OV EAISING AND REMOVING WEIGHTS. Sl/ 



g-reater proportion of the load is thrown on the lower wheel, its spokes, being 

 then in a vertical position, are able to exert all their strength with advantage. 

 The axles being a little conical, in order that they may not become loose, or 

 may easily be tightened as they wear, it is necessary that they should be bent 

 down, so that their lower surfaces may be horizontal, otherwise the wheels 

 would press too much on the linch pin. For this reason, the distance between 

 the wheels should be a little greater above than below, and their surfaces of 

 course slightly conical. (Plate XVIII. Fig. 2Q8.) 



It has been proposed to fix the wheels to their respective axles, to continue 

 the axles as far as the middle of the carriage only, and to cause them to turn oa' 

 friction wheels or rollers; a plan which may succeed if the apparatus is not 

 too complicated for use; but in fact the immediate friction on the axles is not 

 great enough to render this refinement necessary. If both opposite wheels 

 were fixed to a single axis, one of them would be dragged backwards and the 

 other forv/ards, whenever the motion deviated from a straight line; and a -si- 

 milar effect actually takes place in those carriages which are supported on a 

 single roller. 



The effect of the suspension of a carriage on springs is to equalise its motion, 

 b]^ causing every change to be more gradually communicated to it, by means 

 of the flexibility of the springs, and by consuming a certain portion of every 

 sudden impulse in generating a'degree of rotatory motion. This rotatory mo- 

 tion depends on the oblique position of the straps suspending the carriage, 

 which prevents its swinging in a parallel direction; such a vibration as would 

 take place if the straps were parallel, would be too extensive, unless they 

 were very short, and then the motion would be somewhat rougher. The ob- 

 liquity of the straps tends also in some measure to retain the carriage in a ho^ 

 rizontal position : for if they were parallel, both being vertical, the lower 

 one would have to support the greater portion of the weight, at least accord- 

 ing to the common mode of fixing them to the bottom of the carriage, the 

 spring, therefore, being flexible, it would be still further depressed. But 

 when the straps are oblique, the upper one assumes always the more vertical 

 position, and consequently bears more of the load ; for when a body of any 

 kind is supported by two oblique forces, their horizontal thrusts must be 

 equal,, otherwise the body would move laterally; and in order that the hori- 



VOL. I. F f 



