102 WHEEL CARRIAGES. 



The advantage of wheels over sledges may be 

 understood from the following considerations. A 

 sledge, in sliding over a plane, suffers a friction 

 equivalent to the distance through which it moves ; 

 but if we apply to it an axle, the circumference of 

 which is six inches, and wheels of eighteen feet in 

 circumference, it is clear, that when the carriage 

 moves eighteen feet over the plane, the wheels 

 make but one revolution ; and as there is no slid- 

 ing of parts between the plane and the wheels, but 

 only a mere change of surface, no friction can 

 take place there, the whole being transferred to 

 the nave acting on the axle so, that the only slid- 

 ing of parts has been betwixt the inside of the 

 nave and the axle ; which, if they fit one another 

 exactly, is no more than six inches ; and hence it 

 is plain, that the friction must be reduced in the 

 proportion of one to thirty-six. Another advan- 

 tage is also gained; by having the surfaces confined 

 to so small an extent, by which means they may 

 be more easily kept smooth, and fitted to each 

 other. The only inconvenience is the height of 

 the wheels, which must in all cases be added to 

 that of the carriage itself. 



By means of this circulatory motion, the friction 

 becomes very much less than what it would be if 

 the weight were drawn along the ground upon a 

 sledge ; insomuch, that a four wheeled carriage 

 may be drawn with five times as much ease as one 

 that slides as a sledge upon the same surface. 



By applying wheels to a carriage, the friction is 

 lessened in the proportion of the diameters of the 

 axles and hollow parts of the naves to that of the 

 wheels. 



Large wheels have also the advantage over small 

 ones in overcoming obstacles, because they act as 



9 



