130 TUCKER— EFFECT OF DIURNAL VARIATION 



nineteen years, exhibit a difference of o".5 between declinations 

 measured by day and night. 



Corrections to the adopted constant of nutation were computed 

 from this series, from both day and night observations combined, and 

 from the two periods of the day separately. 



The first of the solutions mentioned gives a correction of -\- o".03 

 to the constant 9". 22. 



To return to meridian circle results, our fundamental work during 

 the years 1905 to 1908, and in 1916, has given us the observed lati- 

 tude at all hours of day and night. 



Over one thousand observations have been combined in deriving 

 the following diurnal term. More than one quarter of the total 

 number are of zenith stars, close enough to the zenith point to be 

 observed facing either north or south for the measurement of bisec- 

 tion error. 



A somewhat larger number are of stars bright enough for day- 

 light observation, divided into groups for which the means are close 

 to the zenith. 



The stars a Andromedce and Polaris furnish a third of the total 

 number. 



Observations of Polaris and /? Ursce Minoris are the only ones 

 for which corrections for the diurnal variation of refraction are of 

 importance. This correction has been derived from observations of 

 stars at large zenith distances on both sides of the zenith, and the 

 solution is independent of the latitude and its variation and of the 

 nadir readings. The diurnal variation in the atmospheric refraction 

 at this station is approximately one per cent, of the total refraction. 

 A separate solution for the diurnal term had been made from the 

 zenith stars only, before including the results from the other stars. 

 Errors of refraction could play no part in this solution, which gave 

 the same numerical coefficient as the solution from all the stars, within 

 one unit in the third place of decimals. 



The observations are nearly evenly distributed between daylight 

 and night hours. 



All have been corrected for the latitude variation of long periods 

 from the results of the international zenith telescope stations, exclud- 

 ing the z term. 



