208 LECTURE xviir. 



fortunately has the most eligible inclination of 1 in 4, and is 33 yards in 

 thickness, affording the most advantageous means of fixing every part of the 

 machinery with perfect security. The whole length of the plane is 151 yards, 

 besides a lock of 18 yards at the upper end, (Plate XVII. Fig. 216.) 



Inclined planes are also universally employed for facilitating the ascent 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 general inclination of the surface may be superseded by the formation of 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 advantageously 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 has 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 

 Avheelwork. 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 re(juired 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- 

 mediately, or with the interposition of wheelwork, by a winch, by the hori- 

 zontal 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 advantageous mode 

 of employing the str.ength of a labourer, but the bulk of the machine is 



