DEPARTMENT OF MERIDIAN ASTROMETRY. 239 



REDUCTION OF OBSERVATIONS. 



The definitive clock-rates have been applied to all of the right 

 ascensions of the 87,000 San Luis observations, and the formation of 

 the final places is well under way. 



Pending the completion of the examination of the individual micro- 

 scope readings, little has been done toward the final reduction of the 

 San Luis zenith distances. To date, about 71,000 of the 87,000 zenith- 

 distance observations have been critically examined. For the circle 

 opposite the clamp, all the microscope readings have been inspected. 

 From about 170,000 individual readings, 274 errors were detected, 

 mainly 5", 10", or greater. The b" errors, which occurred in the 

 greatest abundance, would have been difficult to detect in the final 

 reduction, so that the zenith distances will be greatly improved by the 

 examination now nearly finished. Incidentally 108 errors in means and 

 41 misapplications of division corrections were caught during the 

 process of the examination, a distinct help to the future reductions. 



For the current work the reductions to mean wire have been kept 

 well up to date. As the material accumulates, a close check has been 

 kept on the fixity of the wire intervals. The errors of runs of the 

 microscopes have been determined as usual and some progress has 

 been made towards taking means and applying to them the runs and 

 division corrections. The readings made last year on circle A for the 

 determination of its eccentricity have been utilized in the formation of a 

 table for the further examination of the readings of single microscopes. 



Some progress has been made in entering new observations on the 

 card catalogue. 



In the report of this department, as given in the Year Book No. 11, 

 the collimation of the meridian-circle telescope was treated as a linear 

 function of the temperature. Some account was taken of a possible 

 lag through adopting the temperatures of the thermometer incased in 

 the barometer box. The probable-error of the resulting collimations 

 was so small that it was deemed unnecessary to employ further refine- 

 ment. Since the return of the instrument from San Luis the collima- 

 tions have been so discordant that it seemed imperative to further 

 investigate them. 



Dr. Albrecht has treated the Albany collimations graphically, on the 

 assumption of a temperature gradient term. Virtually a temperature 

 coefficient of — 0!005 was applied and the residuals grouped according 

 to morning and afternoon observations. This showed a distinct dif- 

 ference, indicating the possibility that the collimation contains a tem- 

 perature gradient term. By graphical methods the relation of colli- 

 mation to change of temperature was determined and a smooth curve 

 drawn. By process of trial, an equation was derived to fit the curve. 

 Attention was called to the possible existence of errors of this nature 

 in the results for collimation obtained at other observatories. 



