SIZE OF COAL. 37 



nace is charged, to show the comparative labour for each 

 kind of fuel. Carefully measure the quantities of fuel and 

 water (the latter in a tested water meter), consumed per square 

 foot of fire grate per hour, also the heating surface of the 

 boiler. A certain proportion of the fuel drawn from the 

 fire? at the end of the trial is assumed to be good, and its 

 weight deducted from the total fuel supplied. The level of the 

 Avater in the boiler should be the same at the end as at the be- 

 ginning of the test. The feed water should be kept as far as 

 possible at the same temperature to avoid subsequent correc- 

 tions, and the feed pump should work regularly. A trial 

 should continue for at least a day on account of irregularities 

 that may cause errors. For ultimate comparisons of fuels, the 

 results should be corrected for moisture in the steam and in the 

 fuel. Other tests are necessary to ascertain whether the 

 proper quantity of air has been admitted for combustion of the 

 fuel, determined by the chemical composition of the gases as 

 they leave the boilers. 



We must remember that coal is seldom burnt as carefully 

 in practice as it is for tests. The engineer wants to get as 

 much steam as possible out of his boiler, and does not always 

 give the coal time to burn properly or economically, so when 

 comparing results, we must consider whether the coal is to be 

 made to fit the work, or the work to fit the coal. Also it must 

 be borne in mind that the returns from careful tests are no 

 criterion as to the work the coal will do when subject to care- 

 less everyday usage. 



Size of Coal. 



For commercial purposes, coal is classified according to 

 its size, but all collieries do not produce the same classes. 

 Shovel-filled, or "shandygaff," is the nm-of-mine coal as 

 broken at the faces. Fork filled, is coal filled into 

 skips at the face with a fork: such a fork has about 10 tines, 

 say an inch and a quarter apart. Fork-filled coal has much of 

 the slack separated out, which slack is shovelled on one side 

 till required by the colliery owners. The miners are not paid 

 directly for this slack, but as compensation they are paid at a 

 higher rate for fork-filled coal. Since this might encourage 

 some men to fill in an undue proportion of slack with the 

 forked coal, in order to increase their tonnage, the allowable 

 percentage of slack in fork-filled coal is limited to ten per 

 cent. The weighmen, by experience, can soon tell if this is 

 exceeded, so if the skips of one man show that he is trying to 

 gain a point, they are put on one side and tested ; if the percent- 

 age of slack has been exceeded then that miner has to seek 

 work elsewhere. Large, lump, round, or screened coal is that 

 which passes over bars placed Jin. apart; though at some col- 



