172 



LECTURE XV. 



ON MACHINERY. 



Having taken a general view of those branches of practical mechanics 

 in which forces are to be resisted, we are next to consider the modifications 

 of forces and of motions ; and in the first place the modes of applyinor 

 forces, of changing their direction and intensity, and of communicating 

 them to different parts of our machines by the intervention of rods, joints, 

 cranks, wheelwork, ropes, or other flexible substances ; in the second place, 

 the structure of these substances, and the metliods by which the union of 

 flexible fibres in general may be effected ; and ir^ the third place, the regula- 

 lation and equalisation of motion, by means of clocks and watches. 



The modes of applying mechanical forces are almost as various as the 

 machines that are constructed, and the purposes for which they are employ- 

 ed: but in general, the strength of men is applied by means of levers, or 

 winches, or by walking wheels, which slide beneath them as they attempt to 

 ascend ; and that of other animals, by a horizontal arm projecting from a ver- 

 tical axis, to which they are harnessed, and sometimes also by causing them 

 to walk on or in a moveable wheel. Many of these arrangements may how- 

 ever be very conveniently considered as belonging to the particular objects 

 for which each machine is constructed, especially to the modes of raising 

 weights by cranes, and of grinding substances by mills. 



When nlotion is simply communicated to a substance placed before the 

 moving body, such materials must be employed as are capable of exerting 

 a repulsive force, or a thrust; and these are generally of the same kind as 

 are sometimes concerned in the operations of architecture, but more com- 

 monly in those of carpentry, particularly metal and wood. But when the 

 body to be moved is behind the moving power, and is pulled along by it, 



