Abstract :
In the copper mining district of northern Michigan a fair-sized mine usually operates two or more shafts along the strike of the lode, these shafts being usually at least a thousand feet apart. The tonnage to be hoisted from each shaft is large, — from 13,000 to 25,000 tons per month. The hoisting engines used are generally steam-driven, non-condensing, duplex engines with Corliss valves, supplied by steam from separate boiler plants where the distance between shafts is great.
