CHAP. FV.] MOMENT OF ROTATION OF FOKCE8. 35 



resultant moment at m or o of the components at a and b par- 

 allel to R, is proportional to the ordinate o n. 



So for any point, the ordinate included by the polygon a b o 

 d ef, and the closing line af, to the scale of length multiplied 

 by the "pole distance " H to the scale of force, gives the mo- 

 ment at that point of the components parallel to the resultant. 



Again, the moment about any point as 0, is equal to the mo- 

 ment of the force in the string S, with reference to that point. 

 If now, we conceive this force acting at n, and decompose it 

 into a force parallel to the resultant and a force perpendicular 

 to the resultant, the first component will pass through and its 

 moment therefore is zero. 



Hence : The moment, at any point, as o, is equal to the com- 

 ponent perpendicular to the resultant of that string cut by a 

 section through o parallel to the resultant , multiplied by the 

 ordinate of the polygon taken parallel to the resultant. 



The practical importance and application of these principles 

 will appear more clearly in the consideration of parallel forces 

 in the next chapter. 



