a domed top made of lead in the first examples. Later boilers were made of small wrought iron plates riveted together. The problem was producing big enough plates, so that even pressures of around were not absolutely safe, nor was the cast iron hemispherical boiler initially used by Richard Trevithick. This construction with small plates persisted until the 1820s, when larger plates became feasible and could be rolled into a cylindrical form with just one butt-jointed seam reinforced by a gusset; Timothy Hackworth's Sans Pareil 11 of 1849 had a longitudinal welded seam. Welded construction for locomotive boilers was extremely slow to take hold. Once-through monotubular water tube boilers as used by Doble, Lamont and Pritchard are capable of withstanding considerable pressure and of releasing it without danger of explosion. Combustion The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines. Solid fuel firing In order to create optimum burning characteristics of the fire, air needs to be supplied both through the grate, and above the fire. Most boilers now depend on mechanical draft equipment rather than natural draught. This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as chimney height. All these factors make effective draught hard to attain and therefore make mechanical draught equipment much more economical. There are three types of mechanical draught: Induced draught: This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet or ejector oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draught fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost