158 LECTURE XVIII. 



corruption from engine : the buckets being attached to the opposite ends of 

 a rope which passes round the drum, and which is drawn by means of its 

 adhesion to the drum. One of the buckets descends empty while the other 

 is drawn up full, and when the motions of the buckets are to be changed, 

 the horses are turned, or the wheels are made to impel the axis in a con- 

 trary direction when any other moving power is employed. 



When a ship's anchor is weighed, the cable itself would be too large to 

 be bent round the capstan ; it is therefore connected with it by means of an 

 endless rope, called a messenger. As the messenger is coiled round the 

 lower part of the capstan, it quits the upper part ; so that its place becomes 

 lower and lower, till at last it has no longer room on the capstan ; it is 

 therefore necessary to force it up from time to time : this is called surging 

 the messenger ; it is commonly done by beating it, and to facilitate the 

 operation, the capstan is made somewhat conical. It has been proposed to 

 employ lifters in different parts of the circumference, which are raised 

 once in each revolution, by passing over an inclined plane, with the inter- 

 position of friction wheels ; a patent has been taken out for the invention, 

 and it has already been introduced 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 sufficient 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 

 connected 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. 61.) 



Wheelwork is employed in a variety 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 suc- 

 cession on the teeth of the next wheel. The simplest combination of wheel- 

 work 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 barulcust described by Hero 

 was a machine of this nature. 



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 sup- 

 ported by the axis of the wheel. 



* See Hamilton's Rep. of Arts, ii. II. 126. 



f Brugmans, Commentat. Gott. 1784, vii. M. 75. 



