were enormous. On one occasion he congratulated himself because one of his steam-cylinders was only three-eighths of an inch out of truth in the bore. Nowadays a good firm would reject a cylinder 1⁄500 of an inch out of truth; and in small petrol-engines 1⁄5000 of an inch is sometimes the greatest "limit of error" allowed.
THE ECCENTRIC
is used to move the slide-valve to and fro over the steam ports (Fig. 23). It consists of three main parts—the sheave , or circular plate S, mounted on the crank shaft; and the two straps which encircle it, and in which it revolves. To one strap is bolted the "big end" of the eccentric rod, which engages at its other end with the valve rod. The straps are semicircular and held together by strong bolts, B B, passing through lugs, or thickenings at the ends of the semicircles. The