266 TRANSIT OF VENUS, 1874. EGYPT. 



LONGITUDE OF MOKATTAM STATION, BY WILLIAM ELLIS, ESQ., 

 OF THE EOYAL OBSERVATORY, GREENWICH, AND CAPTAIN C. 

 ORDE BROWNE. 



In introducing this section our first duty is to acknowledge the great 

 obligations of the Expedition to JOHN FENDER, Esq., M.P., Sir JAMES ANDERSON, 

 and the DIRECTORS of the EASTERN TELEGRAPH COMPANY, who gratuitously gave 

 the use of their submarine cables extending from Porthcurno in Cornwall to 

 Alexandria, for preliminary experiments, as well as for the final series of 

 signals, notwithstanding the serious interruption of the ordinary business 

 thereby entailed. The necessary " joining up " of the five separate cables, and 

 the transmission of time signals through such a length of wire that spealdng 

 was excessively inconvenient, was a very serious matter, but the operations 

 were entirely successful, owing chiefly to the personal superintendence and 

 forethought of Mr. EDWARD BULL, the Superintendent at Porthcurno. 



Material aid was rendered by Mr. CROMWELL VARLEY, F.R.S., who kindly 

 lent his artificial cable, consisting of a large resisting coil and apparatus, by 

 which preliminary practice was obtained at Greenwich in- the special character 

 of signal required. Mr. Varley also suggested the system of signalling, dis- 

 tinguished hereafter as " Varley's method." 



The first object of the preliminary experiments at Porthcurno was to 

 ascertain the possibility of obtaining a reliable signal through the great 

 length of cable between that place and Alexandria, which, it may be remarked, 

 is not only 638 knots longer than the Transatlantic cable used in the deter- 

 mination of the longitude of Washington, but has also a much smaller 

 section of copper.* 



It was further necessary to decide on the arrangement of apparatus to be 

 employed, and to take steps to ensure that the same conditions should be ob- 

 served both at Porthcurno and Alexandria when the actual experiment should 

 take place. It was found that when Thomson's reflecting galvanometer was 

 employed in the usual way, the movement of the beam of light was feeble, 

 but that with an arrangement proposed by Mr. Bull an amply sufficient 

 length of travel was obtained. This arrangement consisted in the attach- 



* The Porthcurno and Alexandria cable is 3,222 knots, and the French cable from Brest to 

 St. Pierre 2,584 knots, in length. The weights of copper wire and gutta-percha coating are respec- 

 tively 120 Ibs. and 175 Ibs. per knot for the English, and 400 Ibs. of copper and of gutta-percha 

 together for the French cable. 



