(72.) The quantity of steam expended in this experimental model in the production of a given number of strokes of the piston was inferred from the quantity of water evaporated in the boiler; and on comparing this with the magnitude of the cylinder and the weight raised by the pressure of the steam, the contrivance was proved to affect the economy of steam, as far as the imperfect conditions of such a model could have permitted. A larger model was next constructed, having an outer cylinder, or steam case, surrounding the working cylinder, and [Pg128] the experiments made with it fully realised Watt's expectations, and left no doubt of the great advantages which would attend his invention. The weights raised by the piston proved that the vacuum in the cylinder produced by the condensation was almost perfect; and he found that when he used water in the boiler which by long boiling had been well cleared of air, the weight raised was not much less than the whole amount of the pressure of the steam upon the piston. In this larger model, the cylinder was placed in the usual position, with a working lever and other apparatus similar to that employed in the Atmospheric Engine. (73.) It was in the beginning of the year 1765, Watt being then in the twenty-ninth year of his age, that he arrived at these great discoveries. The experimental models just described, by which his invention was first reduced to a rude practical test, were fitted up at a place called Delft House, in Glasgow. It will doubtless, at the first view, be a matter of surprise that improvements of such obvious importance in the economy of steam power, and capable of being verified by tests so simple, were not immediately adopted wherever atmospheric engines were used. At the time, however, referred to, Watt was an obscure artisan, in a provincial town, not then arrived at the celebrity to which it has since attained, and the facilities by which inventions and improvements became public were much less than they have since become. It should also be considered that all great and sudden advances in the useful arts are necessarily opposed by the existing interests with which their effects are in conflict. From these causes of opposition, accompanied with the usual influence of prejudice and envy, Watt was not exempt, and was not therefore likely suddenly to revolutionise the arts and manufactures of the country by displacing the moving powers employed in them, and substituting an engine, the efficacy and power of which depended mainly on physical principles, then altogether new and but imperfectly understood.