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Marine

Thomson was an enthusiastic yachtsman, his interest in all things relating to the sea perhaps arising from, or fostered by, his experiences on the Agamemnon and the Great Eastern. Thomson introduced a new method of deep-sea depth sounding, in which a steel piano wire replaces the ordinary hand line. The wire glides so easily to the bottom that "flying soundings" can be taken while the ship is at full speed. Thomson added a pressure gauge to register the depth of the sinker. About the same time he revived the Sumner method of finding a ship's position, and calculated a set of tables for its ready application.

During the 1880s, Thomson worked to perfect the adjustable compass to correct errors arising from magnetic deviation owing to the increased use of iron in naval architecture. Thomson's design was a great improvement on the older instruments, being steadier and less hampered by friction. The deviation caused by the ship's magnetism was corrected by movable iron masses at the binnacle. Thomson's innovations involved much detailed work to develop principles identified by George Biddell Airy and others, but contributed little in terms of novel physical thinking. Thomson's energetic lobbying and networking proved effective in gaining acceptance of his instrument by The Admiralty.

Scientific biographers of Thomson, if they have paid any attention at all to his compass innovations, have generally taken the matter to be a sorry saga of dim-witted naval administrators resisting marvellous innovations from a superlative scientific mind. Writers sympathetic to the Navy, on the other hand, portray Thomson as a man of undoubted talent and enthusiasm, with some genuine knowledge of the sea, who managed to parlay a handful of modest ideas in compass design into a commercial monopoly for his own manufacturing concern, using his reputation as a bludgeon in the law courts to beat down even small claims of originality from others, and persuading the Admiralty and the law to overlook both the deficiencies of his own design and the virtues of his competitors'.

The truth, inevitably, seems to lie somewhere between the two extremes.

Charles Babbage had been among the first to suggest that a lighthouse might be made to signal a distinctive number by occultations of its light, but Thomson pointed out the merits of the Morse code for the purpose, and urged that the signals should consist of short and long flashes of the light to represent the dots and dashes.