MOVEMENT OF SCREE-MATIIRIAL. 235 



Ea'perinient 2. — In this experiment, the object was to determine 

 the actual distance which a stone of given length lying on a given 

 slope will descend in a given time. 



Two slabs of a fine-grained sandstone (called York stone by the 

 mason) were cut, each 3 feet long, 5 inches broad, and 2 inches 

 thick. One face and one side of each were smootJked. One of the 

 stones was fixed, so that its upper surface sloped at an angle of 17'' 

 towards the south. The other was placed on this, the smoothed 

 faces being in contact, and the ends of both stones in line at start- 

 ing. Fine scratches were cut in the same straight line on the 

 smooth sides of the stones, in the middle and at either end, their 

 subsequent displacement determining the amount of the movement. 

 The whole was well protected by a light wooden framework covered 

 witii wire-netting. It was exposed to sunshine for about five hours 

 a day at the commencement of the experiment, diminishing to about 

 two hours a day at the end. Headings were, as a rule, taken once 

 a week, at the same hour in the afternoon, and, except at these 

 times, no artificial shadow was ever allowed to fall upon the stone. 



The experiment began on May 5, 1887, and ended on September 

 22. In this time the total descent was 5| mm.*, i. e. an average 

 of •00161 inch per day, or '599 inch per year. The period of the 

 experiment was naturally divided into three intervals, the first from 

 May 5 to June 9, the second from June 10 to August 25, and the 

 last from August 26 to September 22. During the first interval 

 there were 14 days on which rain fell, the sky was usually overcast 

 and on one day only was fairly free from cloud : the average daily 

 range of temperature was 12°*0 F., and the average daily descent 

 of the stone -00187 inch. The second interval was remarkable for 

 its prolonged summer weather, rain fell but seldom, and the sky was 

 cloudless for days together : the average daily range of temperature 

 for the first eight weeks of this interval was 18°-3 F., and the aver- 

 age daily descent for the whole interval '00119 inch. During the 

 last interval there were 8 days on which rain fell, and on half the 

 days the stone was frequently shaded by passing clouds : the aver- 

 age daily range of temperature was 14°-7 F., and the average daily 

 descent -00258 inch. From this experiment we may conclude 

 that : — 



1. The descent is greatest on those days on which there is bright 

 sunshine intercepted frequently by passing clouds. 



2. Eain slightly increases the rate of descent t, probably, though 

 perhaps not entirely, by diminishing the friction between the 

 stones. 



Assuming the average rate of descent throughout the year to be 

 that above given, namely, -00164 inch per day, the upper stone will, 

 by the creeping movement alone, have advanced far enough to fall 

 over the lower one after a period of about 29^ years. This is, of 



* This distance being correct to within one sixth of a millimetre, it follows 

 that the error in the average daily descent is less than -00004 inch. 



t This follows from a comparison of the movements during the first and 

 second intervals. 



e2 



