CHAP. XIII.] 



OBSERVATIONS FOR ERROR 



313 



an Ob- 

 servatory. 



Observations are sometimes necessary for the determina- Sextant 

 tion of " personal equation " before or after the exchange of tions'^fcTr 

 telegraphic signals with a fixed observatory for telegraphic Time at 

 meridian distance. 



In such a case it is advisable to allow the standard sidereal 

 clock to carry on the time between the forenoon and afternoon 

 observations, and to eliminate the chronometer used for the 

 sextant observations, except during the intervals a.m. and p.m. 

 over which they extended. This is specially important when 

 using the inferior transit. 



The clironometer will be compared A\dth sidereal standard 

 clock immediately before and after exchanging signals, if a 

 telegraphic meridian distance is being carried out. 



The following example illustrates the method of treating Example, 

 the comparison between the sidereal standard clock (Hardy) 

 and a mean solar clironometer (Mercer). 



On June 22, 1889, the following comparisons were made 

 between Hardy and Mercer by coincidence of beats : 



Hardy 

 Mercer 



Before 

 Obseravtiom. 



32 22-5 

 28 230 



After 

 Observations. 



4 09 

 10 05 



38-0 

 22-5 



Before 

 Observations. 



06 

 01 



22-5 



28-0 



After 

 Observations. 



50 09-5 

 44 58 



Mercer showed at time of middle sight of a.m. series 

 „ ,, corresponding ,, ,, p.m. series 



Required, the comparison at apparent noon. 



Mercer at middle sight of a.m. series . . 



,, corresponding ,, p.m. series . . 



Mercer at apparent noon (nearly) . . 



01 10 



Before A.M. Observations. 



h. m. sec. 

 Hardy .. . . 2 32 22-5 



Mercer .. . . 8 28 23-0 



After A.M. Observations. 



li. m. sec. 

 Hardy .. .. 4 09 38-0 



Mercer . . . . 10 05 22-5 



Mercer slow on Hardy 6 03 59-5 Mercer slow on Hardy 6 04 15-5 

 Retardation for sidereal interval 



since first comparison . . 15-936 



Mercer glow on Hardy, allowing 



for acceleration of sidereal time 6 03 59-564 



