﻿174 DE. H. C. SORBT ON THE APPLICATION OF Pfay I908, 



III. Angles of Eest of Sand A^'D of S^iall Pebbles. 



Sand. 



A knowledge of the angle of rest of* different varieties of sand, 

 under various conditions, is of great interest, since its sine is a 

 simple measure of the friction which would otherwise be difficult 

 to ascertain, and must have had a preponderating iufluence in 

 connexion with the drifting forwards and deposition of the material 

 of manj' rocks. It sums up the general effect of the density, size, 

 and shape of the grains, the character of their surface, and the 

 relative lubricating influence of the superficial layer of water. 



I made many experiments in a glass trough, so that I could 

 measure the angle of rest under different conditions. One very 

 important point is that the angle at which moving sand stops and 

 accumulates differs materially from that at which it gives way, 

 after having become stationary ; and this explains many important 

 facts. 



Experimenting with the coarse angular sand of the Millstone 

 Grit, washed to get rid of the decomposed felspar, and sieved so 

 that the grains varied in size from about -03 to -07 inch, and 

 averaged about '05, I found that the angle of rest in water was 

 about 41" when coming to rest, but about 49° when giving way 

 after being at rest. In the case of sand varying in size from -005 

 to '020 inch, and averaging about -010, the angle when coming to 

 rest was about 34°. and after being at rest it gave way at 36°, In 

 the ease of very fine-grained sand from Alum Bay, in which the 

 grains varied from -001 to -005, and averaged about -003 inch, 

 the angles were respectively 30° and 33°. It is thus clear that, to 

 a slight extent, the fine sands acted more as a liquid, the difference 

 between the angles being 2° or 3° instead of 8°, When the above- 

 mentioned coarse and fine sands were mixed in equal quantities, it 

 was difficult to prevent separation, since the coarse grains ran down 

 over the fine, and accumulated at a higher angle than the fine ; but, 

 by using great care to prevent this, I found that the angles were 

 about 34° (or the same as for the fine sand named above) and 38°, 

 or 2° more than when no coarse sand was present. 



Pebbles. 



A rounded quartz-pebble measuring -4 by '25 inch easily ran down 

 a slope of the coarse Millstone- Grit sand at 2d°, and came to rest 

 when it was at about 20°. It scarcely sank at all into the surface 

 of the sand. In the case of the finer sand, the pebble just ran down 

 at 20°, but did not sink much. When in water the very fine Alum- 

 Bay sand is extremely soft and mobile, so that even a pebble 

 measuring only '2 by '15 inch half sank into it, and would not run 

 down the surface until the whole gave way. All these facts agree 

 well with the supposition that the thin film of water adhering to 

 the grain has more and more effect on the properties of the material 



