LONGITUDE OF MOKATTAM. 267 



ment of the small mirror to the coil of a Thomson's syphon recorder, by which 

 its action was rendered much more sensitive than when used on the ordinary 

 galvanometer, by being brought into a strong magnetic field.* 



Experiments were made with and without condensers. When no condenser 

 was used on the cable, the beam came to no fixed zero position, but was 

 subject to continual fluctuation. On the other hand, when a condenser 

 was employed at the sending end, the motion of the beam of light was 

 not sufficient to furnish a reliable signal. 



The following was the arrangement finally adopted : A condenser was 

 employed at the receiving end of the cable only between the line and the 

 recording instrument. The mirror was used on the recorder in the manner 

 proposed by Mr. Bull, who also provided a similar instrument for use at 

 Alexandria in order to ensure the same conditions being observed at both 

 ends of the cable. 



The same amount of line battery was used throughout both at Porthcurno 

 and at Alexandria, namely, 40 cells of Menotti or Daniell. Signals, how- 

 ever, were received at Porthcurno from Alexandria, to test the sensitiveness 

 of the instrument, with 12 Menotti cells, and messages were read from 

 Gibraltar with 3 Menotti cells. To keep up the magnetic field, 10 and 12 

 large batteries of the Daniell form were used, the size of the zinc plates being 

 about 1 foot square, and the resistance of the electro-magnet about 16 ohms. 



The following are the particulars of the cables employed : 



Porthcurno to Vigo, 620 knots. 

 Vigo to Lisbon, 247 knots. 

 Lisbon to Gibraltar, 331 knots. 

 Gibraltar to Malta, 1,120 knots. 

 Malta to Alexandria, 904 knots. 



The electrostatic capacity of the cable is 0'3645 microfarad per knot. 

 Specific inductive capacity of insulator, 0'0729 microfarad. 



All possible assistance in Egypt was rendered by Mr. GIBBS, the Superin- 

 tendent; by Mr. STEVENSON, at Alexandria; and by Messrs. CROSS and EOGIERRI, 

 at Mokattam. 



The Post Office Telegraph Department, represented by Mr. EATON, cordially 



* In the Thomson recorder a coil of very fine wire is suspended vertically between the 

 extremities of a powerful electro-magnet depending on the action of a special batter}-. The coil is 

 brought into the actual line circuit ; hence a slight current through the line causes strong movement 

 of the coil. 



M M 



