MANUFACTURE OF NATURAL CEMENTS. 239 



try, of which the McEntee cracker of the Rosendale district may be 



taken as typical, differ but little in design. 



They consist, as shown in Fig. 44, of two 



concentric cone-shaped castings, both provided 



on their adjacent surfaces with a series of 



grooves and flanges. Gillmore states * that 



the elements of the lower portions of both 



cones make a smaller angle with the common 



axis than the elements of the upper portions, 



with a view to lessen the strain and the effects 



of sudden shocks on the machinery by secur- 

 ing a more gradual reduction of the stone to 



the required size. These lower portions, being 



subject to very rapid wearing, are made of FlQ 44 ._ s 7ctions of 

 chilled iron, and are moreover cast in separate cracker used in grinding 

 pieces, in order that they may be replaced by ^SSfST 

 new ones as occasion requires. The greatest 



diameter of the core at the top, including the flanges, is 9 inches, at 

 the bottom 5^ to 6 inches, and its height is 15 or 16 inches. The 

 diameter of the shell, measured within the largest flanges, is 14 to 15 

 inches at the top and a trifle greater than that of the core at the bot- 

 tom, while its height is 16^ to 18 inches. One cracker of this size, 

 working with a velocity of 80 to 85 revolutions per minute, is sufficient 

 for a mill grinding 250 to 300 barrels per day. It is customary to pro- 

 vide one cracker for every two run of millstones. In the data given 

 on page 237 it will be noted that each of the heavier McEntee crackers 

 now in use is expected to supply three run of millstones. 



Rolls. Rolls are not used in any American natural-cement plant. 

 Millstones. This group includes a series of mills very important in 

 natural-cement practice. The ordinary millstones and buhrstones 

 have been used in this industry since its commencement in 1818, while 

 the Sturtevant "rock-emery" mill and the Cummings grinder repre- 

 sent modern types. 



Ordinary millstones are cheap in first cost, but require considerable 

 attention for dressing, etc. They vary in diameter from 3 to 5 feet, and 

 will grind from 6 to 10 barrels of cement per hour to a fineness of 90 pei 

 cent through 50-mesh. When greater fineness is required, however, 

 millstones cannot do the work economically. 



* "Limes, Hydraulic Cements, and Mortars ", p. 160. 



