They were extremely inefficient by modern standards, but when located where coal was cheap at pit heads, opened up a great expansion in coal mining by allowing mines to go deeper.
Efter nutidig målestok var de ekstremt ineffektive, men når de var placeret dér, hvor kullene var billige, åbnede de op for en stor ekspansion af kulminedriften ved at gøre det muligt at lave dybere miner.