236 SCIENTIFIC AGRICULTURE. 



furnishes the most economical motive power, and one which 

 propels machinery with greater uniformity than any other 

 which we possess. 



Among the instruments used fcr modifying and regulating 

 motion are, the fly wheel, governor, ratchet wheel, universal 

 joint, crank, eccentric wheel, arch head, pendulum, knee joint, 

 fusee, &c. 



Every part of a machine ought to be proportioned to the 

 stress it is to bear, and the strength it requires, and should 

 be no heavier than necesssary: all parts should bear their 

 relative proportion of the work and wear, so that when the 

 machine fails, all parts shall be worn out. Every machine 

 should consist of as few parts as possible ; because, when parts 

 are multiplied, friction is increased in the same proportion, and 

 the machine is more liable to get out of repair. All mechanical 

 obstacles and errors have a less ratio to the motion in great 

 than in small machines ; the former, therefore, work with more 

 uniformity and exactness, but are proportionally weaker and 

 more liable to be broken. 



Motion and rest are both equally accidental states of matter: 

 bodies are no more disposed to lie at rest than to put them- 

 selves in motion : they maintain a state of rest so long as there 

 is an equilibrium of all the furces acting upon them ; and when 

 they assume a state of motion, it is because they are acted 

 upon by some extrinsic force, which is stronger than the com- 

 bined action of all those which tend to keep them at rest. 

 When once in motion, bodies would continue moving forever, 

 if no force obstructed them to destroy the equiblirum between 

 accidental resistances and the propelling force : in other words, 

 they never would come to rest, unless brought to rest by some 

 power superior to that which set them moving. 



Motion may be absolute or relative: absolute motion is a 

 change of place by a body, in relation to some fixed point: 



