REGULATION OF STEAM.

123

part of the stroke of the piston, when we have, as in the figure, thediameter of the eccentric path much greater than the diameter ofthe crank path. It thus becomes apparent that the valve must be

Fig. 67.—DIagram of the Lap of Valves.

altered, the multiple-ported type being adopted. Thus, by intro-ducing three steam ports at each end of the valve, with one centralexhaust port, we gain the same area of port on the valve, while theopening of each port is only one-third of that of the single-portedarrangements, so when the steam is cut off at one-eighth or one-fourth part of the stroke of the piston, a valve should be adoptedhaving three steam ports at each end, with one central exhaust port,thus greatly reducing the stroke of the valve, and consequently thelap; the lead may be presumed equal, at least as far as a particularengine is concerned. The lead is greater for high-speed heavy pistonsthan for engines of the locomotive type, some marine engines hav-ing inch of lead, and even at times more, according to the weightand speed, while for high-speed light pistons ^ of lead sufhces.It must be borne in mind that the length of the connecting rodmaterially alters the chord of the arc of supply on the crank path.With a short rod the chord becomes longer, and vice versa; and asthe versed sine of the chord of the arc of supply on the crank path