1869.] Coal Washing. 491 



The simplest, but by no means the most efficient, form of coal- 

 washing machine is that consisting of a simple trough, or passage, 

 having a smooth channel, and set on a shght incline in the direction 

 of its length, as shown in the accompanying woodcut. At the 



Fig. 1. 



lower end, the trough is fitted with a vertical shding sluice valve, 

 working up through the bottom, and in grooves in the sides, which 

 may be elevated or depressed at pleasure by means of a screw ; and 

 just above this sluice is placed a valve, openmg in the bottom or side 

 of the trough. The mode of working is as follows. The coal to 

 be washed Ls either first crushed by passing it between rollers, or, 

 as is sometimes the case, the trough is placed under the screen, over 

 which the coals are tipped in passing from the trucks, as they come 

 from the pit, into the railway waggons. The finer particles of coal 

 then, passing through the meshes of the screen, fall into the trough 

 at its upper or more elevated extremity, into which also a con- 

 tinuous flow of water is admitted. The coal is thus carried down 

 the channel by the water, and it has to be kept constantly agitated 

 during its passage, either by means of a long pronged fork worked 

 by hand, or by a mechanical apparatus designed for the purpose. 

 This agitation is necessary to ensure the more perfect separation of 

 the coal from its impurities. As it flows down, the coal, being 

 lighter than the shale or pyrites, rises to the top, whilst the heavier 

 particles settle down and remain in the bottom of the trough. The 

 coal then is permitted to pass over the sluice at the lower end of 

 the shoot ; but as the impurities accumulate, the sluice is gradually 

 raised to prevent their passing away with the coal, and this is done 

 until the sluice has been raised to a height nearly level with the 

 top of the sides of the trough. The supply of coal is then tem- 

 porarily turned ofi", and the valve opened in the bottom of the trough, 

 through which the impurities are allowed to pass. The sluice is 

 then lowered, and the same operation is repeated. In some 

 machines of this type the trough is made very long, and a sluice is 

 placed in the middle of its length as well as at the extremity. 



