ANALYSIS OF QUANTITATIVE PHENOMENA, 413 



will be affected by atmospheric refraction and the 

 curvature of the earth. But if in the case of two points 

 the apparent elevation of each as seen from the other be 

 observed, the corrections will be the same in amount, but 

 reversed in direction, and the mean between the two 

 apparent differences of altitude will give the true dif- 

 ference of level m . 



In the next two chapters we really pursue the Method 

 of Reversal into more complicated applications. 



m Button, 'Philosophical Transactions,' abridgment, vol. xiv. p. 422. 



