Determination of the Longitude of Cairo from Greenwich. 391 



made an exchange of signals through the circuit formed 

 by the cable lines extending from Porthcurnow to Vigo, and 

 so through Lisbon, and back by the direct cable to Porth- 

 curnow. By this means experience was obtained by the 

 observers at the opposite ends of a cable 1734 knots in 

 length ; that is to say, more than half the length of the Porth- 

 curnow-and-Alexandria Cable. This was a valuable preli- 

 minary experiment, because the observers at the opposite ex- 

 tremities of the cable were sitting side by side, and able to 

 compare and rectify faults together. 



The first object of the Porthcurnow experiments was to 

 ascertain the possibility of getting a reliable signal through 

 the great length of cable between Cornwall and Alexandria, 

 which, it may be observed, is not only 638 knots longer than 

 the Transatlantic one, but has also a much smaller section of 

 copper*. Further, it was necessary to decide what arrange- 

 ment of apparatus to employ, and to take steps to ensure the 

 same conditions being observed at Porthcurnow and Alexan- 

 dria when the actual experiment took place. It was found 

 when Thomson's reflecting galvanometer was employed in the 

 usual way, that 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 attachment of the small 

 mirror to the coil of a Thomson's recorder, by which the for- 

 mer was rendered much more sensitive than when used on the 

 ordinary galvanometer by being brought into a strong mag- 

 netic field |. 



Signals were made with and without condensers. When 

 no condenser was used, the beam came to no fixed zero posi- 

 tion, but was subject to continual fluctuation. On the other 

 hand, when a condenser was employed at the sending end, the 

 travel of the beam of light was not sufficient to furnish a re- 

 liable signal. 



The following was the arrangement finally adopted: — A 

 condenser was employed at the receiving end only, between 

 the line and instrument. The mirror was used on the recorder 



* The Porthcurnow-and- Alexandria cable is 3222 knots, and the French 

 cable from Brest to St. Pierre 2584 knots in length. The copper and 

 gutta-percha coatings are respectively 120 lbs. and 175 lbs. for the English, 

 and 400 lbs. both of copper and of gutta-percha for the French cable per 

 knot. 



t In the Thomson recorder a coil of very fine wire is suspended verti- 

 cally between the extremities of a powerful electromagnet depending on 

 the action of a special battery. The coil is brought into the actual line 

 circuit. Hence a slight current through the line causes strong movement 

 of the coil ; indeed the object of the arrangement is strength as much as 

 extreme sensitiveness, 



