36 



PROCEEDINGS OF THE AMERICAN ACADEMY 



The scheme of observation, and the plate, were the same as used 

 by Professor Rogers. The transits were taken in sets consisting of 

 an equal number of contacts of both limbs with lines ruled at equal 

 and contrary angles with the middle transit lines, thus eliminating the 

 error of the zero of position. The formula of reduction follows simply 

 from equation (2) hereafter given. Thus, if we call A^j, A/g, the dif- 

 ferences of the observed times of transit of opposite limbs, for the 

 angles p and — p, respectively, we get 



D =: Y COS 8 COS p (A^i -f LQ. 



The corrections for proper motion and differential refraction are so 

 far within the uncertainty of observation, in their effect on the con- 

 cluded diameter, that they have been neglected. Table XI. gives the 

 value of the observed diameter and the number of observations in each 

 pair of sets arranged according to the position angle of the lines em- 

 ployed. Table XII. gives the means, taken with reference to the 

 number of observations, of the results of Table XI. 



TABLE XI. 



The mean value of the diameter from the 194 observations is, 



D= 16''.35. 



