LOCATING THE FOUR EXHAUST SOUNDS.
Leaning out of the cab-window, he watches the crank as it revolves, and compares the noise made by the blowing steam with the crank position. When pulling on a heavy grade is an excellent time for noting imperfections in the working of valves and pistons; for the movements are comparatively slow, while the pressure of steam on the working-parts is so heavy that any leak sounds prominently forth. The observing engineer perceives that the four sounds of the exhaust, due to each revolution of the drivers, occur a few inches before the crank reaches, first, the forward center, second, the bottom quarter, third, the back center, fourth, the top quarter. The first and third position exhausts emit the steam from the forward and back strokes of the right-hand piston: the second and fourth exhausts are due to discharges of the steam that has been propelling the left-hand piston. With these facts impressed upon his mind, he will understand, that if an intermittent blow occurs during the periods when the crank is traveling from the forward center to the bottom quarter, or from the back center to the top quarter, the chances will be that the right-hand piston needs to be examined. For the greatest pressure of steam follows the piston just after the beginning of each stroke, and that is the time a blow will assert itself. Should the blow occur while the right-hand crank is moving from the bottom quarter to the back center, or from the top quarter to the forward center, it will indicate that the left-hand piston is at fault. For at these periods the left-hand cylinder is receiving its greatest pressure of steam.