532 THE POPULAR SCIENCE MONTHLY. 



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 col- 

 lajDse when steam had been let down. The engine and boilers took up 

 so much of the tonnage of the vessel, and used such enormous quanti- 

 ties 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 re- 

 duce 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 construc- 

 tion 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 expan- 

 sion 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 econ- 

 omy. 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 



