Heron of Alexandria: The Engineer of Antiquity

Life and career

Almost nothing is known about Hero's life, including his ethnicity, parents' names or occupations, birthplace, or dates. The first mention of him in extant secondary sources is a quotation of Mechanics by Pappus's Collection (4th century AD), and scholarly estimates for Hero's dates range from 150 BC to 250 AD. Otto Neugebauer (1938) noted a lunar eclipse observed in Alexandria and Rome used as a hypothetical example in Hero's Dioptra, found that it best matched the details of an eclipse in 62 AD, and Aage Drachmann [de] surmised that Hero personally observed the eclipse from Alexandria; however, Hero does not explicitly say this, his brief mention of the eclipse is vague, and he might instead have used some earlier observer's data or even made up the example.

Alexandria was founded by Alexander the Great in the 4th century BC, and by Hero's time was a cosmopolitan city, part of the Roman Empire. The intellectual community, centered on the institution of the Musaeum (which included the Library of Alexandria), spoke and wrote in Greek; however, there was significant intermarriage between the city's Greek and Egyptian populations. It is assumed that Hero taught at the Musaeum, because his writings seem like course notes or textbooks in mathematics, mechanics, physics and pneumatics.

Inventions

Hero described the construction of the aeolipile (a version of which is known as Hero's engine) which was a rocket-like reaction engine and the first-recorded steam engine (although Vitruvius mentioned the aeolipile in De Architectura some 100 years earlier than Hero). It was described almost two millennia before the industrial revolution. Another engine used air from a closed chamber heated by an altar fire to displace water from a sealed vessel; the water was collected and its weight, pulling on a rope, opened temple doors. Some historians have conflated the two inventions to assert that the aeolipile was capable of useful work, which is not entirely false, air containing a trace of water vapor.^{[clarification needed]} However, this engine is far from a pure aeolipile.

• The first vending machine was also one of his constructions; when a coin was introduced via a slot on the top of the machine, it dispensed a set amount of water for ablutions. This was included in his list of inventions in his book Mechanics and Optics. When the coin was deposited, it fell upon a pan attached to a lever. The lever opened up a valve which let some water flow out. The pan continued to tilt with the weight of the coin until it fell off, at which point a counter-weight would snap the lever back up and turn off the valve.
• A wind-wheel operating an organ, marking the first instance in history of wind powering a machine.
• Hero also invented many mechanisms for the Greek theatre, including an entirely mechanical play almost ten minutes in length, powered by a binary-like system of ropes, knots, and simple machines operated by a rotating cylindrical cogwheel. The sound of thunder was produced by the mechanically-timed dropping of metal balls onto a hidden drum.
• The force pump was widely used in the Roman world, and one application was in a fire engine.
• A syringe-like device was described by Hero to control the delivery of air or liquids.
• In optics, Hero formulated the principle of the shortest path of light: If a ray of light propagates from point A to point B within the same medium, the path-length followed is the shortest possible. It was nearly 1,000 years later that Alhacen expanded the principle to both reflection and refraction, and the principle was later stated in this form by Pierre de Fermat in 1662; the most modern form is that the optical path is stationary.
• A stand-alone fountain that operates under self-contained hydro-static energy; now called Heron's fountain.
• A cart that was powered by a falling weight and strings wrapped around the drive axle.
• Various authors have credited the invention of the thermometer to Hero. The thermometer was not a single invention, however, but a development. Hero knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water. The expansion and contraction of the air caused the position of the water/air interface to move along the tube.
• A self-filling wine bowl, using a float valve.

Mathematics

Hero described a method, now known as Heron's method, for iteratively computing the square root of a number. Today, however, his name is most closely associated with Heron's formula for finding the area of a triangle from its side lengths. He also devised a method for calculating cube roots. He also designed a shortest path algorithm, that is, given two points A and B on one side of a line, find C a point on the straight line that minimizes AC+BC.

In solid geometry, the Heronian mean may be used in finding the volume of a frustum of a pyramid or cone.

Bibliography

The most comprehensive edition of Hero's works was published in five volumes in Leipzig by the publishing house Teubner in 1903.

Works known to have been written by Hero include:

• Pneumatica (Πνευματικά), a description of machines working on air, steam or water pressure, including the hydraulis or water organ
• Automata, a description of machines which enable wonders in banquets and possibly also theatrical contexts by mechanical or pneumatical means (e.g. automatic opening or closing of temple doors, statues that pour wine and milk, etc.)
• Mechanica, preserved only in Arabic, written for architects, containing means to lift heavy objects
• Metrica, a description of how to calculate surfaces and volumes of diverse objects
• On the Dioptra, a collection of methods to measure lengths, a work in which the odometer and the dioptra, an apparatus which resembles the theodolite, are described
• Belopoeica, a description of war machines
• Catoptrica, about the progression of light, reflection and the use of mirrors

Works that sometimes have been attributed to Hero, but are now thought most likely to have been written by someone else:

• Geometrica, a collection of equations based on the first chapter of Metrica
• Stereometrica, examples of three-dimensional calculations based on the second chapter of Metrica
• Mensurae, tools which can be used to conduct measurements based on Stereometrica and Metrica
• Cheiroballistra, about catapults
• Definitiones, containing definitions of terms for geometry

Works that are preserved only in fragments:

• Geodesia
• Geoponica

Publications

• Liber de machinis bellicis (in Latin). Venezia: Francesco De Franceschi (senese). 1572.