394 Determination of the Longitude of Cairo from Greenwich 



transit observations on each night, in order to obtain the 

 longitude of Alexandria direct from Greenwich, instead of 

 from Mokattam, by a separate operation. 



Mr. Ellis was at Porthcurnow, and, under the Astronomer 

 Royal's direction, arranged the exchanges between Greenwich 

 and that station with Mr. Criswick, who observed stars and 

 signals at Greenwich. 



A complete exchange of signals was effected without any 

 hindrance or disappointment on four nights, namely Novem- 

 ber 14, November 15, November 21, and November 22, 1874. 

 On three nights stars were visible at Greenwich, and on all 

 four nights at Mokattam and Alexandria. 



The following was the arrangement of line batteries and 

 condensers, &c, on the cable circuit. 



A condenser was employed at the receiving end only 

 between the line and instrument. The mirror was used on 

 the Thomson recorder as arranged by Mr. Bull. 



Nearly the same batteries were used throughout at Porth- 

 curnow and Alexandria, namely 40 cells of Mennotti or 

 Daniel on the line. Signals, however, were received at Porth- 

 curnow from Alexandria to test the sensitiveness of the instru- 

 ment with 12 Mennotti cells ; and messages were read from 

 Gibraltar (1161 miles) with 3 Mennotti cells. To keep up 

 the magnetic field, ten and twelve large batteries of the Daniel 

 form were used, the size of the zinc plates being about one 

 foot square, and the resistance of the electromagnet about 

 16 ohms. 



The following are the particulars of the cables employed: — 

 Porthcurnow to Vigo .... 620 knots. 



Vigo to Lisbon 247 „ 



Lisbon to Gibraltar 331 „ 



Gibraltar to Malta 1120 „ 



Malta to Alexandria 904 „ 



Total length 3222 „ 



The electrostatic capacity of the cable is 0*3645 micro- 

 farad per knot. 



Specific inductive capacity of insulator 0*0729 microfarad. 



The total number of cable-signals observed was 956. 



By the order of the Astronomer Royal, Mr. Ellis worked 

 out the longitude of Mokattam before the return of the expe- 

 dition; and the work, as far as the reduction of the exchange 

 of signals was concerned, was then completed. The longitude 

 was slightly modified by the application of corrections for 

 personal equation and also on the final working-up of the 

 whole of the star-observations, which could not be completed 

 by me in camp. 



