156 ON StTRFACE WAVES tRODUCEb feY SLEDGES. 



secret of the spontaneous formation of cahots had, therefore, 

 been the capacity of certain fragmentary or semi-granular 

 materials (e.g., damp road-stuff and damp snow) to cohere or 

 bind under pressure. 



The action of the sledge itself, which is more readily apparent, 

 I have already explained ; but it will be convenient to repeat the 

 explanation as follows : Meeting obstructions at a very small 

 angle, it is very easily tilted in a vertical sense and caused to 

 pitch. This exercises a rolling-out or compressing effect 

 upon the obstruction, and also causes the prow to meet the 

 track at such an angle as to furrow it more deeply to "leeward" 

 of the excrescence. So much is simple, but it is not so obvious 

 why a sledge going on runners should produce ridges extending 

 the whole width of the roadway, as in the case of the tract from 

 the Saddlestone Quarry. The reason, however, soon becomes 

 evident, when one walks behind the sledge and watches how it 

 swings or skids. The runners, in fact, are deflected laterally, 

 with even greater ease than vertically, and this swinging or 

 skidding tendency is one of the essential inferiorities of 

 runners as compared with wheels. 



The undulations produced by dragging the iron mould back- 

 wards and forwards over the same ground averaged 2ft. 8*52 

 inches in length from crest to crest, the " immersed " length of 

 the sledge being 9-25 inches. The quotient is 3-52 that is to 

 say, the " wave " is three and a-half times as long as the sledge. 

 The cahots on the Quarry Track (see page 152), were three and 

 a-quarter times as long as the sledge. 



I wished to ascertain if the ridges on the Quarry Track 

 travelled down-hill, and I put in iron pegs to mark their 

 position, hoping that observations could be made after my 

 departure ; but the pegs could not then be found. The opinion 

 of Mr. John Mandal, of Mandal's Slate Quarry, Ltd., is that 

 these ridges, when developed, do not travel ; he says they 

 become " too hard to move, being all as if crusted and cemented 

 together." The road, he informs me, has to be re-made from 

 time to time, the ridges being " hacked up " (with considerable 



