68 Economy of Fuel. (January, 
combustion must only be permitted to escape when all the 
heat given out in combustion has been effectually abstracted, 
and all radiated heat would also require to be reserved for 
useful purposes. 
There can be no doubt that coal burnt under a certain 
amount of compression will yield the best results; but the 
important question is how to regulate the admission of air 
so that no more is employed in supporting combustion than 
is actually required for that purpose ; any excess above that 
amount must tend only to lower the temperature, and so 
abstract a certain amount of useful effect from the fuel. The 
air admitted for this purpose must also be so regulated as to 
ensure its complete combination with the fuel, so that no 
air in a free state shall pass away undecomposed. The 
channel for escaping vapours should also be so regulated in 
size as to give no more than sufficient space for their free 
passage in a certain state of expansion, whilst the current 
of air into the furnace must also be maintained at a rate 
sufficient for the combustion of the proper amount of fuel 
for the work to be effected. Now, in order to maintain this 
state of things, it is necessary to produce an upward current 
in the chimney, and in order to do this, the ascending vapours 
must either be forced out by mechanical means, or they must 
be allowed to escape at a temperature above that of the 
atmosphere. In either case, therefore, it is clear that a 
certain amount of heat must be developed in excess of what 
would otherwise be required for the purpose. To obtain, 
therefore, the full theoretic value of the coal burnt for any 
specific purpose must be, under these considerations, abso- 
lutely impracticable; the nearer we attain to that point, 
however, the less cause of complaint there will be of a waste 
of coal taking place in any particular case. As we shall 
presently see, the use of hot-blast in iron smelting was 
followed by a decided saving in the amount of fuel required 
to produce 1 ton of pig-iron, notwithstanding that a certain 
portion of it was consumed in first heating the air for the 
blast. It will readily be understood how the introduction 
of cold air into any furnace must have the immediate effect 
of lowering the temperature, and it is found that the amount 
of fuel necessary to heat the air before admitting it into 
the furnace is less than the amount required to maintain the 
temperature within the furnace when it is fed with cold air. 
A free current for escape of the vapours of combustion is 
followed by the escape of the more volatile portions of the 
fuel used, which are therefore absolutely lost ; whilst, witha 
strong blast, where there is a free escape—asin a locomotive— 
small particles of fuel, wholly uncarbonised, are also carried 
