Up to this time boilers were of the box type and the pressure of steam carried rarely ex ceeded 25 pounds per square inch—in filitily of the early steamers 10 pounds or less was the common practice. But the displacing of box boilers by cylindrical permitted a higher steam pressure, and this in turn demanded another form of engine to utilize it economically. The com pound engine, which was built and patented by llornblower in 1781 and revived by Woolf in 1804, had not been much used, because the conditions had not demanded it but. now it became a neces sity. It consisted at first of two cylinders—and many compound engines are still so built—in which the steam was expanded in two stages, the first expansion taking place in the high-pres sure cylinder, by which the pressure was reduced one-half, more or less, and the second expansion in the low-pressure cylinder, where the pressure was carried down to the atmospheric line or be low it.
The demand for increased speed led to higher steam pressure and greater engine speed. The range of economical expansion in one cylinder being limited, the tri-compound or triple-ex pansion engine was designed to utilize the in creased boiler pressures. The gain was two fold. The new engines, using a higher pressure of steam, were lighter than their predecessors of equal power and they were also more economical. The first large vessel to be fitted with them was probably the Propontis, which, in 1874, was sup plied with engines designed by Mr. A. C. Kirk. By ISSO the use of triple-expansion engines be came common, though compound engines were largely used for another decade and they are still fitted in certain steamers where the conditions favor their economical working.
The continued demand for increased power, particularly in small vessels (torpedo boats and the like), naturally pushed up the steam pres sure again, and, although the locomotive boiler was used to some extent, the advantages of the water-tube boiler soon became apparent. (See
section on Boilers below.) Its capability to furnish very high pressure steam reacted upon engine design and produced the quadruple expansion engine. The water-tube boiler is not yet much used in the mercantile marine. but is rapidly displacing the cylindrical boiler in naval construction. It has not yet brought about the extended use of quadruple-expansion engines in large vessels, but this may follow in the course of time.
The length of the voyage and the vast amount of traffic has caused the transatlantic trade t be the principal field of steamship development. While the gain in size and speed of the vessels in this trade has been continuous from the start, a great impulse was given by the building of the Britannic and Uernianie for the White Star Line in 1874. They at once reduced the average pas sage from Queenstown to New York to about eight days. They were followed in 1879 by the Arizona of the Milan Line. confessedly built to outstrip all competitors, and her success was the begin ring of a race for speed supremaey that has brought out a new 'record-breaker' almost every year since. As the vessels are increased in size it takes less horse power per ton to drive them at a given speed, and this has tended to augment the tonnage of the great liners so that the dimen sions of the Great Eastern are now exceeded by nearly a dozen vessels, built and building. The accompanying table gives the principal features of the largest ships belonging to the•great steam ship companies, transatlantic and transpacific.