In the first place, the centre _S_, fig. 124, of the axle, _A_, of the rocker, and _B_ of the lifting-shaft, must be laid down in their proper positions. If, now, the valve has ⁷⁄₈ in. lap and ¹⁄₈ lead, the lower rocker-pin must be one inch ahead of its middle position when the piston is at the front end of the cylinder, and at the beginning of the backward stroke. We will, therefore, mark the centre, _a_, of the rocker-pin in this position. If from the centre of the axle a circle, _c d e_, be drawn whose diameter is equal to the throw of the eccentrics, this circle will represent the path in which the centres of the eccentrics will revolve. If, now, the distance from the centre of the axle to the centre of the lower rocker-pin, _a_, when the latter is in its middle position, be taken for a radius,[52] and from the position of the rocker-pin at the beginning of the stroke as a centre, the circle representing the path of the eccentrics be intersected at two points, _c_ and _d_, the points of intersection will represent the positions of the centres of the forward and backward eccentrics. Having determined these positions, draw arcs of circles, _f_ and _g_, from these centres with a radius equal to the distance from the centres of the eccentrics to the centres of the pins which connect the rods to the link. It is evident that at the beginning of the stroke the centres of the pins in the link must each be in one of these arcs. But the link is suspended by the hanger, _i h_, which oscillates from the end, _i_, of the lifting-arm, which for any one point of cut-off is stationary; and therefore the point of suspension of the link must always be in the arc, _j k_, described from the centre of the pin, _i_, in the lifting-arm, with a radius equal to the length of the hanger. There are, therefore, three points in the link, each of which must be in one of the arcs which have been drawn, and which will determine the position of the link. This can be done easiest by drawing the link, _L_, fig. 125, on a stiff piece of paper, _m n_, and cutting off the back, _p q_, of it through the centres of the pins, _s_ and _t_, and also cutting out a triangular piece, _u_, the apex of which will correspond with the centre of the point of suspension, _o_. By placing this piece of paper on the drawing it can be moved, so that the three centre points, _s_, _t_ and _o_, will respectively conform with the arcs, _f g_ and _j k_, fig. 124. In this position the piece of paper will then be in the position of the link for the point of the stroke represented. By marking the centres of the link-pins on the arcs _f_ and _g_, and from them as centres, with the length of the rods used to draw the arcs, two other arcs, _v_, _w_, be drawn intersecting each other, the point _d_, where they intersect will be the centre from which the centre line of the link can be drawn with a radius, _l d_, equal to the distance from the centre of the eccentrics to the centre of the link.