ON MACHINERY. 133* 



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

 a vertical 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 however 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 motion 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, 

 chains or ropes are sometimes more convenient. In the union of wood for 

 moveable machinery, it is generally advisable to avoid employing pins or 

 bolts of metal ; for these, by their superior weight and hardness, sometimes 

 injure the wood in contact with them, and become loose. 



When the direction of the motion communicated is also to be changed, 

 levers or cranks may be employed, united by joints or hinges of various 

 kinds. Sometimes a long series of connected rods is suspended by other 

 rods or chains, so as to convey the effect of the force to a considerable dis- 

 tance ; in this case the motion is generally alternate, when, for example, 

 pumps are worked by means of a waterwheel at a distance from the shaft 

 in which the pumps are placed. In this arrangement, there is no necessary 

 loss on account of the alternation of the motion of the rods ; for if they are 

 suspended at equal distances from a number of fixed points, they will move 

 backwards and forwards in the manner of a single pendulum ; but the 

 magnitude of the friction is the principal inconvenience produced by the 

 weight of the series. Where a lever is employed for changing the direction 

 of a great force, its strength may be increased by the addition of a frame 

 projecting in the direction of its depth; and if the lever is bent, a cross 

 piece uniting its arms is still more requisite. (Plate XIV. Fig. 180... 182.) 



For the communication of a rotatory motion, Dr. Hooke's universal joint* 

 is sometimes of use, especially when the inclination is not required to 

 be materially changed ; but if the obliquity is great, the rotation is not 

 communicated equably to the new axis at all points of its revolution. This 

 joint is formed by a cross, making the diameters of two semicircles, one of 

 which is fixed at the end of each axis. (Plate XIV. Fig. 183.) 



The best mode of connecting a rotatory motion with an alternate one 

 is, in all common cases, to employ a crank, acting on one end of a long rod 

 which has a joint at the other. If the rotatory motion of the crank be 

 equable, the progressive motion of the rod will be gradually accelerated and 

 retarded, and for a considerable part of the revoluion the force exerted will 

 be nearly uniform : but if we attempted to communicate at once to the rod 

 its whole velocity in each direction, as has sometimes been done by inclined 

 planes, or by wheelwork, the motion would become extremely irregular, 



* Hooke, Animadversions on Hevelius' Machina Coelestis, p. 73, 4to, Lond. 1674 ; 

 and a Description of Helioscopes and other Instruments, 4to, Lond, 1676, p. 14. 



