The Phosphates of America. 



117 



Several popular methods of grinding now give great satisfaction 

 on the industrial scale, and of these we may mention the plants 

 which comprise 



First. A Blake stone-crusher for reducing the lumps to the 

 size of small marbles, attached to a set of French burr mill-stones 

 fitted with revolving screens up to 90 or 100 mesh. 



Second. The Sturtevant mill and crusher, which is composed 

 of two cylindrical heads, or cups, arranged upon the opposite sides 

 of a case, into which they slightly project, facing each other, and are 

 made to revolve in opposite directions. The rock, being conveyed 

 to the interior of the case through the opening at the top, is re- 



THE STURTEVANT MILL IN CROSS-SECTION. 



tained and prevented from dropping below the revolving heads or 

 cups by a cast-iron screen, and entering, as it must, the heads or 

 cups in revolution, is immediately thrown out again from each cup, 

 in opposite directions, with such tremendous force that the rock 

 from one cup in the collision with the rock thrown oppositely from 

 the other cup is crushed and pulverized, and the grinding, which 

 otherwise would be upon the mill, is transferred to the material, 

 which is at once reduced to powder. 



The mill is composed of four elementary parts a case, two 

 hollow heads or cups, and a screen. 



The principle of its construction is shown in the above cross- 

 section of its elementary parts. 



