LONGITUDE OF MOKATTAM. 287 



The signal received on the latter, as has been mentioned, is of a character 

 different to that received on the former ; it is therefore necessary to consider 

 separately each section of the work. In the first section C at Greenwich 

 exchanged signals with E at Porthcurno, using a land line. B receives land 

 signals O'090 later than C. The correction required by the longitude on 

 this account will be O s '045. In the second section E at Porthcurno 

 exchanged signals with H at Alexandria, using the submarine line. There 

 is a little uncertainty as concerns the difference between E and H in receiving 

 cable signals, but according to the comparisons given on page 286 it would 

 appear that the difference in the method of observing the Varley signal was" 

 probably small. It will perhaps not be far from the truth to conclude 

 that for both methods of cable receiving there was no practical difference. 

 On this assumption no correction is required on account of receiving signals 

 in the second section. In the third section H at Alexandria exchanged 

 signals with B at Mokattam, using a land line. Now B receives land signals 

 O s> 105 later than C, and H O s *043 later than C, or B receives later than H 

 by O062. Consequently the correction required by the longitude will 

 be O s< 031. Or the total correction to be applied on account of personal 

 equation in receiving signals is s '07 6. 



The correction for personal equation between C and B in the manner of 

 observing transits has now to be considered. 



The transit instrument which was subsequently used at Mokattam was 

 mounted upon a pier of masonry in the south ground of the Koyal Observatory 

 at Greenwich, and was used by Captain Browne to determine the error of the 

 Sidereal Clock Dent 1914. The instrument was 185 feet east of the meridian 

 of the Transit-Circle. During the observations each day, the clock Dent 1914 

 was several times compared with the Sidereal Chronometer, sympathetic with 

 the Sidereal Standard Clock and Chronograph, by the intervention of a mean 

 time half-seconds chronometer. Captain Browne made observations of this 

 character on three nights before he went to Egypt, and on four nights after 

 his return. It is not considered necessary to give the details of these 

 observations, which were of the same character and quality as those made at 

 Mokattam. 



Making a correction of O s> 195 for the difference of longitude between the 

 two instruments, the results obtained were that Captain Browne (using the 

 small transit, and observing by eye and ear) made the Sidereal Standard 



