284 The Fhilosophical Society of Glasgow. 



of a vessel has been taken working at a temperature of 270°, and 

 evaporating at that temperature 7^1bs. of water from 100° per lb. 

 of coal. 



Chemical Metliod. 



Sea-water supplied to boUer, tem. 100° 15 lbs 8-33 lbs. 



Water discharged, 270° Ih 83 lbs. 



Water evaporated, 7^ 7^ lbs. 



Total heat evaporating from 100° at 270° ) „„- ^o.= 89i<5°-'i 



A, 2 = 1U92 + XCT._32)-(T2_32) 1095-4 r^^^ " ^'^^ 



Heat discharged 1275° 142° 



Fuel consumed in evaporation, 1* lbs. coal ... 1- lbs. coal 



AicKiu 1 <O0172 lbs. coal + 0-0085 



Fuel consumed in preventing crust, O'lOO lbs coal -; n,s. soda ash 



,,.^„, , (1-017 lbs. coal + -0085 



Totalfuel, l-lr,5 lbs coal j ^^^^ ^^^ 



Thus it is seen that it requires only 172 lbs. coal + 85 lbs. soda 

 ash, containing 50 per cent, soda, to be as efficient in preventing 

 crust, as 1,550 lbs. of coal alone, which evaporates 7 J lbs. water from 

 100° at 270°. And these methods are equally expensive when the 

 soda ash is 16-2 times dearer than the coal. This ratio varies with 

 the efficiency of the fuel and the temperature of evaporation. 



Although when coals are lOs., and soda ash XI 0, Mr. Napier's 

 method is more expensive than the ordinary one of discharging the 

 saturated water, there are many cases where it is probable the owners of 

 vessels would profit by its adoption. In long voyages, for example, a 

 vessfel requiring, by the ordinary mode, 1,155 tons of coal, would, by 

 Mr. Napier's method, require 1,017 tons coal, and 8^ tons soda ash, 

 or 1,025^ tons weight. There would be a saving in money, therefore, 

 of 138 tons coal at xf -\- 129 tons freight >// — 8J tons soda ash at 

 z], or if coals are 10/ per ton, freight X3, and soda ash XI 1 per ton, 

 the saving would be £362. That boilers, however, can be worked till 

 the water in them is nearly saturated with common salt, or that the 

 soda ash can be so accurately proportioned as exactly to neutralize 

 the sulphate of lime, are problems which are believed to be new, and 

 have not yet been attempted. The considerable saving which may be 

 efiected shows that the method is worthy of a trial. 



From the foregoing example of a vessel worked at a temperature 

 of 270°, it is also seen that a quantity of fuel, equal to 15 i per 

 cent, of that which produces evaporation, is consumed by the 

 ordinary blowing-ofi" method, in order to prevent crust, and this 

 amount increases with the temperatm'e. Brine chests have been 

 frequently used for the recovery of this notable loss; but apparently 

 from a misapprehension of the quantity of water necessary to be 

 discharged, and a want of knowledge of the amount of surface rc- 

 'Tjuircd to absorb the discharged heat, of a capacity greatly too small 



