XXll INTRODUCTION. 



For the remaining constants, the following values have been used : 



a = 0.00367 for 1° Centigrade. -■ (International Bureau of Weights and 



Measures : Travaux et Memoires, t. I, p. A 54.) 



y r= k cos 2 <f> = 0.002662 cos 2 4>. (Harkness : T/ie solar parallax, etc., 



see p. xix.) 



R — 6367324 metres. (A. R. Clarke : Geodesy, 8°, Oxford, 1880.) 



U.KM 

 r) = — — — = 0.002396. (Ferrel : Report Chief Signal Officer, 1885, 



pt. 2, p. 393.) 

 In redvicing the barometer to sea-level, h^=^o, and the factor ( i H ^ — -) 



becomes i i + -^ )• Taking the product of this factor and K{\ + aO), and 

 neglecting the term in Z, the formula becomes 

 in metric measures 



Z(metres) = (18444 + 67.53^°^- + 0.003 Z)(j-i-^)(iy) log + §. 

 and in English measures 



Z(feet) = (56573 + 123. 1 Q^F- + o.oo3Z)(^i-^)(i + y) log ^° • 



The form adopted for the tables is that of M. Angot.* 



Taking the formula in English measures, let 



Z I 



56573 + 123-1^ + 0.003 Z I — /3 



Then disregarding the small correction for gravity, m --= log ° gives an 



approximate value of B^, and the correction to be added to the observed 

 pressure to obtain the sea-level pressure is 



C = B, — B ^ B{\o"' ~ \). 



If w, be the value of in corrected for gravit}^ we have 



m 



m. = or, approxnnately, = m — my. 



1+7 



The correction for gravity is therefore made 1)y applying to the approxi- 

 mate value m the small correction my. With this corrected value of m, the 

 reduction to sea-level is given by the expression 



^(10"' — i). 



The above fraction designated m contains tlie altitude Z, the mean 



temperature 6, and the humidity factor ■ In the Smithsonian tables, 



I — (3 



vieteorological and physical, by Ur. A Guj^ot, the distinguished author 

 *A. Angot : Annates du Bzireau Central Meteorologique. Aiin^e 1878, 1. 1, p. C. 13. 



