42 



MECHANICS. 



Fi;r. 39. 



Fi". 40. 



of the chair or cradle is nearer the middle of the rocker 

 than to the ends, the rocking motion will take place ; and 

 when the distance from the centre of gravity to the ends 

 of the rockers is but little greater 

 than the distance to the middle, e, 

 as in fig. 39, the motion w^U be 

 slow and gentle ; but if this differ- 

 ence be greater, as in fig. 40, it will 

 be rapid. When the centre is high, 

 the rockers must have less curvature 

 than where it is low and near the floor. If the centre of 

 gravity be nearer the ends than to the middle, the chair 

 will immediately be overturned. This principle should 

 be well understood in the construction of every thing 

 which moves bv rockin<>-. 



CHAPTER IV. 



SIMPLE MACHINES, OR MECHANICAL POWERS. 

 ADVANTAGES OF MACHINES. 



The moving forces which are applied to various useful 

 purposes commonly require some change in velocity, 

 direction, or mode of acting, before they accomplish the 

 desired end. For example, a running stream of water has 

 a motion in one direction only ; by the use of machinery, 

 we change this to an alternating motion, as in the saw of 

 the saw-mill, or to a rotatory or whirling motion, as in the 

 stones of a grist-mill. The direct or straightforward 

 power of a yoke of oxen is made, by the employment of 

 the plow, to produce a side-motion to the sod, as well as 

 to turn it through half a circle. The thrashing-machine 



