456 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



effective power of 4,000 horses at 33,000 foot-pounds, ought to be 

 reckoned to have only 1,000 horse-power nominal. 



To obtain such power it is necessary to employ generators that are 

 capable of vaporizing considerable weights of water, and having 

 therefore large heating surfaces. 



Tubular boilers are generally employed, of which Figs. 309 and 

 310 represent types. Besides this, a single boiler and a single 

 furnace are not considered enough, the Warrior, for instance, has 

 10 boilers and 46 furnaces on board. 



The amount of fuel consumed is something enormous. We will 

 quote a few figures. 



The Great Eastern, the largest vessel afloat, the gross tonnage of 

 which is 22,500 tons, is furnished both with screw and paddles. The 

 engines of the former are 1,600 horse-power nominal, of the latter 

 1,000 horse-power nominal. Her average consumption of coal a day 

 is 270 tons; with this consumption her paddles revolve 13,000 times 

 and her screw 52,500 times a day. 



The French armour-plated frigate Friedland, which, with its com- 

 plete freight of coal and munitions weighs 7,200 tons, consumes at 

 full speed about five tons of coal an hour, or 125 tons every day she 

 continues to travel. This is an expense which varies according to the 

 price of coal from 160 to 200 a day for the fuel alone. 



The external appearance of marine boilers and engines is not 

 very like that of the steam-engines employed in manufactories. 

 Although all their parts are of relatively large size, they are arranged so 

 as to occupy the smallest possible space. Boilers, condensers, driving 

 machinery, &c., all are set close together. 



The chief special types of Marine Engines are as follows : 

 Trunk-engine. In this engine there is a hollow cylinder fastened 

 to the piston itself, and working steam-tight through the cylinder 

 cover. At the bottom of the hollow cylinder the connecting rod is 

 made fast, the hollow cylinder being large enough for its vibrations, 

 thus doing away with all the parallel motion and piston-rod. Some- 

 times the hollow cylinder is made to pass through both ends of the 

 large cylinder, to equalise the pressure. 



Side-lever Engine. The side-lever engine is an engine something 

 after the fashion of the beam-engine, but having the beam about the 

 level of the bottom of the cylinder, and the top of the piston-rod 



