EXPLANATION OF THE PLATES. 339 



Fig. 10. and 11., parallel motions for steam boat engines, show how the point 

 of connexion of the radius rod may be determined by geometrical construction. 

 Thus, having determined the three points, H, //, h', set off h k equal to C h' ; join 

 C k, and bisect it with the straight line n d, meeting k h d in the point required : 

 for since k d = C d, and h k = C h', we have h d = h' d' ; and hence when d arrives 

 at d', h will coincide with h', and the motion will thus be properly adjusted. 

 Fig. 1. may be constructed in this manner. See art. 492. 



Fig. 12. exhibits a simple parallel motion, and is easily constructed geometrically 

 by finding C, the centre of a circle, passing through the three points D D' D''. 

 Fig. 13. shows how a pump rod may be attached to this motion. 



PLATE XI. 



This plate represents a plan and elevation of an atmospheric engine for raising 

 water from a mine. The beam is supported by a frame of cast iron, designed so 

 that it may be taken apart when it is necessary to move the engine to another 

 mine, (art. 578.) The steam comes from the boiler by the steam pipe S, and is 

 admitted to the cylinder C by a sliding piston in B, (see Fig. 2. Plate vi.) and 

 then the piston in the cylinder C rises to the top of the stroke, and the piston rod 

 forces the end / of the beam up with it ; as the beam rises, it draws the rod F G 

 with it, which near the end of the stroke moves the sliding frame H, (art. 483.) 

 and by the rod O the slide in B, so as to shut the steam off, and open the passage 

 to the air pump A : into the passage a jet of water plays at I. The piston then 

 begins to descend by the pressure of the atmosphere and raises the pump rods, and 

 at the end of the stroke the part F of the rod F G moves the slide, and shuts the 

 passage to the air pump, and opens that for the steam. See Plate vi. Fig. 1. 



The parallel motion 1 is guided by the radius rod cd attached to the frame, (art. 

 491. and 495.) and by connecting the rods h i : the same rod does for both ends 

 of the beam. The sliding frame H is supported by a cross bar beneath H, 

 and another at K, and the slide may be moved by hand by the lever M. Cold 

 water for injection is supplied by the pump E, and water is raised from the hot 

 well by the pump D, and passes to the boiler by the pipe Q, with a small branch 

 pipe at P, to give water to the top of the piston. See art. 400 405. 



1 In the parallel motion apparatus, shown in Fig. 1., the vibration of the beam is not bisected. 

 See p. 233, Note. 



