Watt’s automatic ball governor is shown in Fig. 78 and its function is as follows: When the working strain on an engine is relieved by the throwing out of action of a part of the work being performed, the engine would run too fast, or if more than a normal tax were placed on the engine, it would “slow up.” To secure a regular and uniform motion in the performance of his engine Watt invented the automatic or self-regulating ball governor and throttle valve. A vertical shaft D is rotated constantly by a band on pulley _d_. Any tendency in the engine to run too fast throws the balls up by centrifugal action, and this through toggle links _f h_, pulls down on a lever F G H, and partially closes the throttle valve Z, reducing the flow of steam to the engine. When the engine has a tendency to run too slow the balls drop down, and, deflecting the lever in the opposite direction, open the throttle valve, and increase the flow of steam to the engine. This double acting engine of Watt marks the beginning of the great epoch of steam engineering, and his patent expired just in time to give to the Nineteenth Century the greatest of all natal gifts. [Illustration: FIG. 78.--WATT’S AUTOMATIC GOVERNOR AND THROTTLE VALVE.] Steam engines are divided into two principal classes, the low pressure engine, using steam usually under 40 pounds to the square inch, and the high pressure engine, using steam from 50 to 200 pounds. In the low pressure engine there is the expansive pressure of the steam on one side of the piston, aided by the suction of a vacuum on the opposite side of the piston, which vacuum is created by the condensation of the discharging, or exhaust steam, by cold water. As there are two factors at work impelling the piston, only a relatively low pressure in the boiler is required. In the high pressure engines there is no condensation of the exhaust steam, but it is discharged directly into the air, and this type was originally called “puffers.” Familiar examples of the low pressure type are to be found in our side wheel passenger steamers, and of the high pressure type in the steam locomotive. [Illustration: FIG. 79.--PRINCIPLE OF CUT-OFF.]