25 



§ 17. We shall call the diagonal points, at which 

 the compound forces are applied, ^'Points of Ajjpli- 

 cation,'' and as we confine ourselves to rotaticm from 

 front to rear, there will be for every S two diagonal 

 pairs, or four points in all. The diagonal pairs 

 will replace each other in successive counter-actions 

 between their points. 



§ 18. We shall, for easier illustration, always 

 consider an example in which the opposition of the 

 left upper and right lower forces begins the succes- 

 sive counter-actions. In this the left upper force 

 rotates from the front centre of the border of its 

 plane, by the left, to the rear centre. The right 

 loioer force likewise from the front centre, but by 

 the right to the rear. The action of the other pair 

 of diagonal points will then, mutatis mutandis, al- 

 ternate with these. 



§ 19. The theory of the twists may then, we think, 

 be discussed as follows : 



If we first bend the rod in the simplest manner, 

 i.e., by pressure, without rotation, appHed at j)erpen- 

 dicularly, not diagonally, opposite points, so that it 

 shall take the shape of a C, then a spring will be 

 formed ; and if, while one end of this spring is fixed, 

 the other end be liberated so that it can pass in 

 one line only, the force of the spring will be ex- 



