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 instnmient 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, iii Madison 

 St., Chicago, Illinois, for $5. 



Fig. 28. Combined clinometer and compass. 



123. The trechometer. For measuring the effect of slope upon run-off, 

 a simple instrument called the trechometer (rpixf^, 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 )4 ^ 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 i 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 



