98 



CHEMICAL MANURES. 



core, which depends on the degree of friabihty of the material. 

 The hinged plates BB form safety pieces, they avoid all sudden 

 shock, such as would be caused by the passage of a hard body, 

 a piece of iron, for example, and then yield without any of the 

 essential part of the machine being damaged. This mill is used 

 for the preliminary crushing of rock phosphate, or phosphate in 

 nodules of a certain size. It is fitted with two very powerful fly- 

 wheels, which ensure regularity in working. A crusher of this 



^^ liil^ 



m^mm^ 



Fig. 15.— Double EoUer Mill. 



sort making 200 revolutions a minute can crush 5 tons of phosphate 

 an hour with 3 H.P. Hand cracking, formerly in vogue, is now 

 only adopted occasionally, when the quantity of phosphate in large 

 lumps is not enough to warrant the use of the engine. 



Toothed Boiler Mills. — To crush green (raw) bones for fat ex- 

 traction simple and double mills are used with toothed steel rollers. 

 The rollers of the simple mill consist of a certain number of steel 

 discs fitted with teeth, and of united intermediary rings in juxta- 



