32 POPULAR LECTURES AND ADDRESSES. 



N P, cutting it at any angle (see Fig. 9A). With 

 any point, O, as centre on the line N P, describe 

 a very small circular arc through P p', and let N' 

 be the point in which the line of O p' cuts the 



FIG. 9A. 



axis. Measure N P, N' p', and the difference of 

 levels between P and P'. Denoting this last by 8, 

 and taking a as a linear parameter, calculate the 

 value of 



__ _ 



O P N P N' P' 



