84 THE HABITAT 
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 de- 
grees, 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 com- 
pass may be obtained from the Keuffel and Esser Company, 111 Madison 
St., Chicago, Illinois, for $5. 
ig. 28. Combined clinometer and compass. 
123. The trechometer. For measuring the effect of slope upon run-off, 
a simple instrument called the trechometer (tpéxo, to run off) has been 
devised. This consists merely of a metal tank, 3 x 4 x 12 inches, hold- 
ing 144 square inches of water, with an opening 1% x 12 inches at the base 
in front, closed by a tight-fitting slide. Three metal strips, 2 x 12 inches, 
are fastened to the front of the tank in such a way as to enclose a square 
foot of soil into which the strips penetrate an inch. In the front strip is an 
opening, I inch square, provided with a drip from which the run-off. is 
collected in‘a measuring vessel. In use, the instrument is put in position 
with the metal rim forced down 1 inch into the soil; the tank is filled, the 
graduate put in place, and the slide raised. The run-off for a square foot 
is the amount of water caught by the graduate, and is represented in 
cubic inches per square foot. For obtaining results which express slope 
alone, comparisons must be made upon the same soil, from which all cover, 
dead and living, has been removed. They must be as closely together in 
time as possible, at least during the same day, as rain or evaporation: will 
