Instead of building the boilers in two or three flat-sided divisions up to the water-line, and covering the whole with one slightly convex roof or top, many manufacturers prefer completely distinct boilers of an elliptical form, connected merely by pipes, forming the steam and water communications. This method has the advantage, that if either of the side-boilers should be by any accident rendered unserviceable, it may be shut off", and thus it would not impair the efficiency of the other two, provided that the steam communications should be each furnished with a stop-valve.
Elliptical boilers, variously arranged, are getting much into use in Scotland, especially on the Clyde. The annexed sketches represent the boilers of the Sirius steam-ship, constructed by Messrs. Wingate and Co. of Glasgow. Fig. 1 is a transverse section of the middle boiler and one of the side-boilers, and a front elevation of the other side-boiler. Fig. 2 is a longitudinal section of the middle boiler.
The three boilers are all of the same dimensions, and have each three fireplaces a a a, as shown in the elevation of one of the side-boilers. From each fire-place a flue b b proceeds in a direct line to the back of the boiler, where it is curved upwards, and is returned, and forms an upper flue c, lying directly over the flue b, and extending to about midway of the boiler. Each of the three flues c opens into a transverse flue d, and the transverse flues d of the side-boilers communicate with the transverse flues of the centre boiler. e is the chimney, rising from the upper side of the transverse flue of the centre boiler, and passing through g the steam-chest of the same boiler: h and k are the steam-chests of the side boilers. and communicate with the steam-chest of the centre boiler, the former by the pipe m, and the latter by the pipe n. A the bottom of each boiler is a pipe o, furnished with a cock, and branching into the transverse main -p, by which means the communication between the water spaces of any two of the boilers can be opened or cut off at pleasure; q is the steam main proceeding to the engines; r the safety-valve, upon the spindle of which are placed the weights; s the pipe for the escape of the steam which has passed the safety-valve; t is the feed-pipe, fitted with stop-cocks; and w a glass gauge for showing the height of the water in the boiler; x x are gauge-cocks for the same purpose.