reached the point _b_ the valve will have received its greatest travel, and that when it reaches _p_ the steam-port _c_ will again be closed or the steam cut off. If the small eccentric is employed, the valve will then have its maximum travel when the centre _h_ reaches _s_, and the port will be closed when it reaches _i_. By drawing lines, _o p_ and _o n_, through _i_ and _p_, it will be seen that from the beginning of the stroke until the steam is cut off, if the large eccentric is employed, it, and consequently the shaft and crank, must move over an angle measured by the arc _q t p_. If the small eccentric is used, it and the crank must move through an angle measured by the arc _u t n_. In other words, the crank must turn a considerably greater distance before steam is cut off with an eccentric having a large than with one having a small throw. [63] It will be seen that this causes the position of the centre of the eccentric to be reversed. [Illustration: _Fig. 142._ _Fig. 143._ Scale ³⁄₁₆ in. = 1 inch.] It is also quite obvious from fig. 142 why the port is opened a shorter distance with a small than with a large eccentric. The distances _o s_ and _o b_ are equal to half the throws of the eccentrics, or 1³⁄₄ and 2¹⁄₂ in. The linear advance _o r_ is in both cases ⁷⁄₈ in., and therefore after the port begins to open the valve will be moved by the small eccentric a distance which is equal to 1³⁄₄ - ⁷⁄₈ = ⁷⁄₈ in., and by the large one 2¹⁄₂ - ⁷⁄₈ = 1⁵⁄₈ in. [Illustration: _Fig. 144._ Scale ³⁄₁₆ in. = 1 inch.] QUESTION 234. _What is the effect on the admission of giving an eccentric with a small throw the same angular advance as one with a large throw, and then reducing the lap of the valve so that the lead will be the same in both cases?_