Page:Lectures on Ten British Physicists of the Nineteenth Century.djvu/73

 to reason to much purpose on the time required for any of the works of Nature.

Sir William Thomson's labors in connection with the laying of the Atlantic cables called for his presence on board ship, and thus attracted his attention to the art of navigation, if indeed he could live in Glasgow without being in some measure drawn into it. He became a skillful yachtsman, and he used his yacht for testing improvements in the means of navigation. His achievements in this direction are numerous and important, but the principal ones are his improved mariner's compass, and his improved sounding line. The use of iron in the construction of ships introduces a serious interference with the compass needle; the needle may direct itself towards a point in the ship instead of a point in the Earth. The action of the ship's magnetism must be cancelled; and this is no easy matter in the case of the ordinary mariner's compass. The improved compass of Sir William Thomson had instead of one large needle, a number of very small needles placed parallel to one another; and instead of a heavy continuous card a light card with the centre wholly cut away. It is more steady, more free to move, and more easily protected from the ship's magnetism. His sounding-line consists of a sinker of 20 to 30 pounds, carried by a strong steel wire. The greatest vertical depth of the sinker beneath the surface is recorded by an instrument which measures the greatest water pressure; and it is read after the instrument has been brought back on board ship. In the old method of casting the lead the depth is determined from the length of rope run out. With the old method a ship must be brought to a standstill, if any trustworthy measure is desired in deep water; with the improved line, a steamer may be running at a speed of 20 knots.

Connected with navigation is his invention of a machine for calculating the heights of the tides at a given port. "It is essentially a mechanical contrivance by which the sum of