A History of Engineering in Classical and Medieval Times by Hill Donald
Author:Hill, Donald
Language: eng
Format: epub
Publisher: Taylor and Francis
Figure 8.2: Screw
The method of rotating the screw is also obscure: Vitruvius simply says ‘And so the screws are rotated by men treading — et ita cocleae hominibus calcantibus faciunt versationes’. It is clear that a treadmill was used and it is possible that power was transmitted from this to the screw through a pair of gear-wheels, since these were already in use in mills. We cannot be sure. Medieval commentators on Vitruvius show the screw turned by a crank, but this is certainly an anachronism. Vitruvius recommends that the screw be installed at an angle of about 37 degrees, which would give a high lift with a low output. Remains of Roman screws and their mountings found in Spanish mines indicate an angle of about 15 degrees. The angle chosen would depend upon the relative importance of lift and output.10
The invention of the screw is ascribed to Archimedes by ancient authors, and of course the device usually bears his name. In his thorough study of Greek and Roman mechanics,11 Drachmann mentions that there has been a tendency on the part of some modern writers to doubt this ascription, but he is convinced that the doubts have no foundation. He makes the point, however, that the screws found in Spanish mines show a single spiral of copper instead of the eight wooden windings described by Vitruvius, and asks why Vitruvius described this design when a single spiral is sufficient and takes up less space. His answer is that this was the original Archimedean pattern and that the single spiral was introduced later, probably by the engineers in the copper mines. He also suggests that Archimedes, who is said to have invented the screw during a stay in Egypt, saw the drum or tympanum (see below) in everyday use there and applied his knowledge of spirals to derive the screw from the drum. And since the tympanum has eight compartments, so did the original screw. Drachmann concludes: The invention was a stroke of genius, and it comes almost as a relief to find that after all it was not made out of the whole cloth, so to speak. I have always felt that this was rather too much!’ This explanation is perfectly credible and accords well with documented examples of the process of invention. The screw was in use in Egypt in Ptolemaic times, and spread from there to North Africa, Spain and France during the Roman Empire. Its main use was for irrigation, but it was also used for drainage in mines and for extracting water from the bilges of ships.12 According to Forbes it was still in common use in Upper Egypt and other parts of the Arab world in 1965, but had disappeared from the Delta region.13 The screw is very rarely mentioned by Arabic writers.
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