THE TRANSATLANTIC LONGITUDE. 83 



The cable of 1866 showed for each knot a resistance of 3.89 ohms,' and an insu- 

 lation of 2437 megohms ; the electrostatic capacity being essentially the same as in 

 the other. 



Thus we have in the cable of 1865, as the total resistance to conduction, about 

 7660 ohms ; as the total resistance of the insulator 1 505 000 ohms ; as the total 

 electrostatic capacity about 670.4 farads. In the cable of 1866 the total resistance 

 is about 7270 ohms; the total insulation 1316 000 ohms; the total electrostatic 

 capacity 654.5 farads. 



The battery employed by the telegraph company was composed of what are 

 known as Minotti's cells ; these being a modified form of Daniell's, in which the 

 zinc rests upon a column of wet saw-dust at the bottom of which is a layer of 

 sulphate of copper, and a copper disk being at the base of all. My friend Mr. M. 

 G. Farmer, to whom I applied for information, found by experiment the electro- 

 motive force of one of these cells to vary from 0.75 to 0.95 volt, averaging 0.84; 

 while the average of four Daniell's cells of ordinary construction gave 0.923 volt. 

 Hence he estimates that, after the full strength of the current is developed, one 

 cell should give, upon one cable with earth-connection, about 110 farads in a second. 



The experiments made for measuring the velocity of signals it will be well first 

 to describe in their regular order. 



On the night of November 1, the first essays were made, after the use of an 

 electro magnet had proved hopeless ; but owing to numerous difficulties incident to a 

 first trial, only a few signals were exchanged. These were made by employing a 

 battery of 20 cells at Valencia, having its positive electrode to the cable of 1866, 

 while the two cables were connected at Newfoundland without battery, and the 

 signals thence were given by alternately breaking and closing the circuit. In the 

 first set no communication was made with earth; 18 signals from Valencia, and 7 

 from Newfoundland being recorded at both stations. In the second the zincode 

 of the battery was connected with the ground as well as with the cable ; and of the 

 signals thus given, 13 from Valencia and 3 only from Newfoundland were thus 

 recorded. 



On November 10, the first two series of experiments were successfully made, as 

 previously arranged in the programme, excepting that during the second series the 

 Newfoundland battery remained without change, Valencia using 4 cells, and New- 

 foundland 20. On November 16 the last two series were carried out, with 4 cells 

 at each station. 



On the 16th, an independent series of experiments was also instituted by causing 

 the cables to be connected without battery at Newfoundland, while signals were 

 given and observed at Valencia, with resistances of various amounts introduced in 

 the circuit, and with variations in the battery power. 



The first question to be investigated is, whether the positive and negative signals 

 were transmitted with the same velocity. For deciding this, no knowledge of the 

 actual time of transmission is requisite, but a simple comparison of the records of 

 the same signals at the two stations will afford an answer. This comparison gives 

 us the interval T — T (the diff"erence of the time indicated at the same moment by 

 the two clocks) diminished by the time of transmission in the case of signals given 



