_Answer._ The motion of the valve is susceptible of an almost infinite number of changes, by different variations and combinations of proportions of the working parts of the link motion. These changes are, however, limited by the general laws which govern the motion of eccentrics, and therefore cannot influence the motion of the valve beyond certain limits. Hardly any variation can be made either in the proportions or arrangement of the working parts which will not have some influence upon the movement of the valve. Aside from the proportions of the valve itself, which have already been discussed, the throw of the eccentrics, the length of the rods and of the link, the point of connection of the rods with the link, the point of suspension, the position of the lifting shaft, the length of the arms, the length and position of the rocker arms will each of them effect the distribution of steam. The number of combinations of all these different proportions is of course almost infinite, and therefore any full discussion of them will be impossible here. QUESTION 227. _What are the most important points which require attention in designing a link motion?_ _Answer._ It should be proportioned so that-- First, the lead and the period of admission should be the same for each end of the cylinder, for each point of cut-off, and, if possible, in back as well as forward gear. Second, the width of opening for both admission and exhaust should be as large as possible when steam is cut off short. Third, the exhaust or pre-release should occur early enough and be maintained long enough to reduce back-pressure as low as possible. QUESTION 228. _How can the lead and period of admission be equalized?_ _Answer._ It is impossible to make the periods of admission absolutely alike for every point of cut-off in both fore and back gear. It is therefore customary to disregard the back gear, as engines are worked but little with the link in that position. Even for forward gear the periods of admission cannot be made exactly alike for each end of the cylinder and for each point of cut-off, and therefore it is usual to make the periods of admission alike for half-gear forward, in which position the link is worked most.