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date: 15 December 2019

energy and power

Energy and power are closely related concepts: energy implies the capacity to do work, and power affects the rate at which work is done (energy transmitted per unit of time). The availability of energy, and the rates at which that energy can be converted into heat or mechanical work, for example, constitute fundamental limits to the performance of any economy.

“Energy” derives from the Greek ἐνέργεια‎. The word is used by Diodorus Siculus (20.95.1) for the motive power of one thousand men pushing a battering ram, but for the most part it means “action” or “operation.” In Latin, energia is used by Jerome (Ep. 53.2) to describe the power of a living voice, but it is largely a medieval term. Latin offers a wide range of words for “force” or “power” in a more general sense, such as vis or potentia, which could be used for physical force, but not in the abstract sense in which “energy” can be used. This perspective on the ancient world, and the importance of energy and power, is a wholly modern one.

The ancient world remained almost entirely an organic energy economy rather than employing the mineral energy (fossilised plant matter like coal and oil) that dominates the modern world.1 For any work that needed to be carried out, Greeks and Romans were dependent on human and animal muscle, on the wind (for sailing), and on the force of flowing water. Light (see lighting) came from burning olive oil in lamps, or beeswax or tallow as candles. Heat (for homes, cooking, and industrial processes like refining and working metals, firing pottery, and the like) was obtained by burning wood, charcoal, or other organic material like the solid remains of olives after pressing or dried animal dung.2 Archaeological evidence shows that coal was burned as fuel in Britain between c. 50 ce and the 5th century, and remains have been found at around seventy sites, but coal does not seem to have been used elsewhere during this time period—not least because southern Europe and the Mediterranean do not have easily accessible deposits. More importantly, it was used to produce heat rather than mechanical work and so, unlike in 18th-century England, did not support any sort of industrial revolution.3

This dependence on organic sources of energy had a number of important consequences for the performance and capacity of the ancient economy. Above all, it established the total dependence of ancient societies on the land, as the primary source of food and fodder for the creatures doing the work, as well as the majority of raw materials for industry, and the fuel required for processing them. On the one hand, energy derived from the land is renewable in the short term (crops for food and fodder) and relatively short term (trees for wood and charcoal), unlike mineral energy. On the other hand, there are strict limits to the energy that can be derived from a given area of land, with attempts at increasing the yield through increased labour inputs rapidly reaching the point of diminishing marginal returns, and there are always other demands on the land besides fuel. In the mid-19th century, the energy derived from coal just for domestic fuel consumption in England would have required virtually all available farmland to produce the equivalent from wood, to say nothing of its use in fuelling steam engines.4

This therefore limited the amount of energy at the disposal of ancient societies—estimated at only a fifth or even less of the average daily energy consumption in the modern world. Wood and even charcoal have much lower calorific values than fossil fuels (Sixteen to twenty megajoules per kilogram for wood and thirty for charcoal, as opposed to thirty-five megajoules per kilogram for coal and over forty for oil derivatives), making it harder to reach high temperatures for refining metal ores and the like. Further, biological energy converters (humans and animals) have a low level of efficiency of conversion, so that only 15–20 percent of energy intake is converted into mechanical work as opposed to 35 percent with modern machinery.5 Similar limits apply to the total amount of power that could be exerted in a single task; humans can deliver up to 1 horsepower in a very short burst, but 0.1 or less over any period of time; the average modern car can deliver a sustained 100 horsepower or more, and purpose-built engines and other machinery are vastly more powerful. It was for a long time believed that harnesses were inadequate to make much use of animal traction before the 9th or 10th centuries CE, but this thesis has now been discredited.6 However, the mass of the population had limited access to mechanical power beyond human muscle: one required either substantial amounts of land (for fodder, and to have enough work to make the investment worthwhile) to afford a draught animal, or a suitable watercourse and capital to invest in the equipment to make use of water power.

