James Watt was himself a product of the modern scientific spirit. He was a man so constituted mentally that he could apply scientific methods to problems which his logical and clairvoyant mind could readily and exactly formulate the instant he was led to their consideration in the natural course of his progress. He was the ideal great inventor and mechanic. With inventive genius he combined strong common sense--not always a quality distinguishing the inventor--clear perception, breadth of view, and scientific method and spirit in the treatment of every question. His natural talent was re-enforced by an experience and an environment which led him to develop these ways and this mental habit. His trade was that of an instrument maker, his position was that of custodian and repairer of the apparatus of Glasgow University. He had for his daily companions and stimulus the great men and ozonized atmosphere of that famous institution. He kept pace with advancing science, and was imbued, both naturally and through contact with its promoters, with that ambition and those aspirations which are the life element of all progress, whether scientific or other. He was aware of the nature of the problems seeking solution at the time, and familiar with the state of his own art and that of the great mechanicians about him. Everything was favorable to his progress, so soon as he should be given an opportunity to take a step in advance and to come into sight at the front. The man and the time were both ready, and all conditions, internal and external, social and personal, were favorable to his development. The invention upon which Watt was to improve was at his hand. A word in regard to its status at the moment will throw some light upon that of Watt and his creation. Newcomen had, as we have seen, produced the modern type of steam engine as an original and wholly novel invention. But this machine, marvelous as an advance upon pre-existing forms of the steam engine, was still, as seen in the light of recent knowledge and experience, exceedingly defective. The purpose of a steam engine is to convert into usefully applicable power the hidden energy of fuel, stored ages ago in the earth, by transformation, through the action of vegetation, from the original form, the heat of the sun, into an available form for reconversion, through thermodynamic operations. In this process of reconversion, whatever the nature of the machine used in the operation, there are invariably wastes, both of heat required for conversion into power and of the power thus produced. That machine which effects the most complete transmutation of the heat supplied it into mechanical power, which wastes the least amount of heat supplied and of power produced, is the best engine, and constitutes an advance over every other.