Page:The Development of Navies During the Last Half-Century.djvu/279

 Hence vertical engines, whose height is only limited by that of the protective deck. Every engineer knows that the normal position of a cylinder and piston is perpendicular; circumstances alone have forced him sometimes to be content with the horizontal. At the present time every war vessel of size above a gunboat is supplied with vertical engines. The 'Shannon' and 'Dreadnought,' both built at Pembroke, and the 'Nelson' and 'Northampton, both hailing from Messrs Elder, of Glasgow, were the first important ships to be so fitted. These four ships were all completed between 1875 and 1878.

In 1881 was launched the 'Polyphemus,' a most remarkable ship, but noteworthy more for her hull, which is fully described elsewhere, than for her machinery. She has twin screws, driven by two pairs of compound horizontal engines constructed by Messrs Humphrys & Tennant, of Deptford, who, it may here be remarked, have latterly built and are building some of the most important engines in the navy. The boilers of the 'Polyphemus,' originally of the locomotive type, gave a great deal of trouble, and new ones were substituted for them with good results. The pressure of steam is 110 lbs. per square inch. It will be observed with what leaps and bounds the pressure of steam advanced in the course of comparatively few years. A quite usual pressure now is 155 lbs., and this will probably be very considerably increased before long.

With the extended application of high-pressure steam came, as a natural corollary, a desire to utilise