ECONOMY OF FUEL. 



sole or principal cause of the rapid destruction of marine boilers- 

 If it were so, it would necessarily happen that marine boilers in 

 which expedients are adopted by which fresh water is used, or 

 even those in which the process of blowing out has been regularly 

 observed, and in which the scale is detached before it is allowed 

 to thicken to an injurious extent, would last as long, or nearly as- 

 long, as land boilers. It is found, however, that the boilers in 

 which these expedients are adopted with the greatest effect and 

 regularity are, nevertheless, less durable in a very large propor- 

 tion than land boilers. Thus, while a land boiler will last for 

 twenty years, a marine boiler, similarly constructed, will, even 

 with the greatest care, be worn out in four or five years. 



The cause of this rapid destruction of the boiler is corrosion,, 

 but how this corrosion is produced is a question which has not 

 hitherto been satisfactorily answered. It is contended that this 

 is not to be ascribed to any chemical action of the sea water on, 

 the iron, inasmuch as the flues of marine boilers rarely show any 

 deterioration from this cause, and even in worn-out marine boilers 

 the hammer-marks on the flues are as conspicuous as when they are 

 fresh from the boiler-maker. The thin film of scale which covers 

 the interior surface would rather protect the iron from the action 

 of the water. In fine, the seat of the corrosion is almost never 

 those parts of the boiler which are in contact with the water. It 

 is that part of the metal which includes the steam space that 

 exhibits corrosion ; but even there the effect is so irregular, that 

 no data can be obtained by which the cause can be satisfactorily 

 traced. The part which is most rapidly corroded in one boiler is 

 not at all affected in another ; and in some cases we find one side 

 of the steam-chest attacked, the other side being untouched. 

 Sometimes the iron exfoliates in flakes, while in others it appears 

 as though it were eaten away by an acid. 



43. In the application of the steam-engine to the propulsion of 

 vessels in voyages of great extent, the economy of fuel acquires 

 an importance greater than that which appertains to it in land 

 engines, even in localities the most removed from coal-mines, and 

 where its expense is greatest. The practical limit to steam 

 voyages being determined by the greatest quantity of coals which 

 a steam vessel can carry, every expedient by which the efficiency 

 of the fuel can be increased becomes a means, not merely of a 

 saving of expense, but of an increased extension of steam-power to 

 navigation. Much attention has been bestowed on the augmenta- 

 tion of ,the duty of engines in the mining districts of Cornwall, 

 where the question of their efficiency is merely a question of 

 economy ; but far greater care should be given to this subject, 

 when the practicability of maintaining intercourse by steam 



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