CIRCULATING PIPES, DAMPERS, &c. 17 



account of a less portion of cold air being permitted to pass through the flue at a time. When 

 three such furnaces are employed instead of two, (for it has been observed that four are 

 decidedly objectionable,) which communicate with a common flue, it is evident that there are 

 always two furnaces in action, while the third is being entirely cleared out ; an operation which 

 at sea must be performed at least, in turn, once in two hours, the fire-door being open during 

 the operation, and a considerable quantity of cold air afterwards passing through the grate 

 until the relay of coals is in a proper state of combustion. Here the three furnaces have 

 a decided advantage over two of the same area collectively, in maintaining the power of the 

 engines during the disuse of one of the number : but if the two furnaces be managed properly, 

 want of steam during the above operation will rarely be felt. Of course these observations are 

 referred to vessels of such power, as to have, at least, two distinct boilers, or two distinct sets 

 of furnaces and flues in the same boiler, so that you have the full effect of one or more boilers 

 or sets of furnaces, without any drawback, at all times. From the necessities of construction, 

 three furnaces cannot directly discharge their fire into a flue, whose width is less than that of 

 either of the furnaces individually : the fire having, in one furnace at least, to cross, to a certain 

 extent, that of another before it can enter the flue, this furnace is least active when either of the 

 other furnace-doors are open, the cold air directly entering the flue retarding its effect : the fire 

 of this furnace moreover, when all are in full force, acts directly on the side plates of the cross 

 flue opposite, before it passes with that of the other furnaces into the common flue ; the con- 

 sequence is, that if the boilers are partially incrusted, these plates must be the first to give out, 

 next to those of the furnaces, of which there are many instances. Hence when three furnaces 

 are employed, the flue should be deflected as much as possible, to accommodate each of them. 

 28. From an extensive examination of the boilers of long sea voyage, I am led to the 

 opinion that the fires act more destructively on the sides of narrow furnaces, than of those under 

 similar circumstances of greater width ; it may arise in a great measure from the fuel being 

 heaped up more thickly in the former than the latter : the air not being able to penetrate the 

 mass well in the middle, then passes up the sides, urging an intense flame to act thereon ; this 

 cannot take place so effectually in a wider furnace. A furnace that is very long cannot 

 be properly managed by the firemen at sea : it has every chance of being overcharged, more 

 particularly if the boilers keep steam easily, because then they require less attention, and are 

 charged so as to last as long as possible for the accommodation of the man ; and this practice 

 cannot well be overruled night and day, unless engineers neglect other duties ; for it would be 

 necessary to be constantly in a stoke-room for the purpose. I should say, that if a furnace be 

 six feet in length, it should not be much less than three feet in breadth, to afford a fair chance 

 for firing with as little injury to the side plates as possible, and with the means of keeping the 

 fires in the required condition for the greatest effect : furnaces should not be carried beyond 

 this length more than can be avoided ; it is not necessary that they should ever be wider than 

 three feet, indeed it would be inconvenient : but the nearer it is approached in width for 

 various powers, I think, the better. By adopting short and wide furnaces, short firing-tools 

 become necessary, and of course short stoking-room, thereby gaining on the entire section of 

 the ship for the stowage of coals abaft. The crowns of the furnaces, whether of iron or copper, 

 never decay, and rarely alter their figure, if arched, unless the water is allowed to descend below 

 them. In some of the most neglected and most injured boilers that I have seen, the crowns of 

 the fire-places have remained perfect, when every other part has been attacked by the fire : it 



