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WASHING COAL 



portions of the coal which have passed through the second perforated plate, fall on to 

 a solid bottom, A', whence they are thrown, delivered direct into the pit by means of 

 a fixed shoot, e, 



The crushing-cylinders, F F , are made with a covering of cast iron, mounted on an 

 iron shaft. This covering can be easily replaced when worn out. It has on its sur- 

 face small grooves, which are usually placed longitudinally, parallel with the axis of 

 the cylinder, in order to avoid the slipping of the substances operated on. But it is 

 also necessary to crush fragments of slate which gain admission with the coal, and 

 these consisting of thin, flattened laminae, it would be necessary to bring the crusher 



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closer than would be required to reduce the coal, which is of a more cubical form, to 

 the proper size. 



In order to obviate this difficulty, another series of grooves are formed on the sur- 

 faces of the crusher transversely to those already described, the intersection of the 

 two producing projections in the form of quadrangular pyramids, with slightly 

 rounded tops. In coming between the projections of the crushers, the fragments of 

 slate, being unable to pass, are broken up without reducing the coal to a smaller size 

 than is required. 



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When the coal has undergone a preliminary sifting, which has removod all the 

 pieces exceeding 6 or 7 centimeters in size, one pair of crushers is sufficient. In 

 that case the grating may be dispensed with altogether by discharging the coal direct 

 into the pit, and returning from the sifter to the washer the pieces of coal which have 

 not been able to pass beyond the first perforated plate. 



The small coal resulting from the washer, or from the sifter, by means of the 

 jigger, is delivered into a common pit placed under the washers. The pit is shaped 

 like an inverted quadrangular pyramid, the three faces of which are inclined to ono 

 another at an angle of 45, to facilitate the descent of the substance, and the fourth 

 is usually vertical. It is on the latter that an opening is made, which is regulated by 

 a flood-gate. 



An elevator, formed of an endless chain, with buckets, raises the coal from the 

 bottom of the pit, and places itself sufficiently high to allow of the final discharge, 

 which may take place into the waggon. 



The rate of ascent of the buckets and their capacities are calculated so as to raise 

 160 10 200 tons of coal in the working hours ; but this quantity may be diminished 

 by means of the flood-gate in the pit. 



The coal discharged by the elevator falls on the sorter, which ought immediately to 

 divide it, according to size, and distribute it to the ferry-boats, 



