78 THE TRANSATLANTIC LONGITUDE. 



but, in sending them, an armature-time intervened between the actual clock-signal 

 and its transmission to Greenwich. This was reduced to a minimum by strong 

 tension of the spring, and two series of experiments were made to measure the 

 amount of the delay. 



For this purpose, the relay-magnet being retained in the chronograph-circuit in 

 the same manner as during the transmission of signals to Greenwich, the two ter- 

 minals of the instrument (which are in permanent connection with the armature 

 and its conducting stop, and which, during the sending of signals, are connected 

 with the two wires of the main line) were also brought into communication with 

 the chronograph-circuit on the two sides of the recording magnet. The effect of 

 this arrangement was, that when the clock-signal, which is of course recorded upon 

 the chronograph, released the armature of the relay-magnet by interrupting the 

 galvanic circuit, this armature on its arrival at the outer stop completed a metallic 

 connection by which the chronograph was excluded from the circuit. This was 

 recorded upon the chronograph, like a second interruption, which continued until 

 the tension of the spring was overcome by the re-established current. In this man- 

 ner two signals Avere given in each second; the first by the clock directly, the 

 second by the relay after the lapse of the interval required for the armature to 

 reach the outer stop. Then, if the chronograph-magnet be adjusted with all pos- 

 sible delicacy, the length of the record of the total interruption must be increased 

 by the full amount of the delay in question. Series of observations were made for 

 the investigation of this point on the 4th and 14th of November, and indicate a 

 delay of 0'.02 in the communication of signals, being equivalent to a retardation of 

 the clock by this amount in the currents sent, though not in their record; and 

 implying a diminution both of the longitude and of the transmission time by O'.Ol. 



The longitude as deduced from the two nights' exchanges is : — 



Number of signals. 

 f^ SC Greenwich. Valencia. 



1866, November 5, 0" 41"" 33'.305 0M15 66 210 



13, 33.280 0.110 80 TO 



the mean being, 41 33.29 



The Greenwich observations were made by different persons on different nights, 

 but were aU reduced to Mr. Dunkin in the usual manner. 



The line of telegraph passed through Killarney and Mallow to Dublin, thence to 

 Wexford, St. David's, Cardiff, London, and Greenwich. Its total length must 

 have been very nearly 600 miles (966 kilometers), exclusive of the submarine 

 cable between Ireland and Wales, which is about one-tenth part as long. The 

 length of the cable across the straits of Valencia is about three-quarters of a mile. 



Referring the longitude of Valencia to Feagh Main, as the fundamental point 

 adopted for the great European Arc of Parallel, by means of geodetic reduction of 

 the telegraphic stations, Mr. Airy finds for the longitude of this point west of 

 Greenwich — 



