360 
Utilisation of Waste Substances and 
driven by coals of a cjuality and in quantity greatly superior to 
those used in the contrasted engine. Before results can be fairly 
compared, circumstances must be equal. Now, seeing that an 
essential element in good stoking is uniformity in firing, 
it is obvious that a great help to ensure this vi'ill be the uniform 
quality of the fuel itself. So that of the two it may be almost 
affirmed that it will be better to have the coal of uniform 
mechanical character, even if it be inferior in quality, than 
of varying character, though it be superior in quality. In 
practice, when the coal is of an uneven quality — lumpy and 
small together — a good careful stoker will almost invariably 
be found to sort the coals before using them. Hence in our 
largest factories, where machine firing is not adopted, the 
quality, or rather the kind, of coals used has a close refer- 
ence to the mechanical uniformity of the mass. Thus the class 
of coal known generally throughout the North of England as 
" nuts " is preferred by good stokers ; so much so, that the term 
nuts is often taken to be synonymous with " machine or 
engine coal." This coal, as the name indicates, is made up of 
screened coal, so that the pieces are pretty uniformly of the same 
size — from small to largish " nuts." There is amongst this class 
a good deal of " slack " or small, which is very useful in bank- 
ing-up," and also in " bridging, or backing up." Some first- 
class stokers indeed prefer this " slack " or " small " of coal, as 
it enables them to secure most efficiently the " uniform firing " 
they know to be so essential to success. 
Importance of uniform and rcfjular Firing. Influence of the 
form of Boiler and Furnace upon this. — This "uniform firing" 
precluding, as it does, all "forcing in firing," the stoker is 
in this respect greatly dependent upon the boiler and its furnace 
which he has to work. In the Cornish pumping-engines, where 
the most economical consumption of fuel in proportion to the 
steam raised is perhaps to be met with, there is not only abundant 
steam and water space in the boiler, but ample furnace-bar and 
heating space in the furnace. Under these circumstances there 
is not the slightest excuse, as there is no necessity, for forcing the 
fire ; and indeed with this high class of steam-engine and boiler 
such a thing is never tolerated. But in the case of the class of 
portable steam-engines and boilers now so largely used on farms, 
a large bulk, and by consequence that of the fire-spaces in fur- 
nace and boiler, cannot for obvious reasons be secured as in the 
case of stationary engines and boilers. Hence the greatest argu- 
ment which can be brought forward in favour of stationary as 
against portable steam-engines and boilers is just this : that in 
the stationary class better proportioned steam, water, and fire- 
spaces can be secured, and that these lead to steady uniform 
