antamount to turning the eccentric round on the shaft. In that
form of the link motion in which the link itself is not raised or lowered,
but is susceptible of a motion round a centre in the manner of a double
ended lever, the lead continues uniform. In both forms of the link motion,
as the stroke of the valve may be varied to any required extent while the
lap is a constant quantity, the proportion of the lap relatively to the
stroke of the valve may also be varied to any required extent, and the
amount of the lap relatively with the stroke of the valve determines the
amount of the expansion. In setting the valve when fitted with the link
motion, the mode of procedure is much the same as when it is moved by a
simple eccentric. The first thing is to determine if the eccentric rods are
of the proper length, and this is done by setting the valve at half stroke
and turning round the eccentric, marking each extremity of the travel of
the end of the rod. The valve attachment should be midway between these
extremes; and if it is not so, it must be made so by lengthening or
shortening the rod. The forward and backward eccentric rods are to be
adjusted in this way, and this being done, the engine is to be put to the
end of the stroke, and the eccentric is to be turned round until the amount
of lead has been given that is desired. The valve must be tried by turning
the engine round to see that it is right at both centres, for going ahead
and also for going astern. In some examples of the link motion, one of the
eccentric rods is made a little longer than the other, and the position of
the point of suspension or point of support powerfully influences the
action of the link in certain cases, especially if the link and this point
are not in the same vertical line. To reconcile all the conditions proper
to the satisfactory operation of the valve in the construction of the link
motion, is a problem requiring a good deal of attention and care for its
satisfactory solution; and to make sure that this result is attained, the
engine must be turned round a sufficient number of times to enable us to
ascertain if the valve occupies the desired position, both at the top and
bottom centres, whether the engine is going ahead or astern. This should
also be tried with the starting handle in the different notches, or, in
other words, with the sliding block in the slot or opening of the link in
different positions.

MANAGEMENT OF MARINE BOILERS.