ON RAISING AND REMOVING WEIGHTS. 161 



were to act externally with their fore feet or hind feet only ; but they 

 have seldom, if ever, been applied to practical purposes. In general it is 

 advisable that walking wheels for quadrupeds should present to them a 

 path as little elevated as possible ; and it might probably be of advantage 

 to harness them either to a fixed point or to a spring or weight, which 

 would enable them to exert a considerable force even in a horizontal direc- 

 tion ; but, probably, after all, they might be more advantageously employed 

 in a circular mill- walk. (Plate XVII. Fig. 217.) 



Mr. White's crane* affords a good specimen of an oblique walking 

 wheel ; the force may be varied accordingly as the labourer stands at a 

 point more or less distant from the centre ; and in order to avoid accidents, 

 a break is always acting on the axis of the wheel by its friction, except 

 when it is removed by the pressure of the man's hand on a lever upon 

 which he leans as he walks. The force is also varied in some cranes by 

 changing the pinion which acts on the principal wheel, and an expanding 

 drum has been contrived for the same purpose, consisting of a number of 

 bars moveable in spiral grooves, so as to form a greater or smaller cylinder 

 at pleasure. In order to place the weight in any situation that may be 

 required, the pulley may be made to slide horizontally on the gib or arm. 

 (Plate XVII. Fig. 218.) 



A model of a crane was exhibited some years ago to the Royal Society, 

 in which a large wheel fixed to a short axis was made to roll round on 

 a plane, while the lower end of its axis was connected by a joint with 

 another axis in a vertical position : then the wheel, having to describe a 

 circumference somewhat larger than its own, was turned slowly, and there- 

 fore powerfully, round its axis, and the motion was communicated to the 

 fixed axis. The machine, however, appears to be more curious than 

 useful. 



Sometimes a steelyard has been combined with a crane, for weighing 

 goods at the same time that they are raised by it. A small crane, fixed in 

 a carriage, is convenient for loading and unloading goods. In France, the 

 carts used on the wharfs are generally so long as to reach the ground be- 

 hind when depressed, and to furnish an inclined plane, along which the 

 goods are raised by a lever and axis, or a kind of capstan, fixed in front. 



For taking hold of stones which are to be raised by means of a rope, a 

 hole is sometimes formed in them, wider within than at its opening, and in 

 this a lewis is inserted, consisting of two inverted wedges, separated by a 

 plug, to which they are fastened by a pin. (Plate XVII. Fig. 219.) 



When a rope or chain which is to raise a weight, is so long as to require 

 a counterpoise, the effect of this may be varied according to the length of 

 the rope which is unbent, by hanging it on a second rope or chain, which 

 acts on a spiral fusee, slowly turned by a wheel and pinion. 



The use of cranes is so extensive and so indispensable, that their forms 

 have been often multiplied on account of local circumstances, or even from 

 caprice ; but the constructions which have been described appear to be of the 

 most general utility, and from them it will be easy to judge of others. 



When weights of any kind are simply to be removed from one situation 

 * Trans, of the Soc. of Arts, x. 230, 

 M 