However, while the whole period of classical antiquity faced these “limits of the possible,” the picture was not static. In the 3rd century bce, range of machines designed either to make the most effective and efficient use of existing energy sources or to open up new sources of energy were invented and came into use. This period has indeed been characterised as the next great revolution in production after the domestication of animals: rotary animal-powered mills were much more powerful and efficient than hand querns for grinding grain (see Figure 1), while other machinery employed animal power to mix and knead dough or to raise water into a channel for the purposes of irrigation.7

energy and power

Figure 1. Rotary grain mills, driven by donkeys, from a bakery in Pompeii.

Photo by Carole Raddato. Creative Commons, CC BY-SA 2.0.

Further, there were significant developments in the construction of water mills, both over- and under-shot types, in order to drive different machinery; grain mills—again, most famously in the substantial Roman complex at Barbegal near Arles (see Figures 2 and 3) and the mills on the Janiculum in Rome—and saws for cutting marble.

energy and power

Figure 2. Remains of the water-mill complex at Barbegal, near Arles, in southern France.

Photo by Jeanlucdel. Creative Commons,CC BY-SA 4.0.

energy and power

Figure 3. A model reconstruction of the Barbegal complex, from Arles museum.

Photo by Carole Raddato. Creative Commons, CC BY-SA 2.0.

It was long believed that the Romans made little use of water mills, except in a limited number of such large-scale complexes, in part because the variability of many Mediterranean rivers suggested that they would not deliver a sufficiently reliable flow of water. However, Roman expertise in hydraulic technology enabled both the construction of aqueducts to feed mill complexes, where this was worth the expense (both the Barbegal and Janiculum mills are assumed to have been constructed by the state) and to create suitable mill streams for smaller constructions. It is now clear, from surveying physical remains, references in literary sources, and pictorial representations, that water mills were widespread in Italy and other provinces. The number of references peaks in the 4th and 5th centuries ce, but that is a reflection of the availability of new types of documents, such as saints’ biographies and monastic charters, that were more likely to mention mills; this should rather be taken as an indication of the prevalence of mills in the previous centuries as well.8

Rome, unlike classical Greece, was not solely dependent on muscle power and had greater energy and power resources at its command, but with a water mill able to generate around three horsepower, it was still vastly inferior to the power available to modern societies. It should also be noted that the rate of conversion of solar energy into food, fodder, and firewood is not a constant but depends on climatic conditions; the evidence of climate change in late antiquity also implies a diminished supply of energy for later Roman society.

Bibliography

Adams, C. Land Transport in Roman Egypt: A Study of Politics and Administration in a Roman Province. Oxford: Oxford University Press, 2007.Find this resource:

Bresson, A. “La machine d”Héron et le coût de l”énergie dans le monde antique.” In Innovazione tecnica e progresso economico nel mondo romano. Edited by E. Lo Cascio, 55–80. Bari, Italy: Edipuglia, 2006.Find this resource:

Clark, G., and D. Jacks. “Coal and the Industrial Revolution, 1700–1869.” European Review of Economic History 11 (2007): 39–72.Find this resource:

Dearne, M. J., and K. Branigan. “The Use of Coal in Roman Britain.” Antiquities Journal 75 (1995): 71–105.Find this resource:

Malanima, P. “Energy Consumption in the Roman World.” In The Ancient Mediterranean Environment Between Science and History. Edited by W. V. Harris, 13–36. Leiden: Brill, 2013.Find this resource:

Raepsaet, G. Attelages antiques. Jougs et jouguets. 2d ed. Brussels: Groeninghe Uitgeverij/Drukkerij BVBA, 2016.Find this resource:

Veal, R. “Fuelling Ancient Mediterranean Cities: A Framework for Charcoal Research.” In The Ancient Mediterranean Environment between Science and History. Edited by W.V. Harris, 37–58. Leiden: Brill, 2013.Find this resource:

