THE CONQUEST OF NATURE 



too low and narrow to allow the use of the trolley pole. 

 By the time the last cars have been delivered the first 

 cars of the train have been filled, and the process of 

 gathering may be begun at once, and the loaded train 

 made up for the return trip. With such a locomotive 

 two men can distribute and gather up from one hundred 

 to one hundred and twenty cars in an ordinary eight- 

 hour working-day, hauling from three hundred to three 

 hundred and fifty tons of coal. 



In certain regions, a system of third-rail current- 

 supply is used, this rail being also a tooth rail with which 

 a cog on the locomotive works frictionally. For climb- 

 ing steep grades this system of cogged rails has many 

 advantages over other systems. 



Another type of electric locomotive used in some 

 mines is a self-propelling or automobile one equipped 

 with storage batteries. Such locomotives do away 

 with the inconvenience and dangers of contact rails 

 or trolley wires, but are heavy and expensive. A com- 

 promise locomotive, particularly useful for gathering, is 

 one equipped with both trolley pole and storage bat- 

 teries. This locomotive is so made that the storage 

 batteries are charged while it is running with the trol- 

 ley connection, so that no time is lost in the charging 

 process. Such locomotives have been found very sat- 

 isfactory for many purposes, and but for the imper- 

 fections common to all storage batteries would be 

 ideal in many ways. They can be worked over any 

 improvised track, regardless of distance, which is an 

 advantage over the flexible- cable system where dis- 

 tances are limited by the length of cable; and the 



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