22 BULLETIN 119, U. S. NATIONAL MUSEUM. 



Print of James Watt's Balance Wheel Rotative Engine. 



Watt says in his specification : 



When steam is cut off at one-quarter the stroke there must be an equalizing 

 arrangement to enable the piston to complete its stroke when pumping. For 

 this purpose the upper end of the piston rod is a rack working into a toothed 

 segment. On the other end of the beam, in place of the old horsehead meshing 

 also into this segment, is a pinion secured to the shaft by a flywheel. At any 

 motion of the beam at the early part of the stroke power is expended to give 

 the flywheel motion, and the momentum thus gained is expended again at the 

 last or weakest portion of the stroke, thus equalizing the power. 



Cat. No. 180,630 U.S.N.M. 



Print which Shows the Converting of Reciprocating into Rotary Motion. 



James Watt, 1781. 



It appears that J. Pickard in 1780 took out a patent for converting 

 reciprocating into rotary motion by means of a " crank." It has been 

 said that Watt would not attempt to make any terms with the man 

 nor run the risk of a lawsuit. The use of pins in disks is repre- 

 sented. These were not called cranks in the specification, but " points 

 of attachment of the connecting rods." Another sketch represents 

 an eccentric on the shaft, the connecting rod embracing it and pro- 

 vided with three friction rollers or bearings. Another sketch repre- 

 sents the well-known " Sun and Planet " motion in which a spur wheel 

 rigidly fixed on the end of the connecting rod gears into a spur wheel 

 of equal diameter on the engine shaft and is held in gear by a pro- 

 jecting pin and friction wheel from the back of the planet wheel, 

 traveling in a circular groove concentric with the shaft. A " Spur 

 Planet" on the connecting rod and an internal geared disk on the 

 shaft held in gear by a similar method to that shown in the preceding 

 sketch is also represented. Cat. No. 180,615 U.S.N.M. 



Print of Steam Indicator Diagrams Taken with Watt's Indicator by 

 Edward Cooper, Esq., August, 1840. 



There is one view of a full-power diagram, showing the card and 

 the record made by the pencil during one revolution of the engine. 

 The card is first ruled with perpendicular lines dividing the length 

 of its horizontal movement into 10 equal spaces, corresponding with 

 the stroke of the piston divided into the same number of equal divi- 

 sions. The black horizontal line, the " atmospheric line," shows the 

 position of the pencil when not affected by the st^am or vacuum pres- 

 sure. The column of figures at the left of the diagram shows the 

 pressure in pounds to the square inch; those above the atmospheric 

 line indicate the steam pressure, while those below show the atmos- 

 pheric or vacuum pressure. The irregular black line was made by 



