346 



INTRODUCTION TO THE 



The times were read from a chronometer placed near the observer. In the winter months the 

 doors and windows were left open, and every precaution taken to equalize the internal and 

 external temperatures. This condition could not often be perfectly obtained it being impos- 

 sible, in cold weather, to exclude entirely the heat from the lower part of the building. The 

 external and internal temperatures are given at each observation ; and, in addition to such 

 remarks as were deemed essential to designate the state of the atmosphere, there is given 

 opposite each series an abstract estimate of its quality, shown by the figures following the 

 letter A ; A 10 signifying the most perfect condition. 



The chronometer used was compared with the standard clock of the Observatory immediately 

 before the commencement of each series of comparison. A sidereal chronometer was used until 

 October 12, 1850, and after that time a mean-time chronometer. In the reduction of the 

 observations subsequent to October 12, 1850, the quantity A t applied to the differences in right 

 ascension expresses the correction necessary to reduce the mean to the sidereal interval. 



The adjustment of the instrument was verified by observations of standard stars at proper 

 intervals ; the deviation of the polar axis in elevation, or in azimuth, never exceeding 30 

 seconds of arc. The state of the adjustment is sufficiently indicated by the micrometer readings 

 to the same fixed star. 



The value of a revolution of the micrometer used in the reductions is 15". 3717. It has been 

 determined by transits of stars within 20 of the pole, and by measured distances between well 

 determined stars of the Pleiades. The following table designates the character of the observa- 

 tions upon which the value adopted here depends : 



For the final value (15.37168) the mean was token, rejecting the observations of February 

 11 and 12. No attempt has been made to introduce a correction for temperature. 



At the commencement of the observations, the micrometer contained three transit wires, 

 and a declination wire upon one slide, and three declination wires upon the other ; the latter 

 slide being the one moved by the micrometer screw ; the transit wires being designated by 

 the letters A, B, C, and the central wire on the same slide by /. The three declination wires 

 on the other slide are designated by the numbers 1, 2, 3 ; No. 1 being next to the micrometer 

 screw-head. After December 18, 1851, five declination wires were introduced instead of three ; 

 numbered as in the first arrangement. The values of the intervals between the declination 

 wires were as follows : 



Rev. 



From November 1, 1849, till March 12, 1850 .... 12 = 30.151 



2 3 = 29.913 



1 3 = 60.064 



From October 1,1650, till November 1,1850 . . . . 1 2 = 30.142 



2 3 = 29.947 

 13 = 60.089 



