284 



TEANSIT OF VENUS, 1874. EGYPT. 



TABLE XII. COLLECTION of RESULTS for RETARDATION of the G-ALVANIC CURRENT 



in the SUBMARINE CABLE. 



These values apply to the retardation in the cable portion of the line only ; 

 the land retardation and the effect of personal equation in the land signalling, 

 both in England and Egypt, being eliminated by the method of reduction 

 employed. But they are affected by Mr. Ellis's manner of giving and 

 receiving cable signals at Porthcurno, and by Mr. Hunter's manner of giving 

 and receiving cable signals at Alexandria ; and these influences cannot be 

 separated. The error thereby introduced into the retardation (judging by 

 the numbers given on pages 285 and 286) is presumably small. 



The mean of the results from the " make " and " break " methods gives 

 2 S< 62 for the sum of the cable retardations in both directions, or l s< 31 for the 

 retardation between Porthcurno and Alexandria, or between Alexandria and 

 Porthcurno.* 



The value 4 S< 88 includes the sum of the cable retardations in both direc- 

 tions + the time occupied by the beam at both stations (Porthcurno and 

 Alexandria) in reaching the middle point of its excursion. 



CORRECTION of the GREENWICH-MOKATTAM LONGITUDE for the effects 



of PERSONAL EQUATION. 



The Greenwich-Mokattarn longitude requires correction for the personal 

 equations between the observers who gave and received signals, and for the 

 difference of the manner in which the observers at Greenwich and Mokattam 



Mr. Varley had estimated the retardation each way at 



