ON SURFACE WAVES PRODUCED BY SLEDGES. 151 



the bumping of sledges soon produces a long train of waves in 

 the snow (if the weather be not too cold), and if the traffic be 

 both ways the train of waves extends in both directions. 



For those who do not live in cold countries it is advisable to 

 describe in some detail the motions of a sledge, which differ so 

 greatly from those of a cart. The gliding "runner" meets 

 obstructions at a very slight, or grazing, angle, and is deflected 

 from its course with surprising ease. If the deflection be in a 

 vertical plane the sledge pitches as a boat does. If the deflec- 

 tion be in a horizontal plane the sledge skids, i.e., swings 

 sideways, and is then apt to capsize. Both these pernicious 

 tendencies are minimised in carts by the rotation of the wheel ; 

 indeed, an experience of sledges soon convinces one that the 

 power of a rolling wheel to resist deflection, in both the 

 horizontal and vertical directions, is one of its essential 

 superiorities to runners. 



In ascending and pitching over a mound of snow the sledge 

 compresses it, arid thus consolidates the mound, if the temper- 

 ature be not too low, on account of the well-known property of 

 regelation, which enables one to make snowballs in damp 

 snow. The skidding, or swinging, of the sledge is responsible 

 for the fact that the undulations produced by it are as broad as 

 the driving way, instead of being as narrow as the runners. 



It is not only in snow that sledges produce cahots, as I learnt, 

 after my return from Canada, during a visit to Coniston, 

 September, 1901. The road from Saddlestone slate quarry on 

 the " Old Man " of Coniston is in places so steep that the 

 following system has been adopted for bringing down the 

 slate. Half the load is placed on a two-wheeled cart and 

 half is placed on a sledge, which, being hitched on behind 

 the cart, serves as a drag for the steeper parts of the track. 

 When a gentler slope is reached the sledge is run on to the 

 wheeled truck shown in Plate I. The steep parts of the 

 road down which the sledge passes are all in undulations 

 of symmetrical or nearly symmetrical form, having, like the 

 cahots in snow, rounded crests and troughs. This is the more 



