1865.] 



Pendulum Base Observations for India. 



433 



TABLE III. Showing the Mean Temperature for each experiment, and 

 the Mean of the whole series for each Pendulum. 



"D. Correction for pressure of air. This correction, as shown in the 

 Phil. Trans, for 1832, is thus determined : 



Let ft' denote the reading of the gauge for the mean of the experiments 

 made in air, and ft" the same reading for the mean of the vacuum experi- 

 ments ; also let t denote the mean temperature of all the experiments, 

 both in air and vacuo, then the expression 



ft'-ft" 



32) 



will denote very nearly the mean difference of density between the two sets 

 of experiments. 



Now if N' denote the mean number of vibrations in air during a mean 

 solar day, and N" the mean number of vibrations in vacuo during the same 

 time, then the constant for one inch of reduced pressure will be 



flit _ "Vrr 



C '=pr^r(l + -0023(^-32)). 

 Hence if ft denote the actual mean pressure for a single experiment and 



