The nineteenth century is the age of kinetic energy: the energy of either solid, liquid, gaseous or electrical matter transformed into useful work. It has been stated by that eminent specialist in steam engineering, Prof. R. H. Thurston, that “the steam engine is a machine which is especially designed to transform energy originally dormant or potential into active and useful available kinetic energy;” and that the great problem in this branch of science is “to construct a machine which shall in the most perfect manner possible convert the kinetic energy of heat into mechanical power, the heat being derived from the combustion of fuel, and steam being the receiver and conveyor of that heat.” Watt and his contemporaries regarded heat as a material substance called “Phlogiston.” The modern kinetic theory of heat was a subsequent discovery, as elsewhere explained. The inventors of the last part of the eighteenth century and of the nineteenth century have directed their best labours to construct an engine as above defined by Thurston. First as to the boiler: Efforts were made first to get away from the little old spherical boiler of Hero. In the 18th century Smeaton devised the horizontal lengthened cylindrical boiler traversed by a flue. Oliver Evans followed with two longitudinal flues. Nathan Read of Salem, Massachusetts, in 1791, invented a tubular boiler in which the flues and gases are conducted through tubes passing through the boiler into the smokestack. Such boilers are adapted for portable stationary engines, locomotives, fire and marine engines, and the fire is built within the boiler frame. Then in the 19th century came the use of sectional boilers--a combination of small vessels instead of a large common one, increasing the strength while diminishing capacity--to obtain high pressure of steam. Then came improved weighted and other safety valves to regulate and control this pressure. The compound or double cylinder high-pressure engine of Hornblower of England, in 1781, and the high-pressure non-condensing steam engine devised by Evans in 1779, were reconstructed and improved in the early part of the century.