Among the many inventions of Hero of Alexandria, none have exerted the same amount of fascination as his so-called aeolipile, or "wind ball," which some historians have described as the first steam engine and earliest jet turbine. Hero briefly explains the construction of this device in his Pneumatica. The device is quite simple, consisting of two metal chambers, three tubes, and a rod. The first chamber is a hollow hemisphere or covered cauldron and is meant to be filled with water. When this vessel is heated, the water gradually turns to steam and escapes through a tube or pipe leading to a hollow sphere. This pipe and a rod also attached to the cauldron hold up the sphere and are connected to it in such a way that the sphere can pivot around the axis they form. The steam, after entering the sphere, is allowed to escape through two additional L-shaped tubes set opposite one another along its circumference equidistant from the "poles" formed by the rod and tube connecting the sphere and the hemisphere. The exhaust ends of the L-shaped pipes point in opposite directions so that the force of the escaping steam causes the sphere to rotate around its axis. The classicist J. G. Landels, an expert in ancient science and engineering, reported that his own model of the aeolipile reached speeds of 1,500 revolutions per minute.

There continues to be some dispute over the precise significance of Hero's steam engine. Some scholars have essentially dismissed the device as no more than a toy without any practical application. Others have suggested that Hero intended the aeolipile not to do work but to demonstrate fundamental physical principles. This position seems reasonable given that the introduction to the Pneumatica explores the nature of air, water, and vacuum in general. Nevertheless, it is remarkable that this device, mentioned in no other ancient text, receives no special comment or explanation from Hero amid the other, more banal inventions described in this work. Another recurring debate concerning the aeolipile asks why Hero or some other ancient inventor did not adapt it for useful work, precipitating an ancient industrial revolution. The answer seems to be that the lack of proper fuel, construction materials, and tools impeded further development. Some have also suggested that the slave-based Roman economy did not place a sufficient premium on labor-saving machines to promote further development of the device.

The mere fact that Hero's writings survived him suggests that later Greek and Roman thinkers considered his work to be important. Writing materials and scribes were expensive, so the preservation of Hero's work constituted a significant investment. A lack of sources makes it difficult to assess Hero's impact in antiquity, but Pappus of Alexandria, an early fourth century scholar, suggests that Hero had many followers. To this day, Hero's books remain a vital source for the study of ancient mathematics, engineering, surveying, and natural philosophy because so few other works on these subjects survive.

The impact of Hero's ideas and inventions in later periods is somewhat easier to gauge and seems to have been considerable. Scholars in the Islamic world preserved some of his writings, demonstrating their continued importance. Hero's wind-powered organ may have inspired Islamic engineers to develop the windmill. Many in both the Islamic world and the Byzantine Empire were inspired, directly or indirectly, by the wonderful automata described in the Pneumatica and sought to emulate them. Hero was not forgotten in medieval Europe, but interest in him increased dramatically in the Renaissance. In Italy and elsewhere, demand for automata grew. Humanist scholars also took an interest in Hero, particularly his ideas about the vacuum, and translated the Pneumatica into Latin, Italian, and German. Leonardo da Vinci, Francis Bacon, and Robert Boyle were all familiar with his writings. Hero's aeolipile even contributed to early efforts to design functional steam engines.

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