338 EXPLANATION OF THE PLATES. 



PLATES X. (A) AND (B). 



The figures of these plates are to illustrate the combinations used to produce 

 rectilinear motion, from motion in a circular arc. 



Fig. 1. is the parallel motion used for steam boat engines. The beam A F is 

 below the cylinder ; from G, the end of the cross head, draw a line to A, the centre 

 of the axis of the beam, and it will cut the rod D B in E, and the length of the 

 radius bar C D may be found by art. 492. ; when E B is equal to E D, the length 

 of the bar C D is equal to A B, and this is the best though not always the most 

 convenient form. The rod D' G may be at any height, provided it be parallel to 

 A F ; and B may be at any point in A F, if the position of C is not limited. 



Fig. 2. shows the most common construction for engines with the beam above 

 the cylinder. H is the piston rod connected at G, and C D is the radius bar. 

 The line G A cuts the link B D in E, the proper point for the air pump rod. 

 Fig. 3. shows a plan of the upper side of the beam, where C D, C D are the radius 

 bars ; and the beam is in two parts, as is usual in large engines. 



Fig. 4. is a diagram to illustrate the investigation of the properties of the 

 combination in its most simple form. See art. 489. 



Fig. 5. is a diagram for the apparently more complicated case,- when the rod is 

 fixed to one angle of a parallelogram. See art. 492. 



Fig. 6. shows how to arrange for three piston rods to move parallel, as for 

 Woolf s engine : the points of suspension must be all in the line A G ; and any 

 number of them may be similarly adapted. 



Fig. 7. shows another arrangement for three rods at one end, and two at the 

 other end of the beam. See art. 495. The dotted arcs, Plate x. (B), show how 

 the centre C for the radius bar may be determined geometrically, by drawing a 

 circle through the three positions, D, d, d', of the point of connexion. 



In all the cases the corresponding points are marked by the same letters, and 

 therefore by referring to the investigation of Fig. 4. the relations may be traced : 

 the particular forms of different engine makers will be found by turning the part 

 upside down, altering the place of the parallel bar G D, or altering the proportions 

 of the parts. In every combination where the bar C D is not equal to A B, the 

 variation from rectilinear motion increases with the extent of the angle described. 



Variations of the parallel motion are exhibited in some of the other Plates. 

 Fig. 8. is designed to show that the vibrating pillar motion is exactly the same 

 in principle, and only differs in the particular mode of combination. See art. 494. 

 Fig. 9. is a particular case. 



