OBSERVATIONS FOE TIME. 29 



Between observations upon any two stars the striding level should be 

 placed upon the. pivots of the instrument and readings taken to ascertain 

 'the departure of the axis from a horizontal position. 



In order to avoid unequal expansion of the pivots from unequal heat- 

 ing-, both bull's-eye lamps must be lighted and placed in their stands, in 

 order that both pivots may be equally heated. 



After the comparison of chronometers at the two stations, to be here- 

 after described, a similar set of stars should be observed, if possible. 



REDUCTION OF TIME OBSERVATIONS. 



Certain constants of the transits should be measured before proceeding 

 with the reduction of time observations. The value of a division of the 

 striding level should be measured by means of a level trier. The equati >rial 

 interval of time between each of the threads and the mean of all the threads 

 should be obtained, as it is not infrequently needed in utilizing broken or 

 imperfect observations. These can best be obtained from observations on 

 slow moving stars, but any stars may be used for the purpose. The inter- 

 vals as observed, are reduced to the equator by multiplying them by the 

 cosine of the declination of the star observed. 



The object of these observations is specifically the determination of 

 the error of the chronometer. This error equals the right ascension of a 

 star minus its observed time of transit, corrected for certain instrumental 

 errors. These errors are "as follows: 



CORRECTION FOR ERROR OF LEVEL. 



The level error, designated by b, is ascertained from the readings of 



the striding level. The value of a division of the level in seconds of time 



must have been previously ascertained by means of a level trier. The 



effect of the level error is greatest at the zenith and diminishes to zero at 



the horizon. The correction in seconds of time is given by the following 



equation : 



Cor—b cos (<p- fl >) sec Sz=bB. 



When the declination is north, it is to be regarded as having a plus 

 sign for upper and a minus sign for lower culmination. When south it is 

 negative. 



