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take into consideration the specific objects for which the fuel 
is to he employed. A fuel pre-eminently calculated for steam 
purposes may be one of the least so for other equally exten- 
sive applications. The relative evaporative values of different 
fuels, or their power of producing steam, are, however, too im- 
portant to be passed over without special and separate notice. 
The qualities of fuels, with reference to this point, are so 
remarkably varied, and the necessary excellences so seldom 
united in one species, that it becomes difficult to deduce 
any definite general results. The characteristics essential 
for steam purposes, having especial reference to the steam 
navy, may be stated as follows : — 
1. That the fuel should burn so that steam may be raised 
in a short period, if this be desired ; in other words, it 
should be able to produce quick action. 
2. It should possess high evaporative power ; that is, be 
capable of converting much water into steam with a small 
consumption of coal. 
3. It should not be bituminous, lest the particles of coal 
should become cemented together into tough and infrangible 
cakes, obstructing the passage of air through the bars, and 
its free circulation through the mass of combustible. 
4. It should possess considerable cohesion of its particles, 
so that it may not be broken into too small fragments by 
the constant attrition which it may experience in the vessel. 
5. It should combine a considerable density with such 
mechanical structure that it may be easily stowed away in 
a small space, a condition which, in coals of equal evapo- 
rative values, often involves a difference of more than 20 
per cent. 
6. It should be free from any considerable quantity of 
sulphur, and should not progressively decay ; both of which 
circumstances render it liable to spontaneous combustion. 
7. It should not contain a large per centage of ash. 
