MOVEMENT OP SCREE-MATERIAL. 233 



The creeping of the lead on the roof of Bristol Cathedral through 

 a distance of 18 inches in less than two years is well known, and 

 arose, as Canon Moseley proved*, from the alternate expansions and 

 contractions of the lead during changes of temperature taking place 

 mainly downwards, being assisted by gravity in that direction. 



Let AB represent a bar of lead or other substance resting on an 

 inclined plane. When the 



temperature rises the bar 3 



expands ; but, since it re- C^ 



quires a less force to push 

 a body down, than up, an 

 inclined plane, the part, 

 AC, pushed down the ^ 



plane, is longer than the 

 part, BC, pushed up it. 

 Again, when the tempe- 

 rature falls the bar contracts, and the part, BD, pulled down the 

 plane, is longer than the part, AD, pulled up it. In both cases, 

 then, the descent of one end is greater than the rise of the other, 

 and therefore the bar, as a whole, descends. 



Let a feet be the length of the bar, ji the coefficient of friction 

 between it and the plane, a the inclination of the plane, and e the 

 coefficient of expansion of the bar for a rise of 1° ; then, if the 

 temperature rise r°, and fall subsequently by the same amount, 

 the total descent of the bar, Canon Moseley shows, will be 



aer tan a 



feetf- Hence the descent is greater the longer the bar, the greater 

 the coefficient of expansion and the range of temperature, the higher 

 the slope of the plane, and the less the coefficient of friction. 



Canon Moseley also tested his theory by a simple experiment. A 

 sheet of lead, 9 feet long and J of an inch thick, was placed on a 

 flat wooden surface inclined at an angle of 18° 32'. The average 

 daily movement, from the 16th of Pebruary until the 28th of June, 

 was '1745 inch. The movement was found to be greatest on those 

 days when there were cold winds or passing clouds, there being then 

 many changes of temperature during the day J. 



It appeared to me that, in the same way, all stones free to move 

 on the surface of screes must be slowly creeping downwards, and 

 that this might be the explanation of the fall of the stones in the 

 two instances given above. But it yet seemed desirable to make 

 further experiments with slabs of stone, instead of with lead-sheet- 

 ing : first, because the granular surfaces of rocks might offer effective 



* " The Descent of Glaciers," Roy. Soc. Proc. (1855), vol. vii. pp. 333-342. 



t Ibid. pp. 334, 335. 



+ " On the Descent of a Solid Body on an Inclined Plane when subjected to 

 Alternations of Temperature," Phil. Mag. (1869), 4th series, vol. xxxviii. 

 pp. 99-118. 



a. J. G. S. No. 174. ' R 



