Thornycroft (Natural Circulation) Although John Isaac Thornycroft founded the Steam Carriage and Wagon Company in London during 1864, restrictive laws caused him to instead enter steam powered maritime production.  Given his success in the maritime arena, one can only speculate whether the introduction of the automobile was set back a generation. By 1895, changes in the legislative scene encouraged Thornycroft to build a steam land vehicle, leading to full production of steam trucks and busses in 1898.  The company is still in existence, distributing small engines based on Mitsubishi designs.   At the dawn of the 20th century, however, Thornycroft was a major force in the development of commercial motor vehicles. The drawing depicts a side and top view of the Thornycroft boiler.  A nearly vertical circle of water tubes is arrayed around the firebox, connecting annular upper and lower drums.  Combustion gasses, produced by a coal fire resting on the grate, pass between the tubes to the outer shell where a steam jet derived from the engine exhaust draws the gasses into the exhaust stack and propels them out of the boiler. Supply of air to the burner is termed ‘draft’.   The term induced draft is used when gas flow results from a mechanism forcing the exhaust out of the boiler with the resulting partial vacuum drawing in new air. Forced draft describes mechanisms which force air into the burner, such as the fan in the Doble burner. Induced draft is preferred for solid fuel burners because the exhaust blower maintains the firebox pressure slightly below the atmosphere which causes air to rush in rather than flames rushing out whenever the door is opened to add fuel. The Thornycroft boiler circulates naturally, differences in water density causing the water to descend in the downcomer tubes and rise in the generator tubes.  Downcomers are commonly of larger diameter than generator tubes and shielded from the boilers heat; this case is unusual in that the generator and downcomer tubes are identically sized.  A shield between the firebox and exhaust stack prevents the flow of hot combustion gases between the downcomer tubes which reduces the heat they absorb and allows the water inside to descend.