This chapter focuses on the work of William Thomson; Lord Kelvin on two navigational instruments the magnetic compass and the mechanical depth sounder as a way of thinking about ships and their instruments. The issues that arise illustrate how existing and emerging technologies interact. For example, how the increasing use of iron and steel, the development of faster ships and, in the fighting navies, of bigger guns, had significant effects on the instruments the ships carried and on the ways in which those ships were arranged. The development of the Mark IV sounding machine and its movement forward to the ship's command centre, which recognised its strategic importance in the sorts of action envisaged in the early twentieth century, was relatively unproblematic, involving straightforward changes to the machine and its operation and to the ship. The adoption of the Thomson compass as the Standard Compass on Royal Navy vessels had been more complex.