The word “indispensable” in the passage is closest in meaning to
[#paragraph1]By the eighteenth century, Britain was experiencing a severe shortage of energy. [#insert1] Because of the growth of population, most of the great forests of medieval Britain had long ago been replaced by fields of grain and hay. [#insert2] Wood was in ever-shorter supply, yet it remained tremendously important. [#insert3] It served as the primary source of heat for all homes and industries and as a basic raw material. [#insert4] Processed wood (charcoal) was the fuel that was mixed with iron ore in the blast furnace to produce pig iron (raw iron). The iron industry’s appetite for wood was enormous, and by 1740 the British iron industry was stagnating. Vast forests enabled Russia to become the world’s leading producer of iron, much of which was exported to Britain. But Russia’s potential for growth was limited too, and in a few decades Russia would reach the barrier of inadequate energy that was already holding England back.
[#paragraph2]As this early energy crisis grew worse, Britain looked toward its [#highlight3]abundant[/highlight3] and widely scattered reserves of coal as an alternative to its vanishing wood. Coal was first used in Britain in the late Middle Ages as a source of heat. By 1640 most homes in London were heated with it, and it also provided heat for making [#highlight4]beer, glass, soap, and other products[/highlight4]. Coal was not used, however, to produce mechanical energy or to power machinery. It was there that coal’s potential was enormous.
[#paragraph3]As more coal was produced, mines were dug deeper and deeper and were constantly filling with water. Mechanical pumps, usually powered by hundreds of horses walking in circles at the surface, had to be installed. Such power was expensive and bothersome. In an attempt to overcome these disadvantages, Thomas Savery in 1698 and Thomas Newcomen in 1705 invented the first primitive steam engines. Both engines were extremely inefficient. Both burned coal to produce steam, which was then used to operate a pump. However, by the early 1770s, many of the Savery engines and hundreds of the Newcomen engines were operating successfully, though inefficiently, in English and Scottish mines.
[#paragraph4]In the early 1760s, a [#highlight6]gifted[/highlight6] young Scot named James Watt was drawn to a critical study of the steam engine. Watt was employed at the time by the University of Glasgow as a skilled crafts worker making scientific instruments. In 1763, Watt was called on to repair a Newcomen engine being used in a physics course. After a series of observations, Watt saw that the Newcomen’s waste of energy could be reduced by adding a separate condenser. This [#highlight8]splendid[/highlight8] invention, patented in 1769, greatly increased the efficiency of the steam engine. The steam engine of Watt and his followers was the technological advance that gave people, at least for a while, unlimited power and allowed the invention and use of all kinds of power equipment.
[#paragraph5]The steam engine was quickly put to use in several industries in Britain. It drained mines and made possible the production of ever more coal to feed steam engines elsewhere. The steam power plant began to replace waterpower in the cotton-spinning mills as well as other industries during the 1780s, contributing to a phenomenal rise in industrialization. The British iron industry was radically transformed. The use of powerful, steam-driven bellows in blast furnaces helped iron makers switch over rapidly from limited charcoal to unlimited coke (which is made from coal) in the smelting of pig iron (the process of refining impure iron) after 1770. In the 1780s, Henry Cort developed the puddling furnace, which allowed pig iron to be refined in turn with coke. Cort also developed heavy-duty, steam-powered rolling mills, which were capable of producing finished iron in every shape and form.
[#paragraph6]The economic consequence of these technical innovations in steam power was a great boom in the British iron industry. In 1740 annual British iron production was only 17,000 tons, but by 1844, with the spread of coke smelting and the impact of Cort’s inventions, it had increased to 3,000,000 tons. This was a truly amazing expansion. Once scarce and expensive, iron became cheap, basic, and [#highlight11]indispensable[/highlight11] to the economy.