This instrument was designed by Watt to give his firm some standard of work which would serve as a basis for the power of each engine, on which to charge their customers; their engines being sold by the horse-power. But its usefulness far exceeded the immediate purpose for which it was produced. Its diagram, to the eye of an expert, gave valuable information in respect of the setting of the valves, the tightness of the piston, the dryness of the steam, the degree of vacuum in the condenser, and, generally, of the state of efficiency of the engine. “It would be difficult to exaggerate the part which this little instrument has played in the evolution of the steam engine. The eminently philosophic notion of an indicator diagram is fundamental in the theory of thermodynamics; the instrument itself is to the steam engineer what the stethoscope is to the physician, and more, for with it he not only diagnoses the ailments of a faulty machine, whether in one or another of its organs, but gauges its power in health.”[79] § We have now traced the evolution of the steam engine up to the time when it was first adapted to the propulsion of war-vessels. There we must leave it. In a later chapter we shall consider the evolution of the propelling machinery in its relation, especially, to the military qualities of ships. A few observations will be sufficient to illustrate the conditions, as to design, practice, and material, under which the steam engine made its appearance in the royal navy. After the death of Watt all improvement of steam machinery was strenuously opposed by the combined force of prejudice and vested interest. The great Watt himself had set his face against the use of high-pressure steam, and, such was the lingering force of his authority, years passed before the general public gave assent to the advances made by his talented successors--Hornblower, Woolf, Evans, and Trevithick. Before the end of the eighteenth century the first steps had been made to use the force of steam for driving ships. Before Trafalgar was fought steam engines had made their appearance in the royal dockyards. Then there was a pause; and many years passed by before steam propulsion was admitted to be a necessity for certain classes of war-vessels.