28 APPENDIX. 



July, 1837, (coals used principally, West Hartly,) the steam was generated abundantly at 

 the conclusion, and the flues contained far less soot than I had calculated, which I attribute to 

 less urgent firing : the sides and tops of the interior of the flues were comparatively clean. 

 From this it may be inferred, that were sufficient coals on board, a vessel with boilers of good 

 construction might continue to steam for an indefinite time or distance, so far as the accumu- 

 lation of soot is concerned. It should be observed, that the coal employed in the two former 

 cases was not all Welch coal ; a quantity of Newcastle and Beyrout coal having been 

 procured at Alexandria. 



The flues of marine boilers are rarely on fire ; I never saw but one instance, when the 

 ignited soot was carried through the chimney in a shower which lasted nearly an hour, on 

 board a merchant steam vessel which had only made the voyage from Ireland to St. Andero, 

 and back to Falmouth. I have questioned several engineers of long experience at sea, who 

 have never witnessed any other appearance than that which is common to all steam vessels 

 after being some days at sea ; viz. the flame of carbonic oxide flickering at the top of the 

 chimney, and perceived only at night. The burning of a vast quantity of smoke, and its 

 consequent soot, is to be referred to the great admixture with the air introduced through the 

 fire-doors during the operation of feeding : this has been often exemplified by leaving the 

 doors slightly a-jar, when the smoke is most completely consumed. 



37. FRESH WATER. Charging ordinary boilers with fresh water, prior to starting for 

 a long voyage, is certainly to be recommended ; it cannot be too frequently done, if at no 

 very great expense and labour of a ship's company. I think, however, its advantages have 

 been generally too highly rated : it was rarely done in the 'African;' but from its probable 

 tendency to loosen scale, as well as to prevent the formation of the same to a certain extent, 

 till totally displaced by the salt water, it has its advantages. 



From experiments before detailed, the engines of the 'African' were maintained at twelve 

 revolutions per minute, with an evaporation of 306 cubic feet of fresh water in six hours, by 

 two furnaces, or one boiler, a certain portion of heat being abstracted by the adjoining boiler : 

 now with both boilers in use, the engines are abundantly supplied with steam at twenty-eight 

 or thirty revolutions, a surplus flying to waste ; so that we may reckon that at all times 

 double the above quantity will be evaporated with both boilers. 



That is, 306 cubic feet x 2 boilers = 612 cubic feet evaporated in 6 hours ; and 612x4 

 = 2448 cubic feet = 68'3 tons of fresh water required in 24 hours to meet the necessities of 

 the engines at full speed ; and this is more than treble the contents of both boilers when 

 charged : so that during twenty-four hours the fresh water will have been displaced by more 

 than treble its quantity of salt water : therefore the advantages of fresh water are limited, 

 and a boiler may keep at sea for a considerable time, exclusive of accident, depending alone 

 on the system of change usually adopted in vessels of long voyage. 



Hence the only question of a vessel's continuance at sea, for any consistent period of time, 

 is the fitness of the engines, the conduct of the boilers, and the quantity of fuel which she 

 can stow, compared with her consumption. Consequently the probable capacity of a steam 

 vessel for the Atlantic or any other voyage, so far as her machinery is concerned, may be 

 fairly estimated before she finally sets out for her destination ; the contingencies of weather on 

 the particular service being duly considered. 



