64 GENERAL DISCUSSION OF CLAYS. 



good length, these being arranged paralleled^ and 

 connected at the ends, so that the water, with sus- 

 pended clay, follows a zigzag course. 



This troughing has a slight pitch which is common- 

 ly about one inch in twenty feet, but the amount of 

 pitch depends upon the kaolin, and whether the sand 

 which it contains is fine or coarse. If the kaolin is 

 very fine, and settles slowly, the pitch need not be so 

 great and vice versa. A large quantity of very coarse 

 sand in the kaolin is a nuisance as it clogs up the log 

 washer, and upper end of the trough more quickly and 

 causes so much more labor to keep them clean. As it 

 is, considerable sand settles there, and, to keep the 

 trough clear, sand wheels are used. These are wooden 

 wheels bearing a number of iron scoops on their peri- 

 phery, as the wheels revolve these scoops catch up a 

 portion of the sand which has settled in the trough, 

 and as each scoop reaches the upper limit of its turn 

 on the wheel, it, by its inverted position, drops the 

 sand outside of the trough. These sand wheels are an 

 aid, but it is often necessary, in addition, to keep a 

 man shoveling the sand from the trough. 



If the sand is finer it is not dropped so quickly, but 

 is distributed more evenly along the trough, and does 

 not clog it up so fast. 



The zigzag arrangement of the troughing has been 

 objected to by some, as it produces irregularities in 

 the current causing the sand to bank up in the corners 

 at the bends, and also at certain points along the sides 

 of the troughing.* 



The effect of this is to narrow the channel, and con- 

 sequently to increase the velocity of the current, there- 

 by causing the fine sand to be carried still further to- 



*£. Hotop, Thonindustrie Zeitang, 1893. 



