In the Proceedings of the Royal Society for 1855, Sir William Thomson showed how the effect at the distant end of a cable, caused by the application of a battery at one end, could be calculated and represented graphically in what is called the "curve of arrival." After contact is first made at the sending end between the cable and one pole of the battery (the other pole being to earth), a certain interval of time elapses before any effect is felt at the distant end. This interval of time is denoted by the letter _a_. After the interval of time _a_ has passed, a current begins to issue from the cable at the receiving end, and increases in strength very rapidly. After a further interval of 4_a_ or after a period of 5_a_ from the first application of the battery, it attains about half its maximum strength, and there is very little sensible increase in strength after a time equal to 10_a_ has elapsed. The curve of arrival is drawn by taking distances along O X to represent intervals of time, and distances along O Y to represent strengths of current. Curve No. I. shows the gradual increase in strength of the received current at one end of a cable when the battery is applied to and kept in contact with the other end. For a distance corresponding to the interval of time _a_, the curve does not sensibly deviate from the straight line O X; in other words, no effect is observable at the receiving end during this time. [Illustration] If now, instead of being continuously applied to the battery at the sending end, the cable had been applied to it during a short interval of time, and then disconnected from the battery and connected to earth, the curve of arrival would be of the form shown by curve No. II. Curve No. II. shows the effect of applying the battery during a length of time equal to 4_a_, and then putting the cable to earth. It will be seen that a current gradually diminishing in strength continues to flow out of the cable at the distant end for a considerable time after the battery has been disconnected. This continued discharge is what gives rise to the difficulty experienced in reading the signals sent through long cables. The instrument first used for receiving signals through a long submarine cable (the short-lived 1858 Atlantic cable) was the Mirror Galvanometer, which consisted of a small mirror with four light magnets attached to its back (weighing, in all, less than half-a-grain), suspended by means of a single silk fibre, in a proper position within the hollow of a bobbin of fine wire: a suitable controlling magnet being placed adjacent to the apparatus. The action of this instrument is as follows. On the passage of a current of electricity through the fine wire coil, the suspended magnets with the mirror attached, tend to take up a position at right angles to the plane of the coil, and are deflected to one side or the other according as the current is in one direction or the other.