LONGITUDE OF ALEXANDRIA. 329 



In the section Longitude of Mokattam (page 285) it will be seen that H gives 

 signals O'OOS earlier than B, and receives signals O s< 062 earlier. On this 

 account, therefore, the longitude of Alexandria West of Mokattam requires 

 the corrections + O s< 002 and O'OSl. 



After their return to England, viz., in 1875, April, Captain Browne and 

 Mr. Hunter compared their methods of observing transits in the following 

 manner : 



The transit instrument that had been in use at Mokattam was mounted in 

 the grounds of the Royal Observatory, Greenwich, as before mentioned 

 (page 287), and the beats of the Sidereal Standard Clock were rendered audible 

 to the observer by introducing a sounder into the circuit connecting the Sidereal 

 Standard Clock with the chronograph. It was hoped by this arrangement to 

 obtain the absolute personal equations of both observers, but it was afterwards 

 discovered that the beat of the sounder was subject to a sensible retardation, 

 which destroyed the intended comparison with the Greenwich standard ob- 

 server on those nights, but did not interfere with the relative comparisons of 

 Captain Browne and Mr. Hunter. The following numbers are the Excess of 

 the Sidereal Standard Clock Slow, by B and H respectively, over that by the 

 Greenwich standard observer C + the retardation of the sounder : 



1875. B H 



9 8 



April 12, + o'5o 



13, + 0-41 



14, + 0*41 



17, + o'2g + o*3g 



19. + 0-44 



Mean + 0-40 + 0-41 



It has therefore been considered that there is no sensible difference between 

 their methods of observing transits ; consequently no correction is required to 

 the longitude on this account. 



Mr. Hunter having omitted the correction for diurnal aberration, sensibly 

 8> 02 for all clock stars, a correction of O s> 02 is applied to the longitude of 

 Alexandria West of Mokattam. 



We have then for final result 



