288 Correspondence — Mr. T. MeUard Reade. 



"ground moraine." I lay no claim to liaving invented it; I find, 

 indeed, that it was used by Sir Andrew Eamsay at least as far back 

 as the publication of his " Physical Geology of Great Britain." 

 Perhaps I may be permitted to commend it to Mr. Gardner as a 

 useful English equivalent for the French phrase so much in use. 



Jas. Durham. 



PACKING OP SAND GRAINS AS COMPARED WITH ROUND SHOT. 



Sir, — I am indebted to your correspondent. Prof. A. Harker, for 

 his suggestion and for recalling my attention to this subject (p. 192). 

 No doubt, as he states, round shot could be so packed as to leave 

 much less interstitial space than what would result from my hypo- 

 thetical arrangement. But even if perfect spheres of absolutely the 

 same size could be obtained and friction eliminated, they could not 

 fall together naturally in perfect " pyramidal order," i.e. each shot 

 having points of contact with twelve others ; because their arrange- 

 ment is conditioned by the packing which takes place against the 

 sides of the vessel. If on the other hand they were unconfined as 

 in a heaj), their arrangement would be one of disorder. 



To thoroughly test what practically happens, I filled a rain-gauge 

 measurer up to the mark 30 with No. 4 shot carefully put in layers, 

 and shaken to get them as close together as possible. A second rain- 

 gauge measurer being filled also up to 30 with water, I poured 

 sufficient from it among the shot to fill up the insterstices. I found 

 that 18 remained in the gauge, leaving 12 among the shot, a relation 

 of 4 to 6.^ This shows somewhat less interstitial space than my 

 hypothetical arrangement assumed, but considerably more than the 

 " pyramidal order " arrangement. It is plain to see through the 

 glass that the number of points of contact of the shots vary, and 

 leave variable open spaces in places. This shows that minute 

 differences in size, imperfection of spheroidal shape, and to a large extent the packing 

 against the sides of the vessel and friction, are disturbing elements. Some time ago 

 for the purposes of a paper on sandstones I repeated the experiment mentioned in 

 " Miniature Domes in Sand" on a larger scale, and in a somewhat different way, 

 taking great care to shake the sand well together in layers, as I afterwards did with 

 the shot. Cm'iously enough, the result was within a third decimal place of that 

 I now give for the shot. The grains are mostly well-rounded, but some of them are 

 angular, and the sizes of the grains vary considerably. It is surprising how the sand 

 will keep on packing closer and closer by shaking, whereas the shot is affected to the 

 extent of a reduction of its bulk by only -i^. The question is one of considerable 

 practical interest. I find if a trench for a sewer intersects another trench which may 

 have been filled up for years, and all in apparently homogeneous siliceous sand, the 

 filling in of the old trench discovers itself by falling into the new one, while the sides 

 cut in the ' ' growing ' ' sand remain vertical. Thus it appears that percolation of 

 rain assisted by gravity is slowly moving and packing the grains of sand until they 

 reach the point of maximum consolidation. It is in fact a natural building operation 

 which may be likened to the fitting together of rubble, shot from a cart, to form a 

 wall. Pouring water on fiUed-up sand will consolidate it, and I am in the habit of 

 having this done where a floor has to be made on filled-up sand. It is remarkable 

 how solid sand becomes in time left only to natural influences. I have frequently 

 built large houses on sandhills without failure of foundation. 



Park Coenee, Blundellsands, T. Mellakd Eeade. 



April 5th, 1884. 

 1 If this experiment were repeated with larger vessels, the proportions might diflfer more. 



