826 ON THE MOVEMENT OF SCREE-MATERIAL. 



snow was nearly always driven by the wind from tlie experimental 

 stone before the middle of the day, or melted by the increasing heat 

 of the sun. 



During the 12 weeks from November 23 to February 14, the 

 average daily range of temperature was 7°'5 F., and the average 

 daily descent only -00078 inch. From February 15 to April 3, a 

 period of 7 weeks, the average daily range of temperature (during 

 aU but the first 8 days) was 8-2 F., and the average daily descent 

 (during the whole time) '00147 inch, nearly twice as great as in the 

 preceding period. I believe that this difference was chiefly, though 

 not entirely, due to the influence of snow. 



By contact with the snow the upper stone is more thoroughly and 

 quickly cooled than by contact with the air. Moreover, the lower 

 stone is directly cooled only by the air, and as the movement depends 

 on the difference of the temperatures to which the stones are at 

 any time subjected, the effect of short and repeated contact with a 

 covering of snow is evidently to increase the rate of descent. 



On the other hand, snow, when it lies thickly and for long periods, 

 prevents the stones from fully participating in the range of tempera- 

 ture to which they would otherwise be subjected, and the effects of 

 mere creeping are then reduced to a minimum. 



