LINE OF DIRECTION. 



59 



centre of gravity of the chair or cradle is nearer the 

 middle of the rocker than to the ends, the rocking mo- 

 tion will take place ; and when the distance from the 

 centre of gravity to the ends of the rockers is but little 

 Fig. 39. Fig. 40. greater than the distance to the 

 middle, c, as in Fig. 39, the mo- 

 tion will be slow and gentle ; but 

 if this difference 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 all in- 

 struments which move by rocking. 



