238 



OF THE PARTS OF 



[SECT. vn. 



If great accuracy be required, it will be better to take the positions within each 

 extremity by about one-sixth of the half stroke, the note, page 235, being pre- 

 viously followed in the calculation of the parts. 



To FIND THE LENGTH OF THE CONNECTING ROD. Set the beam at half stroke, 

 that is, parallel to the horizon ; then the distance between the centre of the pin on 

 which the connecting rod is to move and the centre of the shaft, is the length of 

 the connecting rod. 



In concluding this part of our subject we may further observe, that a consi- 

 derable increase of accuracy in the motion will be obtained by the adoption of the 

 expedient recommended in the note of page 230, in each calculation where the 

 length of the stroke is taken into account ; and this will be an advantage of some 

 importance in the construction of large engines. According to Rules n. and v., if 

 the beam and radius bar be adjusted parallel to the horizon at half stroke, and the 

 vibration of the beam bisected, the direction of the motion of the point G where the 

 piston is connected will be precisely vertical at half stroke, and its deviation will 

 take place, alternately right and left, towards each extremity of the stroke. By 

 Rules i. and iv. the three principal positions of the point G, viz. at the two extre- 

 mities and at half stroke, will be precisely in a vertical straight line, and its devia- 

 tion will take place, on alternate sides, between the middle and each extremity. 

 Thus in the adjoining diagram, if the beam and radius 

 bar be fixed parallel at half stroke, G G G G, Fig. 1. 

 represents the form of curve described when calculated 

 by Rule n. or v. ; but when the radius bar is properly 

 fixed as directed, the point G will coincide with the 

 line at each extremity, and at another point not ex- 

 actly but nearly at half stroke, and at half stroke the 

 bars will not in this case be exactly parallel. 



Also Fig. 2. shows the curve according to Rule i. or 

 iv. ; but if the half length of the stroke be diminished 

 by its one-sixth part previously to the calculation of the rule, the position of the 

 curve described will be as represented in Fig. 3., in which the intersections 

 occur at the middle and one-sixth of the half stroke from each extremity. 

 These rules will cause the bars to be precisely parallel to the horizon at half 

 stroke, and may be fixed in this way, or, perhaps more conveniently, adjusted by 

 the method before described. 



Fiffl 



fiyi 



