19'21.] The Xeed for Ltme and How to Meet it. 205 



to fulfil these conditions. In the case of the small portable 

 machines the stone to be ground is fed into the mouth of the. 

 mill in pieces not much larger than 2.V-in. cubes; it then passes 

 into a circular chamber in which there are two or more 

 massive, manganese steel hammers, flexil)ly attached to a 

 rapidly rotating, central shaft. The stone is immediately 

 broken and re-broken by percussion against both the swinging 

 hammers and the walls of the chamber, some part of which, 

 generally the lower half, is composed of stout, manganese steel 

 bars arranged with narrow spaces between them, through which 

 the stone dust can escape when fine enough. After leaving this 

 screen the ground stone falls into a hopper whence it can be sho- 

 velled or bagged ; it is, however, better to arrange the portable 

 plant so that the stone is fed into the machine almost at ground 

 level, and delivered direct into a cart. This can be readily done 

 by providing a feed elevator to raise the broken stone to the 

 mill, and a second elevator to lift the finished product and shoot 

 it into a cart. 



A plant as described above would cost at the present time 

 about £'600, mounted on wheels, complete with two elevators. 

 The output would be about IJ tons of ground stone per hour, and 

 the power taken to drive the mill and its two elevators would not 

 exceed about 15 B.H.P. It is obvious that on small estates where 

 the initial cost of such a plant is more than the needs of one 

 owner warrant, a co-operative scheme should be possible. Success 

 will depend entirely upon careful selection of the plant and atten- 

 tion to certain details, some of which will now be described. 



There are many makes of disintegrator on the market, but 

 only a very few are really suitable for limestone grinding. It 

 is important that the swinging hammers should be as massive 

 as possible, and for this reason there cannot be many of 

 them : two are enough, and any number above four is cer- 

 tainly too many. If the machine offered has four hammers, an 

 opposite pair should be dismantled in order to determine whether 

 the mill will not run easier and give the same output as before. 

 Accessibility is an important feature, and the mill chosen must 

 be of a pattern which can be easily opened up for examination 

 of the grinding chamber and tackle provided to take the weight 

 of any heavy parts not swung on a vertical axis. 



The screen bars must be easily renewable, preferably in 

 single units, not cast up together in segments : and should be 

 made of manganese steel like the swimrinir hannuers, as no 

 other material is tough and durabk^ (Miough. 



