DESCRIPTION OF INSTRUMENT.
The indicator consists essentially of a small steam cylinder, whose under side is connected by pipes to the main cylinder of the engine under inspection. Inside the indicator cylinder is a nicely fitting piston, whose upward movement is resisted by a spring of known strength. The piston-rod passes up through the top of the indicator cylinder; and its extremity is connected with mechanism for operating a pencil, and marking on a card a diagram whose lines coincide with the movement of the indicator piston.