86 



CHEMICAL MANUEES. 



used for crushing very hard phosphate, those of the second kind 

 for soft phosphates. The foundation consists of cast-iron cohimns 

 or a hollow cast-iron support, on which the cage of the mill is 

 fitted up. The shaft of the mill is sustained by a movable bearing 

 with collar ; its lower part rests in a socket. The bearing with 

 collar is screwed to the bottom of the mill cage and completely 

 protected from dust. The adjustment of the revolving millstone is 





Fig. 2. — Flatstone Mill. (Lower stone dormant.) 



done by means of an endless screw or by lever transmission with 

 screw, and hand fly-wheel. The number of revolutions is 120 per 

 minute for mills 5 feet in diameter, under which conditions a mill 

 can grind about 5 tons of phosphate per hour, with 20 H.P. The 

 mill is fed by a cup elevator and a shaking hopper, a single work- 

 man w^ith an assistant can attend to two pairs of stones placed 

 side by side. The mills of the same group are generally driven by 



