eccentric must then be turned round upon the shaft until the notch in the eccentric rod comes opposite the pin on the valve lever, and falls into gear:  mark upon the shaft the situation of the eccentric, and put on the catches in the usual way.  The same process must be repeated for going astern, shifting round the eccentric to the opposite side of the shaft, until the rod again falls into gear.  In setting valves, regard must of course be had to the kind of engine, the arrangements of the levers, and the kind of valve employed; and in any general instructions it is impossible to specify every modification in the procedure that circumstances may render advisable.

725. Q.—­Is a similar method of setting the valve adopted when the link motion is employed.

A.—­Each end of the link of the link motion has the kind of motion communicated to it that is due to the action of the particular eccentric with which that end is in connection.  In that form of the link motion in which the link itself is moved up or down, there is a different amount of lead for each different position of the link, since to raise or lower the link is tantamount 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