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 effect 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 1 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 
