Monday, July 24th.—The morning was exceedingly fine, and the ship proceeded steadily at an average rate of 6 knots an hour, with a light favouring wind and a calm sea. Those who were up betimes had just taken a turn or two on deck, watching for the early dawn, when they observed some commotion in the neighbourhood of the Testing-Room, and soon afterwards the ship’s engines were slowed and stopped. According to Professor Thomson’s galvanometer, which is used in the system employed in testing, a ray of light reflected from a tiny mirror suspended to a magnet travels along a scale, and indicates the resistance to the passage of the current along the Cable by the deflection of the magnet, which is marked by the course of this speck of light. If the light of the mirror travels beyond the index, or out of bounds, an escape of the current is taking place in the Cable, and what is technically called a fault has occurred. At 3·15 a.m., when 84 miles of Cable had been paid out, the electrician on duty saw the light suddenly glide to the end of the scale, and then vanish. The whole staff were at once aroused—the news soon flew through the ship. After testing the Cable for some time by signalling to and from the shore, Mr. de Sauty satisfied himself that the fault which had occurred was of a serious character, and measures were taken accordingly to rig up the picking-up apparatus at the bow, to take in the Cable till the defective portion was reached and cut out. Such an early interruption to our progress caused a little chagrin, but the veterans of submarine telegraphy thought nothing of it. Whilst the electricians were testing, to obtain data respecting the locality of the fault, the fires were got up in the boilers of two small engines on deck to work the picking-up machinery. At 4 a.m. a gun was fired by the Great Eastern to call the attention of the Terrible and Sphinx to our proceedings, and they were also informed by signal of the injury. Notwithstanding the skill and experience of the scientific gentlemen on board, there was a great vagueness of opinion among them respecting the place where the fault lay. Some believed the defective part was near the shore, and probably at the splice of the shore end with the main Cable; others thought it was eastward or westward of the same place; and calculations, varied by uncertain indications given by the currents showing that the fault itself was of a variable character, and permitted the currents of electricity to escape irregularly, were made by the scientific staff, which fixed it at points from 22 to 42 miles—one at 60 miles—from the ship. But repeated observations gave closer results. Mr. Varley came to the conclusion that the fault was not very far from the ship; and Mr. Sanders, a gentleman who had much experience in fault-finding, arrived at the conviction that it was not more than 9 or 10 miles astern.
The best test taken by Mr. Saunders, 1·30 a.m., Greenwich time, July 25, after the Cable had been cut down to 78·5 miles, gave—
| Resistance, shore end disconnected, | 2,600 | units. | ||
| “ | “ | to earth, | 312 | “ |
Let a and b be the lengths of Cable-conductor, having resistances equal to the first and second of these numbers; l the length of Cable, and D the distance of the fault. The ordinary formula gives
D=b-√ (a-b)(l-b)
Hence, l being 78·5, and a and b being calculated from the observed copper-resistance of the conductor in the after-tank, and various assumed temperatures of the sea, we should have, were the measurements perfect, results as follows:—
| Hence | 4·42 units at 59° temperature | 6·7 | miles. | |||
| 4·37 | “ | 53° | “ | 10·1 | “ | |
| 4·25 | “ | 40° | “ | 22·0 | “ | |
| 4·02 | “ | 35° | “ | 27·2 | “ | |
This would give 22 miles for the most probable distance of the fault, as 40° is the most probable mean temperature of the first submerged length of 75 miles. The true distance proved to be very nearly 3 miles. The discrepance is owing partly of course to want of absolute accuracy in the measurements, but probably more to the variation of the resistance of the fault during the interval between the two measurements.
Iron chains were lashed firmly to the Cable at the stern, and secured to the wire rope carried round outside the ship to the picking-up apparatus at the bows. As the paying-out stopped, a strain came on the Cable, which was down in 400 fathoms of water, and it needed nice management to keep the ship steady, as she had no steerage way. The Cable, having been shackled and secured, was severed at 8·50 a.m., and flew with its shackling into the sea, plump astern. The stoppers which held the wire rope were released, and the rope was payed-out rapidly as the Cable sunk, in order that the ship’s head might be brought round, if possible, so as to take the Cable in over the bows in a straight line with its course.