========+============================ | Width of Opening | of Steam-port. +------------+--------------- Point of| Eccentric | Eccentric Cut-Off.|5 in. throw.|3¹⁄₂ in. throw. --------+------------+--------------- 6 in. | ⁷⁄₃₂ in. | ⁵⁄₃₂ in. 8 „ | ⁹⁄₃₂ „ | ³⁄₁₆ „ 10 „ | ¹¹⁄₃₂ „ | ⁷⁄₃₂ „ 12 „ | ⁷⁄₁₆ „ | ⁹⁄₃₂ „ 15 „ | ⁵⁄₈ „ | ³⁄₈ „ 18 „ | ³¹⁄₃₂ „ | ¹¹⁄₁₆ „ 21 „ | 1¹⁄₄[64] „ | 1¹⁄₃₂ „ --------+------------+--------------- [64] The valve throws over 1³⁄₄ in. at this point. It will be seen from this that the eccentric with 5 in. throw gives a greater width of opening for every point of cut-off than the one with 3¹⁄₂ in. throw. For the higher admissions this is not important, but when steam is cut off short it will be observed that the width of the opening is very small. At high speeds the small opening is a great disadvantage. QUESTION 236. _Has it been determined what amount of opening is required for given speeds of the piston?_ _Answer._ Not with any degree of accuracy. It is customary to make the area of the ports about one-tenth that of the piston. It is certain, however, that with steam-ports of this proportion an opening considerably less than their whole area is sufficient to maintain steam at boiler pressure in the cylinders. One of the defects of the link motion is that the opening of the port is very small when the steam is cut off short. It is best, therefore, to secure the largest practicable opening of the ports for the lower points of cut-off. QUESTION 237. _What are the proportions of the valves and eccentrics used in the ordinary practice in this country?_ _Answer._ The following report made by a committee of the Master Mechanics’ Association will show the proportions used on thirty-five different railroads, and is a fair indication of the common practice.