18 APPENDIX. 



can be accounted for by the ready transmission of the heat, which is obstructed more on the 

 sides, from the accumulation of scales and the increased difficulty of removing them. 



29. In the arrangement of plates in the furnaces, perpendicular seams are bad, and in long 

 voyages are certain of injury from the fire; the quantity of inetal at the joinings or seams 

 causing an unequal expansion, and retarding the transmission of heat. Plates the entire 

 length of fire-place may be employed, having their seams beneath and in the direction of 

 the bars ; the upper seam, when the furnaces are large, being as much above the fire 

 as possible ; but, on this subject, opinions are divided, some still preferring the former 

 method of construction. Plates of copper may be manufactured with certainty of the 

 entire size required for the sides, in 50 horse boilers; but iron plates of such dimen- 

 sions could not be depended on. Iron plate for furnaces should be manufactured of 

 the best material that can be obtained ; additional expense for such would be amply 

 repaid. The plate used for furnaces mostly owes its premature destruction to the 

 great thickness usually employed: it thus conducts heat more slowly, and is neces- 

 sarily more heated than thinner plate; besides it is much less likely to be sound than 

 the latter, although of an excellent exterior, which conceals the defects of manufacture. I 

 some time since examined, in a foreign port, an excellently constructed boiler of a 200 horse 

 marine engine, and of the very best material, which had been comparatively seldom employed 

 during eighteen months after construction : the interior of the boilers evidenced the greatest 

 care and attention, but the side plates of the fire-places, from the bars upwards, required to be 

 replaced, from no other cause than that of being too thick. These cases are very common, as 

 before observed, in Her Majesty's steam vessels between Falmouth and the Mediterranean, 

 as likewise those employed on other stations, which need repairs in these places several times 

 before the boilers are required to be taken out, from actual decay. 



Many have been formerly misled as to the cause of injured boilers. I have witnessed 

 ingenious expedients of negligent persons to account for the same, in the presence of eminent 

 engineers, who, not having an opportunity of watching proceedings on a long sea voyage, were 

 obliged, to a great extent, to take plausible reasons as their guide ; viz. saltness of the Medi- 

 terranean, 8tc. : allowing water to become too low in the boilers is too frequently the real 

 cause, which, if copper, will soon exhibit the consequence. Thicker plate has been adopted, 

 probably to resist the tendency to bend, but it only increases the evil. Plates for the sides of 

 fire-places then should not, in the largest boilers above the bars, be more than three-eighths 

 of an inch thick ; and in smaller boilers a less thickness would be preferable, as no oxidation 

 takes place here, or even throughout the flues, of any consequence, above the mass of soot 

 therein deposited. This fact is decidedly opposed to the notion generally propagated, that the 

 iron rapidly imbibes oxygen from the cold air admitted, when the fire-doors are opened to 

 replenish fuel : no more injury is experienced practically from this cause, than that found to 

 be sustained by the exterior of a common culinary boiler, the plates not being sufficiently 

 heated to imbibe a destructive portion of oxygen. 



The lower part of the sides, and the bottom of the fire-place, should be thicker in all cases, the 

 better to bear up against oxidation, which here takes place to a great extent, as likewise along 

 the bottom and lower part of the flues ; and the bottom of the boiler, and the lower portion of 

 the sides, should be stouter than the crown, particularly in the wake of the stoke hole flooring, 

 where, as has been before observed, decay very rapidly takes place. 



