250 WAVES OF SAND AND SNOW 



of a wheel or roller upon a projecting stone is pro- 

 portional 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, I presume, as the 

 square of the speed. Hence the effect of slow, 

 heavy wheeled traffic and of light, rapid traffic will 

 be different in respect to the production of ruts 

 and of transverse inequalities respectively, the latter 

 being produced chiefly by rapid driving. The 

 difference would be more noticeable but for the 

 fact that vehicles used for rapid driving are gener- 

 ally provided with better springs than the heavy 

 wagons which go mostly at a foot's-pace. 



While these matters deserve attention, it must 

 be well understood that wheeled traffic does not 

 produce large and regular undulations like those 

 formed by sledges. 



A slight transverse ufidulation in footpaths, 

 such as the Broad Walk in Kensington Gardens, 

 has not necessarily any connection, except that of 

 form, with waves. This path is highest in the 

 centre, so that rain runs off transversely to its 

 length, making transverse grooves. The places 

 first lowered are thus kept damper, and therefore 

 softer, than the intervening spaces, so that the 

 hollows wear away more readily than the convexi- 



