MINING AND PREPARATION OF CLAYS. 65 



ward the settling tank. This difficulty, which is not 

 often a serious one, has been obviated either by hav- 

 ing the troughing longer or by allowing the water and 

 suspended clay, as they come from the log washer, to 

 pass through a section of straight trough, and from 

 this into another one, of the same depth but five or 

 six times the width, and divided by several longitu- 

 dinal partitions. The water and the clay then pass 

 into a third section, twice as wide as the second, and 

 divided by twice the number of longitudinal divisions. 

 By this means the water moves only in a straight 

 course, but as it is being continually spread out over 

 a wider space it flows with an ever decreasing velocity. 

 By the time the water has reached the end of the 

 troughing, nearly all of the coarse grains have been 

 dropped and the water is ready to be led into the set- 

 tling vats, but as a further and necessary precaution 

 it is discharged on to a screen of one hundred meshes 

 to the linear inch, the object of this being to remove 

 any coarse particles that might possibly remain, and 

 also to eliminate sticks and other bits of floating dirt 

 that are sure to find their way in. 



Two kinds of screens can be used, (1) stationary, 

 and (2) revolving. 



The stationary screen is simply a frame with a cop ^ 

 per cloth and set at a slight angle. The water and sus- 

 pended kaolin fall on the screen, and pass through. 

 A slight improvement is to 'have two or three screens 

 which overlap each other so that whatever does not 

 get through the first will fall on the second. If the 

 vegetable matter and sticks are allowed to accumu- 

 late, they stop up the screen, and prevent the kaolin 

 from running through, consequently the stationary 

 screens have to be closely watched-. 



The revolving screens are far better for they are 



