Longitude of Cairo from Greenwicliby Telegraph- Signals. 393 



and as incidentally giving the longitude of another important 

 place in Europe very accurately. He calculated that the re- 

 tardation of the current would be about 1*25 second between 

 Porthcurnow and Alexandria, a calculation which was proved 

 by the result to be remarkably correct. The idea of the 

 establishment of an intermediate station was negatived as 

 impracticable by the Astronomer Royal. 



Mr. Yarley kindly placed his artificial line (furnished with 

 condensers, &c., to represent the conditions of a submarine 

 cable of greater or less length) at the disposal of the Astro- 

 nomer Royal. By means of this apparatus abundant practice 

 was obtained in the observation of a signal similar to that 

 made on the actual cable. When the expedition had gone 

 out to Egypt, and stations had been established with proper 

 telegraph-connexions, the actual exchange of signals took 

 place. Matters were then in the following condition. 



The primary station in Egypt was established on the western 

 spur of the desert range of the Mokattam hills, about 560 feet 

 above Cairo. The real object of the experiment was to obtain 

 the longitude of this point from Greenwich. The longitudes 

 of the subsidiary stations at Thebes and Suez, and if necessary 

 Alexandria also, might be readily obtained from the Mokattam 

 station by exchange of signals through land lines. 



To compare the Mokattam local time with Greenwich time, 

 it was necessary to exchange land-line signals between these 

 two places with the transmitting stations established, as above 

 noticed, at each end of the cable; consequently Mokattam and 

 Alexandria exchanged signals, while Greenwich and Porth- 

 curnow were similarly engaged. Then took place the ex- 

 change of cable-signals between Porthcurnow and Alexandria, 

 which lasted for four hours. Finally Mokattam and Alexandria 

 again exchanged signals. 



While the cable-signals were passing a number of star- 

 transits were observed at Mokattam, as well as both before and 

 after the exchanges with Alexandria. 



Mokattam was my own station. The principle I endea- 

 voured to follow was to multiply star- and telegraph-signal 

 observations at that end of the line where each star-observa- 

 tion must be expected to be the less accurate, and where the rate 

 of the clock was more liable to fluctuation. In some cases an 

 unusually large number of stars were observed, owing to the 

 fact that at each opportunity that occurred it was desirable to 

 secure several good transits, because the skies might cloud 

 over at any time. 



Mr. Hunter was at Alexandria. In addition to his work 

 in transmitting cable and land signals, he undertook star- 



