STATE GEOLOGIST. 189 
bituminous, is charged, together with the stone, in alternate layers. For 
100 pounds of stone, about 10 pounds of good anthracite coal is charged; 
in the plant at New Lisbon, Ohio, it is customary to add sixteen shovels 
of good bituminous coal to a carload of about 3,500 pounds of stone. 
The kilns employed in the natural cement industries have undergone 
practically no change during the last thirty or forty years, being simply 
vertical shafts with contracted bottom, of various, but not greatly differ- 
ing dimensions. All of these kilns are now operated on the continuous 
plan, since not only the output, but also the quality, of the cement is 
superior to that burnt in the periodic shaft kiln. The quality of the 
cement could be further improved by the use of furnaces arranged around 
the circumference of the kiln, so that the fuel would not need be in con- 
tact with the stone and also the distribution of the heat could be effected 
with much greater efficiency. However, the greater expense of operating 
the kilns would probably prohibit its use. Such kilns have been tried by 
American cement makers, and as far as qualitative results are concerned 
were eminently successful. Some improvements have been made in re- 
gard to the distribution of air in the ordinary kiln among which Camp- 
bell’s grate in the kiln bottom might be mentioned. ‘There is no doubt 
but that natural cement could be burnt in the rotary kiln with good suc- 
cess, especially since the time of burning would thus be considerably 
shortened, resulting in a better quality of cement. The fuel consumption 
would, of course, be greater, but the greater uniformity and quality of 
the product would compensate to some extent for the waste in fuel. A 
rotary kiln, of the usual size, 60 feet by 6 feet, should have no difficulty in 
burning on an average 250 barrels of natural cement per day, as no delay 
due to “rings” would occur and the lining would not be attacked as vig- 
-orously as it is in Portland cement kilns. Obviously the cement rock 
would have to be reduced to about pea size, at least, before being fed into 
the kiln. 
Rough Grinding. —As usually practiced, the selected burnt rock is 
sent first to the “cracker” and from there to the millstones. The cracker 
consists of a frustrum of a cone, made of cast-iron with a chilled shell 
which revolves within the inverted frustrum of a right hollow cone. 
Both are provided with corrugations for breaking up the stone. The 
lower parts of the cones make a smaller angle with the vertical axis than 
the upper, thus securing a more gradual reduction of the stone. The 
diameter of the shell is about 15 inches at the top and 6 inches at the bottom. 
The height is about 18 inches. A cracker of this size will grind enough 
stone for about 300 barrels per day, making 80 to 85 revolutions. From 
the cracker the crushed material passes on to the mills. In some plants 
the crushed stone is screened on the way to the mill, while in others 
everything goes through the fine grinding machine. An ingenious 
system of screening has been perfected by Berthellet in which all of the 
