Page:Popular Science Monthly Volume 24.djvu/548

532 and thus secure uniformity of motion with the highest expansive use of the steam.

In 1832 no steamship had essayed the passage across the Atlantic. The marine boilers of 1832 were unfit for resisting any considerable pressure, in fact, so weak were they that they have been known to collapse when steam had been let down. The engine and boilers took up so much of the tonnage of the vessel, and used such enormous quantities of coal, that it was predicted that it would never be possible to cross the Atlantic unaided by sail. In fact, the prediction held good for a long time, for transatlantic steamship lines were compelled to establish coaling-stations at Halifax and Queenstown in order to reduce the coal carried, and allow of a little cargo being taken on. In 1832 all hulls were wood, and salt-water was invariably used in the boilers, much to their injury. The speed rarely exceeded eight knots an hour.

In 1882 the ships are almost invariably of iron or mild steel, and this enables the introduction of an element of safety impossible with the use of wood: I refer to the compartment and cellular system of naval construction. The use of iron and steel has made the construction of ships of great length possible.

The boilers are of enormous strength, and carry from 80 to 125 pounds pressure. The cylinder or cylinders are now adapted to the economical utilization of all the expansive force due to the pressure used. To secure this, more than one cylinder, is used; all the expansion could be had in one cylinder, but the difference in temperature of the cylinder, due to the temperature of the steam before and after expansion, would cause undue condensation. The substitution of the propeller for the paddle-wheel for sea-navigation and the high speeds required by the former have done much to reduce the size and weight of the marine engine; and have also had a marked effect on the economy. The paddle-wheel has practically disappeared, except on rivers.

A piston-speed of 800 feet a minute is often attained in daily practice. Hence, enormous powers are secured with comparatively little loss of carrying-space.

The marine governor of to-day is almost endowed with prophecy. It anticipates the pitching of the ship and withdrawal of the screw from the water, and cuts off steam just before its occurrence, thus avoiding the dangerous racing of the engine when the screw leaves its work. This, for a long time, has been almost the only danger in bad weather; the racing of the engines subjected the screw-shaft to strains for resisting which the shaft was inadequate. The twisting off of the propeller-shaft of an Atlantic steamer is not an uncommon occurrence. Condensation is now had almost in all cases by the surface condenser, thus returning all the water to the boiler to be used again. It might be well to speak here of a steamship built in 1882. Steamships are now making long voyages at a high rate of speed, voyages which till