ON RAISING AND REMOVING WEIGHTS.

217

'greater proportion of the load is thrown on the lower wheel, its spokes, beingthen in a vertical position, are able to exert all their strength with advantage.r lhe axles being a little conical, in order that they may not become loose, orway easily be tightened as they wear, it is necessary that they should be bentdown, so that their lower surfaces may be horizontal, otherwise the wheelsWould press too much on the lincli pin. For this reason, the distance betweenthe wheels should he a little greater above than below, and their surfaces ofcourse slightly conical. (Plate XVIII. lig. 5228.)

It has been proposed to fix the wheels to their respective axles, to continuethe axles as far as the middle of the carriage only, and to cause them to turn onfriction wheels or rollers; a plan which may succeed if the apparatus is notfro complicated for use; but in fact the immediate friction on the axles is notgreat enough to render this refinement necessary. If both opposite \i iewere fixed to a single axis, one of them would be dragged backwards and theother forwards, whenever the motion deviated from a straight line, ant a similar effect actually takes place in those carriages which are supported on asingle roller.

j ^ le effect of the suspension of a carriage on springs is to equalise its motion,y Causing every change to be more gradually communicated to it, by meansfr le flexibility of the springs, and by consuming a certain portion of everysadden impulse in generating a’degree of rotatory motion. This rotatory mo-1() n depends on the oblique position of the straps suspending the carriage,' v hich prevents its swinging in a parallel direction; such a vibration as woulde place if the straps were parallel, would be too extensive, unless they| v cre very short, and then the motion would be somewhat rougher. The ob-Tuty of the straps tends also in some measure to retain the carriage in a ho-Il? '°ntal position : for if they were parallel, both being vertical, the lower? Ue would have to support the greater portion of the weight, at least accord-Ul g to the common mode of fixing them to the bottom of the carriage, the^ Ul 8 'i therefore, being flexible, it would be still further depressed. Buten the straps are oblique, the upper one assumes always the more vertical

. _ _ , , 11 . J '

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

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

ec l Ua h otherwise the body would move laterally; and in order that the hon-

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i.