ON SURFACE WAVES PRODUCED BY SLEDGES. 157 



labour), and the material strewn in the hollows. The symmetri- 

 cal form of the ridges supports the impression of their 

 immobility, particularly as the sledge only traverses them in one 

 direction. It must not be forgotten that the condition of the 

 track varies with the weather. When the materials are 

 moderately damp the cahots would be quickly formed, and at first 

 may travel, but, dry weather supervening, the ridges cake or 

 set in a hard mass like concrete, which could not well participate 

 in a wave-like motion while in that condition. 



OTHER EXAMPLES OF TRANSVERSE INEQUALITIES OF 

 SURFACE IN ROADS AND PATHS. 



The principal inequality produced by wheeled traffic on a soft 

 road or track is, of course, the longitudinal furrow called a rut, 

 but in harder roads a sort of ripple is often produced, although I 

 have never seen the ripple, if so it deserve to be called, 

 attain to regularity. On the Macadamised ways much used in 

 the West End of London the first step in their formation seems, 

 generally, to be the kicking out of a stone by the horse's hoof. 

 The wheel soon enlarges the hole, and when the inequalities 

 thus produced are examined it is seen that they are generally a 

 succession of arcs of wheel-tyres separated by a rather large 

 stone firmly wedged. If the stones employed were smaller I 

 think the inequalities would be less troublesome. In wood 

 pavements if an initial inequality be caused by a bad, soft, block, 

 or by taking up and re-laying, this is speedily multiplied in both 

 directions by the bumping of the wheels. 



The action of a wheel to knock holes in a stony road is 

 exactly the reverse of that of a roller employed for smoothing 

 out the inequalities of paths. The latter, however, is drawn 

 slowly. It is easy to see that the depressing effect of a wheel or 

 roller upon a projecting stone is proportional to its weight and 

 to the time it remains upon the excrescence that is to say it 

 is inversely proportional to the speed, whereas the impact 

 against a projecting stone will increase when the speed increases 



