16 APPENDIX. 



profit thereby to a considerable amount. I have, on a summer's day, found it sensibly 

 cooler in an engine-house closed, containing a cylinder eighty inches in diameter, and ten 

 feet stroke, while the steam-gauge indicated thirty-four Ibs. per square inch on the 

 safety-valve, than I experienced without : the boiler-house nearly the same. Is not economy 

 in fuel as loudly called for in marine engines, without reference to stowage, health, and the 

 labour and dirt of shipment ? 



The advantages above proposed are now carried into effect to a certain extent in Her 

 Majesty's steam vessels, by coating the boilers, whether of iron or copper, with "felt," laid 

 on a thick covering of a mixture of white and red lead ; the same treatment has been observed 

 with regard to the sides of the respective boilers where they come together, when smoke-joints 

 do not interfere : the advantage of this does not consist only in the preservation of these 

 sides, so liable to decay from the water which descends there leaving its salt adhering to the 

 plates, but it operates to prevent the heat of one boiler from being transmitted to another 

 when not employed ; indeed, the practice of coating boilers of marine engines was observed as 

 far back as 1818 in the ' Caledonia,' Mr. Watt's private steam vessel, and also in the ' James 

 Watt' in 1823: the cylinders of the former were also coated in the same manner and 

 covered with reeded mahogany. By combining the arrangements before mentioned, so as to 

 have the respective boilers and flues distinct, smoke-joints may be entirely dispensed with : 

 these are generally dangerous, although often unavoidable, both from the size to which the 

 boilers are required to be carried, to meet the power of the engines, and from the necessity 

 which sometimes exists of adapting them to the peculiar construction of the vessels. 



27. From the frequent necessity of opening the furnace doors, when under weigh, to re- 

 plenish and arrange the fires, a large portion of cold air is allowed to pass through the flues ; this 

 ready passage for the air checks its entrance through the bars of the adjoining furnace or fur- 

 naces, and in most instances throws them for the time nearly out of use : thus the boilers are not 

 only deprived of the effect of the open furnace, but, to a great extent, of all others connected 

 with the common flue : this is so certain, that boilers having four furnaces joining in one flue, 

 as formerly fitted to vessels of 100 horse power and upwards, have been supplanted by others 

 in which two furnaces only discharge their fire into a flue which meets that of the other 

 two furnaces at the chimney, having traversed the water separately before they meet. The 

 former plan of boiler is always remarkable for a bad or indifferent draught; it requires more 

 area of heated surface to generate sufficient steam, occupies of course more space, contains 

 a larger quantity of water, and is, in addition, considerably heavier than the latter style 

 of boiler, which is distinguished for its power in producing steam, and for the ease of working 

 it. Hence it follows that the more distinctly furnaces perform their work, the greater the 

 effect obtained ; but this has its limits ; for if a flue is too direct, a great portion of heat passes 

 up the chimney without effect, the fire not having traversed sufficiently through the water to 

 be absorbed by it. In land engines this defect is not experienced, the fire passing around as 

 well as through the boiler ; while in vessels the fire must be entirely confined to the flues, which 

 are surrounded with water. 



Some of the best judges of the construction and working of marine boilers recommend that 

 the fire should be divided by as many furnaces as possible within certain limits, in order that 

 the feeding a fire should not occupy too much time, and that it might be performed more 

 frequently, and in consequence more regularly; and, it is presumed, with better effect, on 



