is added 

 the angle of the slope. A good field compass, reading to twelve points, is 

 sufficient. It should be checked, of course, by the declination of the needle 

 at the place under observation. A convenient instrument is the one already 

 mentioned, in which compass and clinometer are combined, since these are 

 regularly used at the same time. 



125. Surface. The most important consideration with respect to surface 

 is the presence or absence of cover, and the character of the latter. With 



