214 
W. A. CANNON 
the amount of heat faUing upon the surface of the soil decreases with 
the decrease of its angle to the incident rays. The theoretical efficiency 
of different exposures, for several angles, is given in figure i. It should 
be noted, however, as before suggested, that the amount of heat actu- 
ally affecting the soil may hold different relations from that shown in 
the figure, for various reasons, some of which have been presented. In 
addition to these, it can be said, that the angle with the vertical is of 
itself important as a factor in that it directly affects convection currents. 
2 The general truth of the temperature-angle relation can be easily demonstrated, 
in a manner suggested by Prof. A. E. Douglas, University of Arizona, in the following 
experiment. Take a piece of heavy pine board, about 4X6 inches in size, and bore 
a shallow hole in the middle of one side. Connect this hole by a groove to one end. 
The groove is to carry the thermometer, whose bulb rests in the hole above mentioned. 
Fasten the board to uprights in such a manner as will permit it to swing on a hori- 
zontal axis, which, if continued, would reach the bulb of the thermometer. To the 
right hand upper corner attach a semicircular piece of white cardboard, ruled 
radially giving angles of 15°, 30°, and 45°. A nail is driven in the axis of the semi- 
circle and allowed to project so that its shadow, falling on the cardboard, will give 
the angle of exposure desired. A thin sheet of copper is fastened over one side of the 
pine board, covering the thermometer. Finally, the wooden frame is enamelled 
white, and the copper sheet is painted black. 
In order to check the observations made with the exposure pyrometer a black 
bulb thermometer, in vacuum, is used, and is read alternately with the other 
apparatus. 
In practice, the test is made as follows. All instruments are allowed to come to 
shade temperature. Exposures to the sun of one minute each are made, and the 
instruments are allowed to return to the shade temperature, or close to it. The order 
of exposures, in the illustration given below, are as follows: i, black bulb thermom- 
eter; 2, temperature at an angle of 90°; 3, black bulb; 4, temperature at angle of 
60°; 5, black bulb; 6, temperature at angle of 30°; 7, black bulb; 8, temperature at 
angle of 90°; 9, black bulb. 
The following data present results of a test on Feb. 6, 1915. The temperature 
rise in intervals of one minute and the time of each exposure are given. 
Time 
90° 1 60° 
30° 
90° 
Black Bulb 
10:44 
5.1° c. 
10:47 
2.2° C. 
10:49 
1.8° c. 
10:55 
5.5° ' 
10:58 
1.1° c. 
5-3° 
II :02 
II :o6 
2.3° 
11.08 
5-3° 
^ The second reading of the black bulb thermometer was omitted. 
