238 
OF THE PARTS OF 
[sect. VII. 
« 
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 w T here 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 ii. 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. 
/ 
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j G G 
Fit] 1 
ix 
Fiij2 T 
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