DRESSING OF ORES 



knee-joint levers. Tho fixed jaw ,fig. 647, is a plate of cast iron, bedded in lead, 

 and held in the frame B, by tapered cheek pieces c c'. The vibrating jaw plate D 

 corresponds in this respect with the fellow plate A. The excentric shaft E gives 

 motion to the toggles F F, which press forward the breaking jaw, D, about f ths of an 

 inch. In the depression of the lever the jaw is drawn back by means of a strong 

 India-rubber spring. 



A stone dropped between the convergent jaws will be operated upon by a succes- 

 sion of bites, until it is crushed small enough to pass out at the bottom. The distance 

 between the jaws at the bottom limits the size of the fragments. The frame of the 

 machine is formed of one single casting, and although provided with bolt holes, yet is 

 usually heavy enough to require nothing further than its own weight to retain it in 

 working position, The apparatus should be fixed high enough either to allow a wag- 

 gon to be placed under the jaws, or otherwise that the reduced stones may gravitate 

 to the rollers of a crushing mill. 



The width of the opening at the top of the jaws determines the size of the machines, 

 and at the same time it represents the largest stone which can possibly be dealt with. 

 The quantity of stone reducible in a given time must necessarily depend upon its 

 degree of hardness, as well as upon its structure. In addition, the result is either 

 lessened or increased, according to the minimum distance between the jaws, and the 

 speed given to the excentric shaft. 



647 



The approximate duty rendered by stone-breakers of various size, at a speed of 

 200 revolutions per minute, is as follows : 



At the Kirkless Hall Colliery, in Lancashire, it has been found that one ton of 

 limestone falling thro ugh jaw pieces seven inches apart at the top is reduced into 

 fragments which can be passed through a gauge one and three-quarters of an inch in 

 diameter at a cost not exceeding threepence per ton, the coal for maintenance of 

 engine being charged at three shillings and sixpence per ton. The various parts of 



