_Answer._ By the _travel_ we mean the motion of the valve back and forth, or in other words its stroke. If the arms of the rocker are of the same length, the travel of the valve is equal to the throw of the eccentric. For the preceding illustrations we have selected an eccentric with three inches throw, which is the travel of the valve. [Illustration: _Fig. 27._ Scale ³⁄₁₆ in. = 1 inch.] QUESTION 49. _How is the steam made to work expansively with a slide-valve?_ _Answer._ By giving the valve what is called _lap_. That is, by allowing the edges of the valve when it is in the center of the valve-seat to overlap the edges of the steam-ports, as shown in fig. 27. Where this overlap, _L L_, is on the outside of the valve, it is called _outside lap_; when on the inside, _l l_, _inside lap_. When a valve has lap, those portions of the _face_[14] _h_, _i_, and _h′_, _q_, which cover the steam-ports, being wider than the ports, therefore occupy some time in moving over them, during which time the steam is enclosed in the end of the cylinder, as there is then no communication either with the steam-chest or the exhaust-port. This action is shown very clearly by the motion-curves in fig. 26. The valve in this case has ¹⁄₄ inch lead. At 4¹⁄₂ inches of the stroke of the piston the valve has moved as far as it will go in that direction, and the steam-port has its maximum width of opening. From that point the valve will begin to close the steam-port, and at 14¹⁄₂ inches of the stroke the port will be entirely covered, and the steam therefore be _cut off_. The port will remain closed until the piston has moved 21³⁄₄ inches, when it will be observed from the motion-curve _r s t u v w x_, that the port _c_ is opened to the exhaust and the steam escapes, or, as it is technically called, the _release_ takes place. From the time the steam is cut off to the time it is released, it works _expansively_ in the cylinder. [14] The valve-face is the surface of the valve in contact with the valve-seat. QUESTION 50. _What relation is there between the amount of lap_[15] _and the degree of expansion?_ [15] In speaking simply of _lap_, outside lap is always meant. _Answer._ The greater the lap with any given travel, the shorter will be the period of admission of steam, and, consequently, the more time and space for expansion. QUESTION 51. _What is the effect of inside lap?_