STEAM NAVIGATION

The art of applying the power of steam to the propulsion of boats and ve els in general, as well for inland communication by rivers and lakes, as for the general purposes of national commerce on the seas and oceans by which the various parts of the globe are separated. The manner in which the steam engine is rendered an instrument for the propulsion of ve els must, in its general features, be so familiar to every one as to require but short explanation. A shaft is carried acro the ve el, being continued on either side beyond the timbers; to the extremities of this shaft on the outside of the ve el are attached a pair of wheels, constructed like under-shot water wheels, having fixed upon their rims a number of flat boards called paddle boards. As the wheels revolve, these paddle boards strike the water, driving it in a direction contrary to that in which it is intended the ve el shall be propelled. The moving force imparted to the water thus driven backwards by reaction on the ve el propels it. On the paddle shaft are constructed two cranks or winches, placed at right angles one to the other, so that whenever one of them is thrown into the highest or lowest position the other is horizontal. These cranks are worked by strong iron rods called connecting rods, which are themselves either driven directly by the pistons of two steam engines, or are worked by those pistons; thus the medium of working becomes similar to those used in the ordinary land engines. {See Steam Engine.) The two cranks being placed at right angles, it follows that when one piston is at the top or bottom of its stroke, and the crank driven by it in the highest or lowest position, the other will be at the middle of its stroke, and the crank driven by it will be in the horizontal position. One of the pistons is therefore always in a position to produce the most advantageous effect on the crank at the moment that the other piston loses all power over the crank driven by it; and in the same manner it may be seen that while the power of one piston is augmented from zero to its greatest effect, the power of the other is decreasing from its greatest effect to zero. Thus the combined action of the two pistons is nearly uniform in its efficiency. If one engine only were used the motion of the wheels would be unequal, being most rapid when the piston is at the middle of the stroke and slowest at the extremities. The steam engines used for nav igatlon may be either condensing engines or non-condensing engines. If the latter are used, steam must be used having a pre ure above the atmosphere of from 15 to 20 lbs. per square inch. Boilers in which steam is produced under this pre ure are considered in Europe so unsafe, that non -condensing engines with low-pre ure boilers are almost universally used for navigation. In America, however, high-pre ure boilers with non-condensing engines are generally used. The arrangement of the parts of marine engines is different in several respects from land engines. Steam [s. 1179]