When, in 1855, the subject of laying a cable under the Atlantic ocean began to be seriously considered, Thomson, who was then only 31 years of age, discussed in a series of masterly papers the theory of signaling through such conductors, showing _inter alia_ that the instruments used on land-lines would be inoperative on cables, and also that the same speed of transmission could not be attained on cables as on ordinary telegraph lines. It was shown at the same time, that these differences are due to the fact that, unlike an air-line, the cable is an electrical _condenser_ in which the copper core is separated from the waters of the ocean by a layer of gutta percha, a nonconducting material. As a submerged cable is, therefore, a long Leyden jar of great electrical capacity, it follows that a signal sent in at the American end will not reach the other instantly; for while the current flows along the conductor, it has also to charge up the cable as it progresses, which operation retards the signals, and also deprives them of the clearness and sharpness with which they were sent. The phenomenon is analogous to the diffusion of heat along a bar, the temperature of the various cross-sections rising in gradual succession until the distant end is reached. The mathematical investigations of Thomson showed the necessity of working slowly, and of using weak currents as well as very delicate receiving instruments. The interval of time required for the transmission of a signal from Newfoundland to Valentia is about one second. Some years later, in 1858, Thomson had the opportunity of putting his theoretical views to the test of experiment on a grand, commercial scale, and had the satisfaction of finding that all his conclusions were confirmed. Electricians of the early period distrusted the inexperienced young man who had never erected a mile of telegraph line or even served for a month in a telegraph office; but their distrust was followed by admiration when they saw the efficient manner in which he handled every problem and dealt with every difficulty that occurred while laying the cable of 1858. It was generally admitted that, had it not been for the brilliant work of the young Glasgow Professor, many years would have passed away before the Old World and the New would have been brought into telegraphic communication. Like all interested in the enterprise, Thomson was greatly shocked when the news reached him that signals could no longer be transmitted through the cable, which, after costing so much money, so much thought and labor, now lay a useless thing in two and a half miles of water. Attempts were made to raise it, but without success. During its short life of less than a month, 366 messages were flashed through the cable, aggregating 4359 words of 21,421 letters.