Steam engine indicator used on SS Great Eastern

This engine indicator was used on the huge iron steamship Great Eastern, which is famous for its role in laying the first telegraph cable across the Atlantic Ocean to enable rapid communication between Europe and America. The indicator was used by chief engineer James Rorison to monitor the engines that turned the ship's two side paddles and its screw (propeller). To perform a test, Rorison would screw the indicator into a fitting on an engine cylinder. He would then open a tap to let steam enter the indicator's small cylinder and activate the piston within it. The piston moved up and down as the pressure inside the engine changed. A screw attached to the piston moved with it and shifted a pencil up and down against a piece of paper wrapped around the revolving outer drum; the pencil thus drew a closed loop called an 'indicator diagram'. Measuring the area inside the loop allowed Rorison to calculate the engine's power. Indicator diagrams from the Great Eastern show that its engines operated quite effectively, but that the opening and closing of the valves (to admit and shut off steam) affected the pressure in the cylinders for longer than was ideal.The Great Eastern was at least six times bigger that other ships made in the 1850s. Its engines were huge, and the indicator is also impressive in size. Its design is close to that of the earliest rotating-drum indicators made by John McNaught: its recording drum is concentric with its cylinder rather then being offset as in the standard McNaught design and other indicators. This 1830s design, adapted for use with the ship's engines, was probably selected for its simplicity and robustness.James Watt invented the indicator around 1790. Using Watt's indicator, the engine driver had to observe the motion of a pointer attached to the piston. Watt's assistant John Southern had the brilliant idea of adding a pencil and a moving tablet with paper attached to produce indicator diagrams, the first graphs ever drawn by scientific instruments. The rotating drum design is a compact version of the moving tablet design. It is interesting that this indicator, based on one of Watt's ideas, was used to monitor steam engines made by James Watt & Co for a ship that played a significant role in ushering in a new age of international communication. It is likely that "sometime chief Engineer of the vessel" James Rorison, confident of its place in history, wrote the neatly framed text on the presentation card that is still in the indicator's fitted box.