HOW LIGHT EXERTS PRESSURE 23 



The length of the wave formed is AB. Now let 

 the cord return to its straight form, as in (a). 

 Bisecting AD in F, DC in G, etc., let us again pull 

 it out ; but this time, as in fig. j(c], forming two 

 waves in the length AB. Let F be drawn up 

 through FP = DM in (), and let G be drawn down 

 by an equal amount, and similarly with H and K. 

 Then D, C, E remain in the original line. The pull 



(a) A >3 



" & C H K 



required on F in (c] is greater than the pull on D 

 in (b\ for the slope of the cord is greater, and its 

 tension is more inclined to FP. If, as we suppose, 

 the displacement is only small, the slope in (c) is 

 double that in (b] and the pull along FP is double 

 that along DM. Then the work done at Fin (c} is 

 double that done at D in (&). But in (c) there are 

 twice as many points drawn aside. With twice as 

 many points and double work for each, we must 

 put in four times as much work in all ; or, where 



