66 On Practical Geodesy. 



from which we find I, — l^^ = 3600 091 



and I, — l^^ = 3600-632 ; giving an error of 



0"'\ in the first case, and an error of 0'''6 in the second case. 



In Chambers' " Practical Mathematics " the formulae diflfer 

 from the above in having the factors (1 -|- e^ * cos l^, 

 (1 + e^ • cos'^ ZJ, replaced by (1 + 2 € ' cos^ ZJ and 

 (1 + 2 € • cos^ I) which are greater ; and .'. obviously the 

 results must be the more erroneous. 



Their method of finding the diflference of longitude is by 

 means of the formula 



s ' sin A, s • sin A, 



sin r' • cos l^^ R^^ • sin V • cos I, 

 sin A, sin A„ 



' cos l,^ " cos I, 



from which we obtain the values 



0) = 4499"-838 = 4500"-355 

 having a difference = 0"'5VJ. 



