in Ex hausting Air from Tubes, Sj-c. 



7 



two levers when moved at the centre, because a very 

 powerful, and, at the same time, accurate, movement, is 

 given to the valve, which valve is also retained, or locked, 

 upon its seat, until the movement is reversed, and this 

 without any material strain upon the connecting-rods and 

 eccentric movement. 



I also prefer that the gearing of the valves be worked 

 very promptly, and that after the valves are respectively 

 opened or shut, the motion should remain what is termed 

 dead until the motion is required to be reversed, and I 

 effect this by a plate of metal, out of which is cut a space 

 described by the intersection of two circles, as shown in 

 fig. 1, sheet No. III., which is used for the inlet valves, 

 which may all be moved by one eccentric being opened 

 and shut at equal intervals,— that is, shortly after the 

 change of the stroke of the piston. 



Fig. 2, shows the form of the plate for moving the 

 gearing of the upper outlet valves, and 



Fig. 3, that for the lower outlet valves. 



In figs. 4, 5, and 6, the eccentrics are represented in 

 their places on their driving-shaft, and are referred to by 

 the like letters and figures ; a, representing the driving- 

 shaft, b \ and b^, pulleys, or friction-rollers, working on 

 centres attached to the pieces c, c, and c, which are bolted 

 or screwed to the driving-shaft. d^, d\ and d^, are the 

 three plates represented separately in figs. 1, 2, and 3. 

 E, E, and E, are pulleys, or friction- rollers, acting as guides 

 to their respective plates. The distance from the acting 

 circumference of the rollers, b, to the centre of the driv- 

 ing-shaft must be made equivalent to the semi-diameter 

 of the circles by which the spaces in the plates, d, are set 

 out. Therefore, when the driving-shaft carries round the 

 rollers, b, as long as each travels in the segment of the 

 circle corresponding to it, the plate remains at rest, but 

 when the roller arrives at the other segment, the plate is 

 moved until such segment corresponds with the arc de- 

 scribed by the pulley, whicli then moves on, leaving the 

 plate at rest until the pulley arrives at the other segment, 

 the quantity of motion given by the eccentric being that 

 between the centres of the two arcs of the respective open- 

 ings in the plates, d^, and With the plate fig. 1, 

 the motions are at equal intervals, but with the plates 

 figs. 2 and 3, in which the arcs of circles are of unequal 

 lengths, the alternate motions are in unequal times. 



