Watt found it impossible to determine the duty of his engines at all times by measurement of the work itself, and endeavored to find a way of ascertaining the power produced, by ascertaining the pressure of steam within the cylinder. This pressure was so variable, and subject to such rapid as well as extreme fluctuations, that he found it impossible to make use of the steam-gauge constructed for use on the boiler. He was thus driven to invent a special instrument for this work, which he called the "steam-engine indicator." This consisted of a little steam-cylinder containing a nicely-fitting piston, which moved without noticeable friction through a range which was limited by the compression of a helical spring, by means of which the piston was secured to the top of its cylinder. The distance through which the piston rose was proportional to the pressure exerted upon it, and a pointer attached to its rod traversed a scale upon which the pressure per square inch could be read. The lower end of the instrument being connected with the steam-cylinder of the engine by a small pipe fitted with a cock, the opening of the latter permitted steam from the engine-cylinder to fill the indicator-cylinder, and the pressure of steam was always the same in both cylinders. The indicator-pointer therefore traversed the pressure-scale, always exhibiting the pressure existing at the instant in the cylinder of the engine. When the engine was at rest and steam off, the indicator-piston stood at the same level as when detached from the engine, and the pointer stood at 0 on the scale. When steam entered, the piston rose and fell with the fluctuations of pressure; and when the exhaust-valve opened, discharging the steam and producing a vacuum in the steam-cylinder, the pointer of the indicator dropped below 0, showing the degree of exhaustion. Mr. Southern, one of Watt's assistants, fitted the instrument with a sliding board, moved horizontally backward and forward by a cord or link-work connecting directly or indirectly with the engine-beam, and thus giving it a motion coincident with that of the piston. This board carried a piece of paper, upon which a pencil attached to the indicator piston-rod drew a curve. The vertical height of any point on this curve above the base-line measured the pressure in the cylinder at the moment when it was made, and the horizontal distance of the point from either end of the diagram determined the position, at the same moment, of the engine-piston. The curve thus inscribed, called the "indicator card," or indicator diagram, exhibiting every minute change in the pressure of steam in the engine, not only enabled the mean pressure and the power of the engine to be determined by its measurement, but, to the eye of the expert engineer, it was a perfectly legible statement of the position of the valves of the engine, and revealed almost every defect in the action of the engine which could not readily be detected by external examination. It