No. 2.] SPENCER — COPPER-MINING. 77 



from the workings ; for although cars may run up very quickly 

 even from a great depth, yet but few can ascend each time, while 

 with the man-engine a constant line of men is ascending, and a 

 mine can thus be cleared of 200 to 500 men more quickly than by 

 other means. Moreover if the machine breaks it cannot fall more 

 than 10 feet, or perhaps not at all. 



Almost all the steam engines in the Lake Superior Copper 

 region are high-pressure and are attached, either directly to the 

 winding drum, or else have friction gearing. 



Most of the copper mines are comparatively free from water, 

 and what does find its way into them is chiefly from the surface, 

 or from the upper levels. Consequently the pumps — which are 

 plungers — are relatively small, and the expense of working is 

 inconsiderable. The plungers are usually placed at every 200 to 

 300 feet apart, with cisterns, and so the great pressure of the 

 columns of water is avoided, and the cost of construction of 

 stronger pipes much reduced. In some of the mines the lower 

 levels do not of themselves contain enough water for mining 

 purposes. 



As noticed before, the mines are cool, and out of the leading 

 draughts of air, have an average winter and summer tempera- 

 ture of 60° Fahr. Artificial ventilation is seldom resorted to, 

 unless it be to change the current of air in a shaft from down- 

 cast to up-cast. Most of the mines have adits or shafts of 

 unequal height, and sufficient air naturally circulates, as long 

 as the connections are good. In winter, doors require to be 

 placed in some of. the passages, the currents of air becoming 

 too rapid, as the difference of temperature at the surface and 

 in the mine is very great, the thermometer at the surface some- 

 times indicating from 30° to 47° below zero. 



When the rock brought to the surface is taken to the rock- 

 house, it is hand-picked, and the poorer portions rejected. After 

 the larger masses are broken up by steam-hammers, the whole 

 of the cupriferous rock is put through Blake's rock-breakers and 

 crushed to a small size, after which it is sent to the stamp mills. 

 The larger pieces of copper that can be detached from the rock by 

 hammers are cleaned and shipped as barrel-work, but they usually 

 contains as much as ten per cent, of gangue. Under the stamp 

 the rock is crushed in presence of water, and washed through sieves 

 having holes a quarter of an inch in diameter. The fine material 

 is washed down into hydraulic separators called jiggers (Collom's 



