10 MOTION. 



If a body be acted upon only by one force, or 

 by several in the same direction, its motion will be 

 in the same direction in which the moving force 

 acts ; such is the motion of a boat which a man 

 draws to him with a rope. This is called simple 

 motion. But if several powers, differently directed, 

 act upon it at the same time, as it cannot obey 

 them all, it will move in a direction somewhere be- 

 tween them ; this is denominated compound motion. 



Suppose a body A (plate 1. fig 1.) to be acted 

 upon by another body in the direction A B; while 

 at the same time it is impelled by another in the 

 direction A C ; then it will move in the direction 

 A D; and if the lines A B, AC, be made of 

 lengths proportionate to the forces, and the lines 

 CD, D B, drawn parallel to them, so as to com- 

 plete the parallelogram ABCD; then the line 

 which the body A will describe, will be the diago- 

 nal AD; and the length of this line will represent 

 the force with which the body will move. It is 

 evident, that if a body be impelled by equal forces 

 acting at right angles to each other, that it will 

 move in the diagonal of a square; but whatever 

 may be the direction, or degree of force by which 

 the powers act, the above method will always give 

 the direction and force of the moving body. 



It follows from this, that if we know the effect 

 which the joint action of two powers have upon a 

 body, and the force and direction of one of them, 

 it is easy to find that of the other. For suppose A D 

 to be the direction and force with which the body 

 moves, and A B to be one of the impelling forces, 

 then, by completing the parallelogram, the other 

 power A C is found. 



Instances in nature, of motion produced by 

 several powers acting at the same time, are innu- 



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