Steam locomotives that used up to 20% of the U.S. coal production were replaced by diesel locomotives after World War II, saving a great deal of energy and reducing manpower for handling coal, boiler water and mechanical maintenance. Improvements in steam engine efficiency caused a large increase in the number of steam engines and the amount of coal used, as noted by William Stanley Jevons in "The Coal Question". This is called the Jevons paradox. Electricity consumption and economic growth are strongly correlated. Per capita electric consumption correlates almost perfectly with economic development. Electrification was the first technology to enable long-distance transmission of power with minimal power losses. Electric motors did away with line shafts for distributing power and dramatically increased the productivity of factories. Very large central power stations created economies of scale and were much more efficient at producing power than reciprocating steam engines. Electric motors greatly reduced the capital cost of power compared to steam engines. The main forms of pre-electric power transmission were line shafts, hydraulic power networks and pneumatic and wire rope systems. Line shafts were the common form of power transmission in factories from the earliest industrial steam engines until factory electrification. Line shafts limited factory arrangement and suffered from high power losses. Hydraulic power came into use in the mid 19th century. It was used extensively in the Bessemer process and for cranes at ports, especially in the UK. London and a few other cities had hydraulic utilities that provided pressurized water for industrial over a wide area. Pneumatic power began being used industry and in mining and tunneling in the last quarter of the 19th century. Common applications included rock drills and jack hammers. Wire ropes supported by large grooved wheels were able to transmit power with low loss for a distance of a few miles or kilometers. Wire rope systems appeared shortly before electrification. Recovery of heat for industrial processes was first widely used as hot blast in blast furnaces to make pig iron in 1828. Later heat reuse included the Siemens-Martin process which was first used for making glass and later for steel with the open hearth furnace. (See: Iron and steel below). Today heat is reused in many basic industries such as chemicals, oil refining and pulp and paper, using a variety of methods such as heat exchangers in many processes. Multiple-effect evaporators use vapor from a high temperature effect to evaporate a lower temperature boiling fluid. In the recovery of kraft pulping chemicals the spent black liquor can be evaporated five or six times by reusing the vapor from one effect to boil the liquor in the preceding effect.