to 2, the rod end
moved from _a_ to _b_; by continuing this process we are enabled to
discern the motion for the whole of the stroke. The backward stroke will
be the same, for corresponding crank-pin positions, for both strokes;
thus, when the rod end is at 7 the crank-pin may be at 7 or at 17. This
fact enables us to find the positions for the positions later than 6, on
the other side of the circle, as at 17, 16, 15, etc., which keeps the
engraving clear.

[Illustration: Fig. 284.]

In Figure 284 a pinion, P, drives a gear-wheel, D, on which there is a
pin driving the sliding die A in the link L, which is pivoted at C, and
connected at its upper end to a rod, R, which is connected to a bolt, B,
fast to a slide, S. It is required to find the motion of S, it moving in
a straight line, dotted circle H' representing the path of the pin in
the sliding die A, arc H representing the line of motion of the upper
end of link L, and lines N, O, its centre line at the extreme ends of
its vibrating motion. In Figure 285 the letters of reference refer to
the same parts as those in Figure 284. We divide the circle H' of pin
motion into 24 equidistant parts marked by dots, and through these we
draw lines radiating from centre, C, and cutting arc H, obtaining on the
arc H the various positions for end Z of rod R, these positions being
marked respectively 1, 2, 3, 4, etc., up to 24. With a pair of
compasses set to the length of rod R from 1 on H, as a centre, we mark
on the line of motion of the slide, line _a_, which shows where the
other end of rod R will be (or in other words, it shows the position of
bolt B in Figure 284), when the centre of A, Figure 284, is in position
1, Figure 285.

[Illustration: Fig. 285.]

From 2 on arc H, we mark with the compasses line _b_ on line M, showing
that while the pin moved from 1 to 2, the rod R would move slide S,
Figure 284, from _a_ to _b_, in Figure 285. From 3 we mark _c_, and so
on, all these marks being above the horizontal line M, representing the
line of motion, and being for the forward stroke. For the backward
stroke we draw the dotted line from position 17 up to arc H, and with
the compasses at 17 mark a line beneath the line M of motion, pursuing
the same course for all the other pin motions, as 18, 19, etc., until
the pin arrives again at position 24, and the link at O, and has made a
full revolution, and we shall have the motion of the forward stroke
above and that of the back