_Answer._ It should approximate to that shown in fig. 136b. In this diagram the vertical lines represent inches of the stroke, and the scale on the left the steam pressure in pounds per square inch. The atmospheric and vacuum lines are also indicated, as already explained in answer to Question 55. The points at which the different periods of the distribution begin are indicated by the letters _a_, _b_, _c_, _d_, _e_ and _f_. These are in the order in which they occur: _a_, pre-admission; _b_, admission; _c_, expansion; _d_, pre-release; _e_, release; and _f_, compression. The lines forming the outline of the diagram will be designated for convenience of description as follows: The line from _a_ to _b_, the _admission line_. The line from _b_ to _c_, the _steam line_. The line from _c_ to _d_, the _line or curve of expansion_. The line from _d_ to _e_, the _exhaust line_. The line from _e_ to _f_, the _line of back pressure_. The line from _f_ to _a_, the _line or curve of compression_. [Illustration: _Fig. 136b._] The diagram represents a distribution of steam produced by a valve having ⁷⁄₈ in. outside and ¹⁄₁₆ inside lap, and operated by the link motion represented in fig. 103. The eccentrics have 5 in. throw, and the steam-ports are 1¹⁄₄ and the exhaust 2³⁄₄ in. wide. The valve as shown by the diagram is cutting off at 8 in., or one-third of the stroke. Pre-admission begins when the piston still has ¹⁄₂ in. to move before reaching the end of its stroke. Admission of course begins with the stroke, expansion at 8 in., pre-release at 18¹⁄₂ in., release at the end of the stroke, and compression at 17¹⁄₂ in. of the return stroke. The valve is supposed to be set without any lead, or “_line and line_,”[61] as it is called at full stroke. When the steam is cut off at 8 in. of the stroke, the valve has 2⁵⁄₈ in. travel and ³⁄₁₆ in. lead. The steam pressure in the boiler is supposed to be 100 pounds above the atmosphere. Of course, when the valve cuts off at different points of the stroke, the periods of distribution will be somewhat changed; but from the above diagram the principal features of a good distribution can be explained. [61] That is, the steam edges of the valve correspond with the steam edges of the port at the beginning of the stroke.