The steam engine is one of the most important mechanical contrivances used in trade and industry. With its discovery came the great industrial development of the world. The first steam engine was invented by James Watt in 1781. For a long time he seems to have been practically the only engine-builder doing business and his patents probably prevented others from entering this field until about the beginning of the nineteenth century. The steam engine of today is the controlling feature of our industrial civilization. It furnishes the motive power for all our factories, and without it scarcely one of the articles we use in every-day life could be produced in sufficient quantity to satisfy human needs.

The steam pressure of the first engines was very low. Watt ran his engines with a pressure of only seven or eight pounds more than atmospheric pressure. The boiler pressures in current use have steadily risen during the past century as better materials and better workmanship made higher pressures safe and advisable. Today 125 lbs. per square inch is a very common pressure for ordinary stationary engines; 150 to 175 lbs. pressure is frequently met with in large power plants; and in special cases 200 lbs. pressure is employed. This increased pressure, of course, enables the steam engine to yield a much larger output of power per ton of total weight and the limit is not yet reached. As it has been possible to increase boiler pressures, so also the working parts and the structure of steam engines have been improved and strengthened, until now the weight of engine per horse-power of capacity and the cost of the plant are much less than in Watt's time.