160 LECTURE XVIII. 



of heights by men or by animals ; they may either be uniform, as roads, 

 or the general inclination of the surface may be superseded by the for- 

 mation of separate steps or stairs. The inclination of the surface may be 

 governed by the proportion of the strength of the animal to its weight, 

 the force required to support any weight on a plane being to the whole 

 weight as the height of the plane to its length ; and if the plane be a little 

 less inclined than the exact equilibrium would require, the animal will be 

 able to acquire a sufficient velocity at first to carry it easily up the ascent 

 with a motion nearly equable. The strength of a labourer may be advan- 

 tageously employed in ascending a given height by a flight of steps, and 

 placing himself on a stage which may raise a weight by its descent ; but 

 it appears that the force of other animals is less calculated for exertions 

 of this kind. 



The screw is not often immediately applied to the elevation of weights ; 

 although sometimes a number of screws have been used for raising by slow 

 degrees a large and unmanageable weight, for instance, that of an obelise : 

 and a perpetual screw is frequently employed in giving motion to wheel- 

 work. Such machines possess a considerable mechanical advantage, but 

 they are subject to much friction, and are deficient in durability. Mr. 

 Hunter's double screw might be applied with advantage, if the extent of 

 the motion required were extremely small ; but this limitation confines its 

 utility within very narrow bounds. 



A crane is a machine for raising weights by means of a rope or chain 

 descending from an arm which is capable of horizontal motion, and passing 

 over a pulley to be wound up on an axis. The axis is turned, either im- 

 mediateiy, or with the interposition of wheelwork, by a winch, by the 

 horizontal bars of a windlass, or by a walking wheel, and sometimes by 

 the force of wind, of water, or of steam. A walking wheel is an advan- 

 tageous mode of employing the strength of a labourer, but the bulk of the 

 machine is sometimes inconvenient and detrimental ; when, however, the 

 man walks upon the wheel, and not within it, this objection is in great 

 measure obviated. A walking wheel requires to be provided with some 

 method of preventing the dangerous consequences of the rapid descent of 

 the weight, in case of an accidental fall of the labourer : for this purpose, 

 a catch is usually employed, to prevent any retrograde motion ; a bar has 

 also sometimes been suspended from the axis of the wheel, on which the 

 man may support himself with his hands, and other similar precautions 

 have been adopted. Sometimes the plane of a walking wheel is but little 

 inclined to the horizon, and the man walks on its flat surface. In 

 either case the labour of horses, asses, or oxen, may be substituted for 

 that of men : but for cranes this substitution would be very disadvan- 

 tageous, since much force would be lost in stopping frequently so bulky a 

 machine as would be required. The employment of a turnspit dog is an 

 humble example of the same operation, and even goats appear to have 

 been sometimes made to climb in a similar manner. In a walking wheel 

 used for raising water at Carisbrook Castle, in the Isle of Wight, the woik 

 was performed by the same individual ass for the whole of forty-five years 

 preceding 1771. Walking wheels have also been invented, on which horses 



