Newcomen's Engine
In the Newcomen engine the piston moved a walking-beam to which was attached a pump-rod. Steam was used merely to balance the air-pressure on the piston and allow the pump-rod to descend by its own weight. The steam was condensed in the cylinder, and the pressure of the air forced the piston down. Thus the work of raising water in the pump was done by the air. Newcomen's first engine made twelve strokes a minute, and at each stroke lifted fifty gallons of water fifty yards. He used this engine in pumping water from the mines, and also made engines for lifting coal.
At first the steam was condensed by throwing cold water on the outside of the cylinder. But one day the engine suddenly increased its speed and continued to work with unusual rapidity. The upper side of the piston was covered with water to make the piston air-tight, and it was found that this water was entering the cylinder through a hole that had worn in the piston, and this jet of cold water was rapidly condensing the steam. This was the origin of "jet condensation."
After this steam and water were alternately admitted to the cylinder through cocks turned by hand. A boy, Humphrey Potter, to whom this work was intrusted, won fame by tying strings to the cocks in such a way that the engine would turn the cocks itself and the boy, Humphrey, was free to play. This device was the origin of valve-gear.[1]
[1] Any device by which a steam-engine operates the valves which admit steam to the cylinder is called "valve-gear." One form of valve-gear is the link motion invented by Stephenson. This form will be described in connection with the locomotive. A simple valve-rod, worked by an eccentric such as is used on most stationary engines, is also a form of valve-gear.
Newcomen's engine was extensively used. The tin and copper mines of Cornwall were deepened. Coal-mines were sunk to twice the depth that had been possible. But as the mines were deepened the cost of running the engines increased. The largest engines consumed about $15,000 worth of coal per year. The Newcomen engine required about twenty-eight pounds of coal per hour per horse-power, while a modern engine consumes less than two pounds. Again, because of increased cost, mines were being abandoned. Such was the situation when James Watt came into the field of action.
Watt had learned the mechanic's trade in one year in a London shop, and, because he had not passed through an apprenticeship of seven years, the Guild of Hammermen, a labor-union of his time, refused him admission, and this refusal meant no employment. He found shelter, however, in the University of Glasgow, and was there provided with a small workshop where he could make instruments for sale.
Watt's Engine
A small Newcomen engine belonging to the University of Glasgow was out of repair. London mechanics had failed to make it work. The job was given to Watt. That he might do a perfect piece of work on this engine, he made a study of all that was then known relating to steam (Fig. 13).