140 PROCEEDINGS OP THE AMERICAN ACADExMY 



the instrument. The board bearing the thermograph is moved about 

 the pin by drawing upon a cord, and is brought back to place by an 

 opposed spiral spring. Should it not be desired that the thermograph 

 remain continuously exposed to the radiation from the rock, it may be 

 turned away from it by a pull upon the cord. AVhen a reading is to be 

 made, the cord being released, the spiral spring draws back the board, 

 wliich is stopped at the proper position by a pin fixed in the table. 

 The rock under examination is given a flat surface by grinding it 

 upon a sheet of glass with emery. The sun's rays are directed upon 

 it by a heliostat, the diameter of the solar beam being limited to 

 26 mm. by a circular diaphragm. 



On November 4, 1888, a slab of white marble was made the subject 

 of experiment. The distance from the slab to the thermograph strip 

 was 217 mm. The mean deflection obtained was 82.5 scale divisions. 

 Had the marble slab reflected perfectly in every direction the solar 

 rays that fell upon it, the ratio of the radiation received at the thermo- 

 graph strip to the solar radiation vrould have equalled the ratio of 

 the area of the cross section of the solar beam employed to the area 

 of the hemisphere of which the distance from the marble to the ther- 

 mograph strip is the radius ; or, 



82.5 ttIB^ 



Solar Radiation o ^ 217^ 



From which we find. Solar Radiation = 45980 scale divisions, upon 



the supposition of perfect reflection by the marble. 



The thermograph was next placed in the direct solar beam, with a 



resistance in the galvanometer circuit, and the true solar radiation was 



found to be measured by 84170 srale divisions of deflection. Hence, 



45980 

 of the solar rays falling upon it, the marble slab reflected grp^ part, 



or 54.6%. In the same manner, it is found that a surface of black 

 slate reflects 38% of the solar rays. 



We learn, then, that of the solar rays falling upon rocks a large 

 portion is absorbed and conducted through the mass of the rock, the 

 remaining portion being reflected. In both of the cases given, the 

 sunlight was flashed upon the rock when the reading was to be made, 

 the rock remaining, practically, at the temperature of the room. It 

 appears, therefore, that the solar rays are about equally divided into 

 two portions, one of which is reflected directly, the other absorbed, 

 conducted through the mass of the rock, and radiated in long waves. 

 If the rocks were in large masses, and continuously acted upon by the 



