(114.) Among the various ways in which the steam engine has ministered to the social progress of our race, none is more important and interesting than the aid it has afforded to navigation. Before it lent its giant powers to that art, locomotion over the waters of the deep was attended with a degree of danger and uncertainty, which seemed so necessary and so inevitable, that, as a common proverb, it became the type and representative of everything else which was precarious and perilous. The application, however, of steam to navigation has rescued the mariner from much of the perils of the winds and waves; and even in its actual state, apart from the improvements which it is still likely to receive, it has rendered all voyages of moderate length as safe and as regular as journeys over land. We are even now upon the brink of such improvements as will probably so extend the powers of the steam engine, as to render it available as the means of connecting the most distant points of the earth. The manner in which the steam engine is commonly applied to propel vessels must be so familiar as to require but short explanation. A pair of wheels, like common under-shot water-wheels, bearing on their rims a number of flat boards called _paddle boards_, are placed one at each side of the vessel, in such a position that when the vessel is immersed to her ordinary depth the lowest paddle boards shall be submerged. These wheels are fixed upon a shaft, which is made to revolve by cranks placed upon it, in the same manner as the fly-wheel of a common steam engine is turned. It is now the invariable custom to place in steam vessels two engines, each of which works a crank: these two cranks are placed at right angles to each other, in the same manner as the cranks already described upon the working axles of locomotive engines. When either crank is at its dead point, the other is in full activity, so that the necessity for a fly wheel is superseded. The engines may be either condensing or high-pressure engines; but in Europe the low-pressure condensing engine has been invariably used for nautical purposes. In the United States, where steam navigation had its origin, and where it was, until a recent period, much more extensively practised than in Europe, less objection was felt to the use of high-pressure engines; and their limited bulk, their small original cost, and simplicity of structure, strongly recommended them, more especially for the purposes of river navigation.