268 TRANSIT OF VENUS, 1874. EGYPT. 



co-operated in completing the land line arrangements between Greenwich and 

 Porthcurno. 



The longitudes of all the stations in Egypt at which observations of the 

 Transit of Venus were made in connexion with the British Expedition depend 

 fundamentally on that of Mokattam, the Mokattam Station being temporarily 

 connected with Alexandria by erecting a line from the citadel of Cairo to that 

 station. As it is unadvisable, with due regard to the safety of a submarine 

 cable, to attach thereto a land wire, it became necessary to divide the whole 

 line from Greenwich to Mokattam into sections, and make exchanges of 

 galvanic signals independently on each. The line naturally divided itself 

 into three sections : Greenwich Porthcurno, Porthcurno Alexandria, and 

 Alexandria Mokattam. The days selected for exchanging signals were, 

 1874, November 14, 15, 21, and 22. On each of these days work was com- 

 menced by making exchanges of signals between Greenwich and Porthcurno, 

 and between Alexandria and Mokattam. These went on nearly simul- 

 taneously, and they were followed by exchanges between Porthcurno and 

 Alexandria, which occupied several hours. Finally, for security, a further 

 exchange was made between Alexandria and Mokattam, but that between 

 Greenwich and Porthcurno was not repeated. The exchange of November 14 

 combined with that of November 15, and the exchange of November 21 

 combined with that of November 22, gave, however, all requisite information 

 in regard to the rates of the chronometers used at Porthcurno. 



The signals were given at all stations by means of an ordinary pair of 

 positive and negative keys. Those received on land lines were observed on 

 an ordinary upright galvanometer ; those received on the cable were observed 

 by use of a reflecting galvanometer, constructed, as before mentioned, by 

 fixing a light mirror to a Thomson's syphon recorder (the instrument 

 employed for the speaking work). The cable of the Eastern Telegraph 

 Company, as before mentioned, consists of sections as follows : 1st, Porth- 

 curno to Vigo ; 2nd, Vigo to Lisbon ; 3rd, Lisbon to Gibraltar ; 4th, Gibraltar 

 to Malta ; and 5th, Malta to Alexandria. But, for the longitude work, the 

 whole of these were joined up into one continuous cable, without break or 

 instrument of any kind in circuit, and the signals were passed directly from 

 Porthcurno to Alexandria, or the reverse, through the whole length of cable. 

 The clock used at Greenwich for giving and receiving signals was the Sidereal 

 Standard (by which Greenwich time was determined). At Porthcurno a 

 mean solar chronometer (C. Frodsham 3205) was used for signalling with 

 Greenwich, and a sidereal chronometer (Reid and Sons 1207) for signalling 

 with Alexandria. At Alexandria a mean solar chronometer (Hewitt 890) 

 was alone used. At Mokattam the sidereal clock Dent 1914 was used 



