The steam-engine is a machine of reciprocating, converted into rotary, motion by the crank. The progress of mechanical engineering during the nineteenth century is measured by the improvements of the steam-engine, principally in the direction of saving fuel, by the invention of internal combustion or gas-engines, the application of electrical transmission, and, latest, the practical development of steam turbines by Parsons, Westinghouse, Delaval, Curtis, and others. In these a jet of steam impinges upon buckets set upon the circumference of a wheel. It was clearly indicated by the Italian engineer Bronca, in 1629, but he was too early. The time was not ripe, and there were then no machine tools giving the perfection of workmanship required. Their advantages are that their motion is rotary and not reciprocal. They can develop speed of from 5000 to 30,000 revolutions per minute, while the highest ever attained by a reciprocating engine is not over 1000. Their thermodynamic losses are less, hence they consume less steam and less fuel. It is a very interesting fact that the basic invention upon which not only steam turbines and electric dynamos, but, indeed, all other parts of mechanical engineering, depend, is of such remote antiquity that we know nothing of its origin. This is the wheel which Gladstone said was the greatest of man’s mechanical inventions, as there is nothing in nature to suggest it. Duplication of parts has lowered the cost of all products. Clothing is one of these. The parts of ready-made garments and shoes are now cut into shape in numbers at a time, by sharp-edged templates, and then fastened together by sewing-machines. Mechanical engineering is a good example of the survival of the fittest. Millions of dollars are expended on machinery, when suddenly a new discovery or invention casts them all into the scrap heap, to be replaced by those of greater earning capacity. Prime motors derive their energy either from coal or other combinations of carbon, such as petroleum, or from gravity. This may come from falling water, and the old-fashioned water-wheels of the eighteenth century were superseded in the nineteenth by turbines, first invented in France and since greatly perfected. These are used in the electrical transmission of water-power at Niagara of 5000 horse-power, and form a very important part of the plant. The other gravity motors are wind-mills and wave-motors. Wind-mills are an old invention, but have been greatly improved in the United States by the use of the self-reefing wheel. The great plains of the West are subject to sudden, violent gales of wind, and unless the wheel was automatically self-reefing it would often be destroyed. Little has been written about these wheels, but their use is very widely extended, and they perform a most useful function in industrial engineering.