. The railroad and engineering journal . exhaust ofsteam to and from the cylinder of an ordinary engine ? Answer. I. It must admit steam to one end only of thecylinder at one time, so that the pressure, which moves thepiston, will not be e.\erted on both sides of it at the same time. 2. It must cover the steam-ports so as not to permit steam toescape from both steam-ports at once. 3. It must allow the steam to escape from one end of thecylinder before it is admitted to the other end, so as to give thesteam, which is to be exhausted, time to escape before the pistonbegins its return stroke. 4.
. The railroad and engineering journal . exhaust ofsteam to and from the cylinder of an ordinary engine ? Answer. I. It must admit steam to one end only of thecylinder at one time, so that the pressure, which moves thepiston, will not be e.\erted on both sides of it at the same time. 2. It must cover the steam-ports so as not to permit steam toescape from both steam-ports at once. 3. It must allow the steam to escape from one end of thecylinder before it is admitted to the other end, so as to give thesteam, which is to be exhausted, time to escape before the pistonbegins its return stroke. 4. It must not allow live steam * to enter the exhaust-portfrom the steam-chest. 5. In order to utilize the expansive force of (he steam, thevalve must close each steam-port on the outer or steam sidebefore it is opened on the exhaust side. Question 107. Now does the valve shown by figs. 49-51 fulfilthese conditions ? Answer, i. The lap on the outside of this valve being greaterthan that inside makes it impossible to open either one of the. Fig. 52 steam-ports for the admission of steam, until after the otherport is opened to the exhaust. Thus the valve cannot be movedfrom the position shown in fig. 49 to that shown in fig. 51, so asto open the porttata, without first opening the port b,which allows the steam in d to escape intoe- The outsidewidth of the valve, as indicated by the dotted line m k, fig. 49,is greater than the distance over the outside edges of the steam-ports—shown by the dotted line op -so that it is manifestly im-possible for the valve to uncover both steam-ports, and thusadmit steam into each at once. 2. The width of the exhaust cavity iV in the valve, measuiedfrom /to /, is less than the distance over the inner edges of thesteam-ports c and </—consequently, these ports cannot both com-municate with the exhaust cavity simultaneously. 3. If the valve shown in fig. 51 is moving in the directionindicated by the dart at //, it is obvious that the steam-p
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