Wikander, Ö. Exploitation of Water-Power or Technological Stagnation? A Reappraisal of the Productive Forces in the Roman Empire. Lund: Gleerup, 1984.Find this resource:

Wikander, Ö. “Sources of Energy and Exploitation of Power.” In Oxford Handbook of Engineering and Technology in the Classical World. Edited by J. P. Oleson, 136–157. Oxford: Oxford University Press, 2008.Find this resource:

Wilson, A. “Machines, Power and the Ancient Economy.” Journal of Roman Studies 92 (2002): 1–32.Find this resource:

Wilson, A. “Raw Materials and Energy.” In The Cambridge Companion to the Roman Economy. Edited by W. Scheidel, 133–155. Cambridge, U.K.: Cambridge University Press, 2012.Find this resource:

Wilson, A. “Quantifying Roman Economic Performance by Means of Proxies.” In Quantifying the Greco-Roman Economy and Beyond. Edited by F. de Callataÿ, 147–167. Bari: Edipuglia, 2014.Find this resource:

Wrigley, E. A. Energy and the English Industrial Revolution. Cambridge, U.K.: Cambridge University Press, 2010.Find this resource:

Notes:

(1.) Cf. E. A. Wrigley, Energy and the English Industrial Revolution (Cambridge, U.K.: Cambridge University Press, 2010).

(2.) R. Veal, “Fuelling Ancient Mediterranean Cities: A Framework for Charcoal Research,” in The Ancient Mediterranean Environment between Science and History, ed. W. V. Harris (Leiden: Brill, 2013), 37–58.

(3.) M. J. Dearne and K. Branigan, “The Use of Coal in Roman Britain,” Antiquities Journal 75 (1995): 71–105. Bresson suggests that the limited supplies of coal in the Mediterranean, and hence the higher cost and limited heat output of combustible fuel, is one reason why steam power was never developed in antiquity, although the principle was discovered by Hero of Alexandria in the 1st century ce. See A. Bresson, “La machine d”Héron et le coût de l”énergie dans le monde antique,” in Innovazione tecnica e progresso economico nel mondo romano, ed. E. Lo Cascio (Bari, Italy: Edipuglia, 2006), 55–80.

(4.) G. Clark and D. Jacks, “Coal and the Industrial Revolution, 1700–1869,” European Review of Economic History 11 (2007): 39–72.

(5.) P. Malanima, “Energy Consumption in the Roman World,” in The Ancient Mediterranean Environment Between Science and History, ed. W. V. Harris (Leiden: Brill, 2013), 13–36.

(6.) G. Raepsaet, Attelages antiques. Jougs et jouguets, 2d ed. (Brussels: Groeninghe Uitgeverij/Drukkerij BVBA, 2016). See a brief summary of debate in C. Adams, Land Transport in Roman Egypt: A Study of Politics and Administration in a Roman Province (Oxford: Oxford University Press, 2007), 74–77.

(7.) Ö. Wikander, Exploitation of Water-Power or Technological Stagnation? A Reappraisal of the Productive Forces in the Roman Empire (Lund: Gleerup, 1984); Bresson, Innovazione tecnica e progresso; A. Wilson, “Machines, Power and the Ancient Economy,” Journal of Roman Studies 92 (2002): 1–32; and A. Wilson, “Raw Materials and Energy,” in The Cambridge Companion to the Roman Economy, ed. W. Scheidel (Cambridge, U.K.: Cambridge University Press, 2012), 133–155.

(8.) Ö. Wikander, “Sources of Energy and Exploitation of Power,” in Oxford Handbook of Engineering and Technology in the Classical World, ed. J. P. Oleson (Oxford: Oxford University Press, 2008), 136–157; A. Wilson, “Quantifying Roman Economic Performance by Means of Proxies,” in Quantifying the Greco-Roman Economy and Beyond, ed. F. de Callataÿ (Bari, Italy: Edipuglia, 2014), 161–163.

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