valve, the longer end connected with the crosshead then at one end of 

 the stroke, and if the crosshead be at the other end of the stroke, the upper 

 wrist will be as far removed to the opposite side of the centre line. 



It will also be observed that the position of tlie wrist operating the valve 

 rod will remain the same, in whatever part of the link bar the reversing 

 bar of rod D F may be situated, whether in full gear forward or backward, 

 or at any intermediate point, and the lead of the valve will be the same 

 at both ends of the stroke. The throw of the eccentric for this valve gear 

 will be shorter than the travel of the valve, inasmuch as part of the valve 

 movement is obtained from the crosshead. In practice it is found that 

 about two-thirds of the lead of the valve is obtained from the crosshead, 

 and the eccentricity of the eccentric is lessened to that extent. At first 

 sight it would seem as though the whole was due to the cross-head, but it 

 must be borne in mind that during the last half of each stroke of the piston 

 the eccentric motion is in a direction opposite to that of the crosshead, 

 thus combining the two movements to extend the travel of the valve. 

 The effect of this combination is to accelerate the movement of the xalve 

 at the opening of the ports, and to retard it at and towards the end of the 

 throw of the valve, thus giving a longer admission of steam with a given 

 lap of valve than by the eccentric motion alone. It would be quite prac- 

 ticable to operate the rocker arm link of one engine from the crosshead of 

 the opposite engine, the lead being obtained from its own crosshead, and 

 probably the greatest objection to such an arrangement would arise from 

 the fact that the disability of one engine through an accident would render 

 the valve movement of the other engine inoperative. 



136 



