Watt had been thinking about steam for four or five years before he saw one of Newcomen's engines. Then it was only a model of one, brought to him from the university for repair. When he had repaired the model, he started it to going. It made a few strokes and stopped. There was no more steam. The boiler seemed big enough, so he blew up the fire. The engine now ran all right, but it required much fuel and used up quantities of steam, though the load on the side of the pump was light. Most men would have thought nothing of this, and would have sent the model back to the university. But that was not Watt's way. Everything he did not understand was for him a subject for study, and he never stopped until he understood. So he set to work to discover why the engine used so much steam.

Steam was used, you will remember, to make a vacuum in the cylinder. Watt found that to drive out the air and water, enough steam had to be let into the cylinder to fill it four times. Why was this? First, the cylinder was exposed to the air, which chilled it. The cold cylinder itself, before it was warm, changed considerable steam into water. Second, cold water was poured into the cylinder to condense the steam, and this made the cylinder cold again. Watt estimated that three fourths of all the steam used was thus wasted in heating and reheating the cylinder. Here was the trouble with Newcomen's engine. Watt saw that, to remedy this defect, a way must be found to keep the cylinder always as hot as the steam which entered it, and the vacuum must be made in the cylinder, without cooling it.