870 



The Need for Lime : ITow to Meet It. 



[Jan., 



and mortar. Such mills are represented by " The Griffin," 

 " Bradley " and *' Fuller Lehigh." In these, the stone is 

 crushed, percaissed and ground, while at the same time, the 

 ground product is being constantly stirred up and thrown 

 against a screen which determines the maximum size of a 

 particle which can pass out of the mill. If an excessively fine 

 product is not demanded, such mills offer a cheap and efficient 

 means of reducing the stone, provided care is used in their 

 selection and types which involve the use of bearings within 

 the grinding chamber are avoided, since it is obviously difficult 

 to exclude dust from them if they are running in the midst of it. 



The second type is the Ball Mill, in which the stone is 

 broken by percussion in a chamber in which there are a number 

 of steel bails. The inside of the chamber is lined with steel 

 plates set to form a series of steps upon which the balls fall 

 as the chamber rotates. The outside is encircled by a wire 

 screen. The ground product is continuously subjected to screen- 

 ing, so that only those particles which are still too large to go 

 through the screen are retained in the mill for further reduc- 

 tion. This mill produces normally a product very similar to 

 that obtained by the use of the first type described, but owing 

 to the rather more rigorous screening, the proportion of 

 *' fines," that is, of material very much finer than the mesh of 

 the screen used, is rather smaller than with the first type. 

 This is not an advantage from the agricultu.ral point of view. 



The third type is known as the Short Tube Mill. It 

 consists of a slowly-revolving steel drum, with a slightly 

 inclined axis, in which originally pebbles were used as the 

 grinding medium. The stone is fed in slowly at one end, and 

 finds its way through the drum to the other end as the drum 

 rotates, the rate of feed being adjusted so that by the time the 

 stone reaches the far end of the machine, it is ground by 

 attrition between the pebbles to the required state of sub- 

 division. 



The last few years have produced two modifications of this 

 class of mill. In the first place, steel or chilled cast iron balls 

 have, to a large extent, replaced pebbles, and in the second 

 place, a direct air draught through the drum is frequently pro- 

 vided to bring about air separation of the finest dust as it is 

 formed. In this way the output of a given mill can frequently 

 be increased by 15 to 20 per cent, while scarcely interfering 

 with the quality of the product. Unless the fine dust is removed 

 in this w\ay it continues to be dragged round until it finds its 



