All About Engines

Raising Steam 59 volume for a given area of surface, than any other form. If a liquid or a gas is forced into a vessel of any other shape it tends to become a sphere. Thus, an oval boiler would tend to become cylindrical, and in a cylindrical boiler the flat ends tend to bulge— in fact, all flat sides have a tendency to curve outwards under internal pres- sure ; and during these deformations some of the joints would probably give way. But as Fig. 21.—How a boiler is strengthened lengthways it would be very difficult to bend the plates for a spherical boiler—they would have to be pressed—the usual form adopted is that of a cylinder. The flat ends are prevented from bulging in two ways : long Fig. 22.—How the ends of a boiler are strengthened rods passing right through from end to end, called “longitudinal stays” (Fig. 21), and brackets or “gusset stays” (Fig. 22) fastening the flat ends to the curved sides. After Newcomen’s copper boilers with lead tops, wrought-iron was used, but this has now given way to steel, the plates varying from | inch to ij inches in thickness. Holes for the rivets can be