DIRECTION OF PROPULSION. 



6i 



Direction of Propulsion and Distance through which 

 the Centre of Gravity of the Body is Moved. — The direction 

 of the propulsion given by a limb is necessarily through its column of bones. 

 If we examine all the illustrations of the progressive movements of the 

 horse, from Fig. 39 to Fig. 151, we shall see that in every case, just before 

 a limb leaves the ground, it is directed backward and downward, as, for 

 instance, the near hind in Fig. 54, and the off fore in Fig. loi. Hence the 

 direction of propulsion in these cases must be forward and upward. The 

 speed at which the body is moving will greatly influence the direction of 

 the propulsion. Thus in Fig. 23, which is one of the series (Fig. 22 to 

 Fig. 26) that shows the running high leap of a man, the impetus from the 

 right leg is given vertically ; yet the centre of gravity is projected fonvard at 

 about an angle of 45° to the ground. The reason for this is, that in this 

 case there are two forces of projection, namely, that derived from the 



a d 



Fig. 27. — Angle of Projection of Centre of Gravity. 



extension of the right leg, and that due to the speed at which the pedestrian 

 ran up to the jump before he " took ofif." We have here the operation of 

 " the parallelogram of forces." Thus, if the line a bin. Fig. 27 represents the 

 horizontal force (derived from the speed) and a c the vertical one (obtained 

 from the right leg), and if we draw c d parallel to a b, and bd parallel to a c, 

 we shall have the resultant force represented by the line a d, and the angle 

 of elevation, equal to the angle dab. 



The upward motion given by the limb to the body is necessary to keep up 

 the centre of gravity, which, if we wish the labour to be accomplished with a 

 minimum amount of muscular effort, should be maintained as nearly as 

 possible at one uniform height from the ground ; for the distance throu^-h 

 which the centre of gravity is moved, will be a measure of the work done. 

 Let us suppose at each step of a yard long by a horse, that the centre of 



