206" ^ LECTURE XVIII. \ 



duced in the navy. Some experienced judges, however, are of opinion, that 

 it would be better and more simple to employ a capstan so much tapered, that 

 the tension of the rope itself, guided only by a pulley, might always be suffi- 

 cient to bring the messenger into its place. 



The capstan, which consists of two cylinders of different sizes on the same 

 axis, with a rope passing from the smaller one over a pulley, which is con- 

 nected with the weight, and returning to be wound up by the larger one, is 

 very powerful in its operation ; but it requires a great length of rope for a 

 small extent of motion. (Plate IV. Fig. 51.) 



Wheel work is employed in avariety of ways for raising weights: its powers 

 are in all cases derived from the same principles as the actions of levers, each 

 wheel and pinion being considered as composed of a series of bent levers, of 

 which the axis is the common fulcrum, and which act in succession on the 

 teeth of the next wheel. The simplest combination of wheelwork used for 

 this purpose constitutes a jack ; a bar which is furnished with teeth on one 

 side, being raised by the last pinion. Such instruments were not unknown even 

 to the ancients; the barulcus described by Hero was a machine of this nature. 



(fif A series of buckets connected by ropes, and passing over a wheel, is often 

 employed for raising water to a small height; and sometimes even for solid 

 substances in the state of powder, in particular for raising flour, in a corn mill; 

 and in this case the flour must be brought within reach of the buckets by means 

 of a revolving spiral, which pushes it gradually forwards. When a weight of 

 ^ any kind is raised in buckets distributed through the circumference of a wheel, 

 the force, required for retaining the weight in equilibrium, is as much less 

 than the weight, as the diameter of a circle is less than half the circumference, 

 the remainder of the weight being supported by the axis of the wheel. 



PuUies, and their combinations in blocks, are universally employed on 

 hoard of ships. They are very convenient where only a moderate increase of 

 power is required ; but in order to procure a very gxeat advantage, the number 

 of separate pullies or sheaves must be very much multiplied; a, great length 

 of rope must also be employed; and it is said that in a pair of blocks with 

 five pullies in each, two thirds of the force are lost by the friction and the 



