G.—ENGINEERING 147 
certainly involve more loss of central station capital than the economies 
would repay, but in view of future expansion there seems a need for an 
examination of the present policy, which is only the old isolated station 
plan with interconnection by the grid superimposed. ‘The opportunities 
afforded by the grid permit of a great change in the general plan, and a 
change, moreover, that can be introduced by gradual steps, if the final 
scheme is outlined at the beginning. The present rate of expansion 
indicates that in ten years’ time the station power will be at least double 
its present figure, and while the utilisation of spare plant which the 
grid permits will slow down the increase of plant for two or three years, 
after that the normal growth will give opportunity for a new policy. 
There may be some waste of capital, where stations have been designed 
with a view to large expansion, in that certain permanent parts are now 
unnecessarily large ; but the proportion of such parts, taken all over, 
is only a small item, which the saving in fuel costs will quickly repay. 
No scrapping of existing plant need be done unless there will be a gain 
by so doing. 
WasTE Coat. 
The term ‘ waste coal ’ will here be used to include all coal in the seam 
that is not at present sold, but is or can be brought to the surface, and 
coal of poor quality that will be profitably used in the pit-head station, 
instead of being extensively cleaned for sale. This quantity varies with 
the kind of seam and with the purpose for which the coal is used. In 
Durham and S. Wales, where much of the coal is converted into coke, 
there is little waste, as even small fragments can be coked, and the coal 
is won with small admixture of dirt. But in most other parts the dross 
has a larger ash content and is less saleable. Machine cutting produces 
a larger proportion of dirt than hand winning, some of the mixed coal and 
dirt being left in the pit as not worth raising, but the actual cost of working 
is much less. Ifa use is found for the waste, this disadvantage of machine 
cutting will be removed, and the full advantage of the reduced cost of 
cutting will be gained, while no coal need be lost. 
The use of dry-cleaning processes results in a rather larger proportion 
of waste than does the wet process, and if this waste has no value, the 
cheapness of the process is neutralised by the loss of coal; but again 
a use and a market for the waste will be in favour of dry-cleaning. Wet 
processes are from one aspect a wrong action. ‘The water that is 
unavoidably left in the coal and often ignored is quite as detrimental 
to the calorific value as an equal percentage of ash. It is just as useless 
as fuel, and it has further to be evaporated, in which process it absorbs 
1100 B.Th.U. per pound, whereas one pound of ash would require to 
be heated to a temperature of about 2000° F. to absorb that amount 
of heat. There is, of course, the additional trouble of removing the ash, 
but the avoidance of water in the dross and small coal is a definite 
advantage. Hence dry-cleaning will be more widely adopted if the 
waste can be used. The waste from dry-cleaning is often cleaned again 
and some saleable coal recovered, but if the whole waste has a value and 
is used, the cost of additional cleaning will be saved. 
Of the dirty coal that is at present raised and remains as the residue 
of cleaning operations, some is dumped on to waste land and some into 
