of motion, and the length of the rod is from B to C;
when the eccentric centre moves to E, the centre line of the rod will be
moved to position P, the line Q will have assumed position R, and point
C will have moved from its position in the drawing to G on line _x_. If
the eccentric centre be supposed to move on to F, the point C will move
to H, the radii B C, E G, and F H all being equal in length. Now when
the eccentric centre is at E it will have moved one-quarter of a
revolution, and yet the point C will only have moved to G, which is not
central between C and H, as is denoted by the dotted half circle I.

[Illustration: Fig. 265.]

On the other hand, while the eccentric centre is moving from E to F,
which is but one-quarter of a revolution, the rod end will move from G
to H. This occurs because the rod not only moves _endwise_, but the end
connected to the eccentric strap moves towards and away from the line
_x_. This is shown in the figure, the rod centre line being marked in
full line from B to _x_. And when B has moved to E, the rod centre line
is marked by dotted line E, so that it has moved away from the line of
motion B _x_. In Figure 266 the eccentric centre is shown to stand at an
angle of 45 degrees from line _q_, which is at a right angle to the line
of motion _x x_, and the position of the rod end is shown at C, J and H
representing the extremes of motion, and G the centre of the motion.

[Illustration: Fig. 266.]

If now we suppose the eccentric centre to stand at T, which is also an
angle of 45 degrees to _q_, then the rod end will stand at K, which is
further away from G than C is; hence we find that on account of the
movement of the rod out of the straight end motion, the motion of the
rod end becomes irregular in proportion to that of the eccentric, whose
action in moving the eye C of the rod in a straight line is increased
(by the rod) while it is moving through the half rotation denoted by V
in figure, and diminished during the other half rotation.

In many cases, as, for example, on the river steamboats in the Western
and Southern States, cams are employed instead of eccentrics, and the
principles involved in drawing or marking out such cams are given in the
following remarks, which contain the substance of a paper read by Lewis
Johnson before the American Society of Mechanical Engineers. In Figure
267 is a side view of a pair of cams; one, C, being a full stroke cam
for operating the valve