STEAM NAVIGATION. 



between distant points of the globe, will perhaps depend on the 

 effect produced by a given quantity of fuel. So long as steain 

 navigation was confined to river and channel transport and to 

 coasting voyages, the speed of the vessel was a paramount con- 

 sideration, at whatever expenditure of fuel it might be obtained ; 

 but since steam navigation has been extended to ocean voyages, 

 where coals must be transported sufficient to keep the engine in 

 operation for a long period of time without a fresh relay, greater 

 attention has been bestowed upon the means of economising it. 



Much of the efficiency of fuel must depend on the management 

 of the fires, and therefore on the skill and care of the stokers. 

 Formerly the efficiency of firemen was determined by the abundant 

 production of steam ; and so long as the steam was evolved in 

 superabundance, however it might have blown off to waste, the duty 

 of the stoker was considered as well performed. The regulation of 

 the fires according to the demands of the engine was not thought 

 of, and whether much or little steam was wanted, the duty of the 

 stoker was to urge the fires to their extreme limit. 



Since the resistance opposed by the action of the paddle-wheels 

 of a steam-vessel varies with the state of the weather, the con- 

 sumption of steam in the cylinders must undergo a corresponding 

 variation ; and if the production of steam in the boilers be not 

 proportioned to this, the engines will either work with less effi- 

 ciency than they might do under the actual circumstances of the 

 weather, or more steam will be produced in the boilers than the 

 cylinders can consume, and the surplus will be discharged to waste 

 through the safety valves. The stokers of a marine engine, there- 

 fore, to perform their duty with efficiency, and obtain from the fuel 

 the greatest possible effect, must discharge the functions of a self- 

 regulating furnace, such as has been already described : they must 

 regulate the force of the fires by the amount of steam which the 

 cylinders are capable of consuming, and they must take care that no 

 unconsumed fuel is allowed to be carried away from the ash-pit. 



44. Formerly the heat radiated from every part of the surface 

 of the boiler was allowed to go to waste, and to produce injurious 

 effects on those parts of the vessel to which it was transmitted. 

 This evil, however, has been removed by coating the boilers, 

 steam-pipes, &c., of steam- vessels with felt, by which the escape 

 of heat from the surface of the boiler is very nearly, if not alto- 

 gether, prevented. This felt is attached to the boiler surface by 

 a thick covering of white and red lead. This expedient was first 

 applied in the year 1818 to a private steam vessel of Mr. Watt's, 

 called the Caledonia ; and it was subsequently adopted in anotber 

 vessel, the machinery of which was constructed at Soho, called 

 the James Watt. 

 144 



