high temperature, on the principle of the high pressure engine, now employed for various purposes. But he gave up ihe idea from fear of bnrstinp the boiler, and the difficulty of making tight joints. These facts are sufficient to
prove that he had at this time some idea of improving the steam engine; and he himself modestly says, "My attention was first directed in 1759, to the subject of steam engines by Dr Robison, then a student in the university of
Glasgow, and nearly of my own age. Robison at that time threw out the idea of applying the power of the steam engine to the moving of wheel carriages
and to other purposes; but the scheme was not matured, and was soon abandoned on his going abroad." His active mind, thus prepared, was not
likely to allow the defects of the model which was put in his hands to pass unobserved. This interesting model, which is still preserved among the apparatus of the Glasgow university, has a cylinder whose diameter is two inches, the length of stroke being six. Having repaired it, he tried to set it a-going, the steam being formed in a spherical boiler whose diameter was about nine inches. In the course of these trials he found the quantity of steam, as likewise that of the cold injection water, to be far greater in proportion, than what he understood was required for engines of a larger size. This great waste of steam,
am? consequently fuel, he endeavoured to remedy by forming cylinders of bad conductors of heat, such as wood saturated with oil, but this had not the desired effect. At last the fact occurred to him, that the cylinder was never sufficiently cooled down in order to obtain a complete vacuum. For some time before this it had been found by Dr Cullen that under diminished pressure there is a corresponding fall of the boiling point It now became necessary to ascertain the relation which the boiling point bears to the pressure on the surface of the water. He was not possessed of the necessary instruments to try the boiling points under pressures less than that of the atmosphere, but having tried numerous points under increased pressures, he laid down a curve whose ordinates represented the pressures and abscissas the corresponding boiling points, and thus discovered the equation of the boiling point These consider-
ations led Watt, after much reflection, to the true method of overcoming the difficulties in the operation of Newcomen's engine. The two things to be effected were, 1st, to keep the cylinder always as hot as the steam to be admitted into it, and secondly, to cool down the condensed steam and the injection water used for condensation to a temperature not exceeding 100 degrees. It was
early in the summer of 17G5 that the method of accomplishing these two objects was first matured in his mind. It then occurred to him that if a communication were opened between a cylinder containing steam and another vessel exhausted of air and other fluids, the steam would immediately rush into the empty vessel, and continue so to do until an equilibrium was established, and by keeping that vessel very cool the steam would continue to enter and be condensed. A difficulty still remained to be overcome, how was the condensed steam and injection water, together with the air, which must necessarily accompany, to be withdrawn from the condensing vessel. Watt thought of two methods, one by a long pipe, sunk into the earth, and the other by employing a pump, wrought by the engine itself; the latter was adopted. Thus was laid open the leading principle of a machine the most powerful, the most regular, and the most ingenious, ever invented by man.
Watt constructed a model, the cylinder of which was nine inches diameter, making several improvements besides those above alluded to. He surrounded the cylinder with a casing, the intervening space being filled with steam to keep the cylinder warm. He also put a cover on the top, causing the piston rod to move through a hole in it, and the piston was rendered air-tight by
