and the lead of the valve: the points in which this line intersects the
circle of the eccentric are the points in which the centre of the eccentric
should be placed for the forward and reverse motions. When the eccentric
rod is attached directly to the valve, the radius of the eccentric, which
precedes the crank in its revolution, forms with the crank an obtuse angle;
but when, by the intervention of levers, the valve has a motion, opposed to
that of the eccentric rod, the angle contained by the crank and the radius
of the eccentric must be acute, and the eccentric must follow the crank: in
other words, with a direct attachment to the valve the eccentric is set
_more_ than one fourth of a revolution in advance of the crank, and with an
indirect attachment the eccentric is set _less_ than one fourth of a circle
behind the crank. If the valve were without lead or lap the eccentric would
be exactly one fourth of a circle in advance of the crank or behind the
crank, according to the nature of the valve connection; but as the valve
would thus cover the port by the amount of the lap and lead, the eccentric
must be set forward so as to open the port to the extent of the lap and
lead, and this is effected by the plan just described.

521. _Q._--In the event of the eccentrics slipping round upon the shaft,
which you stated sometimes happens, is it necessary to perform the
operation of setting the valve as you have just described it?

_A._--If the eccentrics shift upon the shaft, they may be easily refixed by
setting the valve open the amount of the lead, setting the crank at the end
of the stroke, and bringing round the eccentric upon the shaft till the
eccentric rod gears with the valve. It would often be troublesome in
practice to get access to the valve for the purpose of setting it, and this
may be dispensed with if the amount of lap on the valve and the length of
the eccentric rod be known. To this end draw upon a board two straight
lines at right angles to one another, and from their point of intersection
as a centre describe two circles, one representing the circle of the
eccentric, the other the crank shaft; draw a straight line parallel to one
of the diameters, and distant from it the amount of the lap and the lead:
the points in which his parallel intersects the circle of the eccentric are
the positions of the forward and backward eccentrics. Through these points
draw straight lines from the centre of the circle, and ma