90 BULLETIN 173, U. S. NATIONAL MUSEUM 



of indicators. Since 1862 the advance in design has been largely in 

 the refinement of various parts, lightening the piston and pencil 

 mechanism, steam jacketing the cylinder, and making the parts more 

 accessible and the whole instrument more rapid to use. The latest in- 

 dicator in the Museum's collection is equipped with an appliance for 

 taking an uninterrupted record of diagrams of the successive strokes 

 of an engine. 



WATT STEAM-ENGINE INDICATOR, c. 1796 

 U.S.N.M. no. 309680; copy; made in the Museum; not illustrated. 



This is a copy of an indicator in the Science Museum at London, 

 which belonged to one of the Boulton, Watt & Co. agents at Man- 

 chester, England. 



The instrument consists of a vertical brass cylinder fitted with a 

 piston, 1 square inch in area, provided with a rod attached to the 

 piston and projecting from the upper end of the cylinder. Between 

 the piston and the cap at the upper end of the cylinder and fastened 

 to both is a helical spring that is lengthened and shortened by varia- 

 tions in pressure below the piston and causes the height of the piston 

 to be a measure of the pressure. Tlie lower end of the cylinder is 

 fitted with a tapered plug cock by which it could be attached to the 

 tapered sockets provided in the cylinders of the early engines. At- 

 tached to the upper end of the piston rod is a pencil that traces a curve 

 on paper fastened to a small board, which is moved across the point 

 of the pencil by the motion of the engine. The motion of the engine 

 was transmitted to the paper by a cord from a point on the bridle of 

 the engine, and the return motion of the board was caused by a weiglit 

 and cord. 



The first form described by Watt was an adoption of the vacuum 

 gauge consisting of a spring-loaded piston nicely fitted to a cylinder. 

 A pointer connected to the piston indicated the pressure within the 

 cylinder. With the slow-moving engines with which this was used, 

 readings of the pressure could be made at known points in the engine's 

 stroke, and the data plotted. 



DIAGRAM TAKEN WITH WATT INDICATOR 



U.S.N.M. no. 180629; print; deposited by J. E. Watkins; not illustrated. 



This diagram was taken with a Watt indicator on a low-pressure 

 condensing engine by Edward Cooper, August 1840. It represents the 

 full-load diagram of the engine. The maximum pressure above the 

 atmospheric line is 5 pounds per square inch, and the exhaust pressure 

 is 13 pounds per square inch vacuum. The card is practically rectan- 

 gular and is computed to have indicated 120 horsepower. Additional 

 notes on the card describe the engine as having a 7-foot stroke, a speed 

 of 171/2 revolutions per minute, and a rating of 60 horsepower. 



