INTODUCTION Engineering Is One of the Skills for Which The

Cambridge University Press 0521792975 - Surveying Instruments of Greece and Rome M. J. T. Lewis Excerpt More information INTODUCTION Engineering is one of the skills for which the Romans are most renowned. Some of their works, such as bridges carrying roads or water, are visually spectacular because of their sheer scale and daring. Others are equally impressive for the less obvious reason that they required very precise surveying. Examples which leap to mind are roads which cut across country as straight as an arrow, kilometre-long tunnels whose headings met deep underground without significant error, and aqueducts on gradients that can average in for twenty-five kilometres or in , for eight. Such feats of engineering would have been impossible without good surveying techniques and good instruments. That these existed has of course long been rec- ognised, and many historians of technology have commented on them, although there has been no fundamental discussion of the evidence for many years. The regular conclusion has been that the standard instrument for laying out straight lines and right angles was the groma, that the standard instrument for levelling was the chorobates, and that Hero’s dioptra was a non-starter. Constant repetition has almost sanc- tified this opinion into a dogma. But while it is partly true, it is also partly wrong, and it is very incomplete in that it is biased towards the Romans and ignores much of the evidence available in Greek. One of my aims is to remedy the deficiency, and in the process to restore to the Greeks their rightful share of the credit. ‘The Greeks had the brains, the Romans had good drains’, runs the jingle, in tune with the perception, common to the ancient Romans and to more recent generations alike, that it was Rome which borrowed the bright but unrealised ideas of Greece and brought them to fruition. Yet the Greeks were engineers too, even if their achievements in this field were often more modest and less immediately obvious. In contrast to the wealth, peace and unity of the Roman Empire at its height, the geog- raphy of Greece was divisive and its political units were smaller and incessantly at loggerheads. There was therefore less opportunity for major undertakings. But this did not inhibit creativity; and in the realm © Cambridge University Press www.cambridge.org Cambridge University Press 0521792975 - Surveying Instruments of Greece and Rome M. J. T. Lewis Excerpt More information INTODUCTION of instrumental surveying, as in so much else, it was the Greeks who developed not only the theory but much of the practice too, and their pioneering contribution deserves our wholehearted respect. This book embraces a thousand years, from the archaic Greek tunnels of the sixth century to the Roman aqueducts which were still being built in the fifth century . During that time span, two major revolutions which altered the political map of the Mediterranean also increased the demand for surveyors and indirectly affected their instruments. First, the death in of Alexander the Great ushered in the Hellenistic Age, when his huge empire was carved up by his generals into what, compared with the previous norm of small units, were super-states. Of these, Egypt, ruled by the Ptolemies, at first dominated the scientific and mechanical scene, with a profound input from the royal research institute, the Museum of Alexandria, founded about . At the same time Archimedes was engaged in largely independent but equally mould-breaking work in Syracuse. After a hundred years or so the influence of Alexandria waned. Its scientific lead seems to have been assumed by the emergent kingdom of Pergamon in Asia Minor in the realm of surveying and by the city state of Rhodes in the related pursuit of instrumental astronomy. The Hellenistic Age was ended by the coming of Rome. But it had seen the foundations of geometry laid, most notably by Euclid, and it had also seen the rise, rooted in that geometry, of the theory of surveying and of precision instruments. Surveyors could now answer questions asked by natural philosophers, such as the size of the earth and the heights of mountains, and more practically they could serve the state both in its military and its civil role. The second revolution was the rise of Rome and its acquisition between and , both by force of arms and by diplomacy, of most Greek territories. Its surveying instruments and even some of the uses (notably road building) to which it put them differed from those of the Greeks, but they evidently went back just as far in time. With the Roman take-over of the eastern Mediterranean these two largely independent traditions met and to some extent melded. Roman engineering expanded eastwards, Greek theory (but not perhaps Greek surveying instruments) was exported west: Vitruvius, for example, the major Latin source of the first century , derived his material on surveying almost entirely from Greek sources. Nor was Greek enterprise © Cambridge University Press www.cambridge.org Cambridge University Press 0521792975 - Surveying Instruments of Greece and Rome M. J. T. Lewis Excerpt More information INTODUCTION stifled. Alexandrian science experienced a revival, exemplified by Hero in the first century , by Ptolemy the astronomer in the second and by Theon in the fourth. With its empire established and for two centuries in relative harmony, Rome was at leisure to undertake its mightiest engineering works. The profession of the ancient surveyor, whether Greek or Roman, may be divided into four distinct categories. The land surveyor (geometres or geodaistes in Greek, finitor, mensor, agrimensor or gromaticus in Latin) carried out relatively localised work on the ground surface. He might record the exact shape of an existing expanse of ground such as a field or an estate and calculate the areas enclosed. He might divide land into plots, normally rectangular, whether in the country for distribution to settlers, or in a town for setting out a grid of streets, or in a military context for laying out a fort. His work is the subject of the Roman compilation known as the Corpus Agrimensorum. Land surveying was concerned essentially only with horizontal measurement, not with vertical. The cartographical surveyor (chorographos, geographos) made maps, usually of larger areas than the land surveyor, for example of regions or provinces or even of the whole known world. At least in theory, this might involve establishing latitudes and, indirectly, longitudes by a combination of astronomical and terrestrial methods, and the spheric- ity of the earth had to be taken into account. A related pursuit was the enquiry, originally philosophical, into the size of the earth and the heights of mountains. The military surveyor (mensor) supplied practical information to the commander and his engineers, who might call in particular for two precise dimensions that were highly dangerous to measure directly in the presence of the enemy: the height of a city wall in order to prepare ladders or a siege tower of the right height, and the width of a river in order to prepare a pontoon bridge of the right length. The engineering surveyor (mensor or librator) investigated terrain with a view to imposing man-made features on it. Roads and aqueducts are the most obvious instances, but hand in hand with aqueducts went drainage and irrigation channels and navigable canals; and with them, on occasion, went tunnels and mine adits with their particular © Cambridge University Press www.cambridge.org Cambridge University Press 0521792975 - Surveying Instruments of Greece and Rome M. J. T. Lewis Excerpt More information INTODUCTION problems of maintaining direction and gradient underground. Harbour works could also require the services of a surveyor. These categories were by no means mutually exclusive. Surveyors in the different branches could employ the same instruments and similar techniques, and could even be the same men. Some of their instruments and techniques were also shared by astronomers who, especially with the rise of mathematical astronomy, wished to find the angular distances between stars and planets and the apparent diameters of the sun and moon, and to establish the celestial coordinates of stars and planets relative to the equator or the ecliptic. The various branches of surveying will not receive equal treatment in this book. The work of the agrimensor has already been much dis- cussed, and little can be added to our still imperfect understanding of its principal instrument, the groma. Nor can much new be said of map making, considerable though it evidently was. 1 Ptolemy, its greatest exponent, claimed (Source ) that places could be accurately located by coordinates determined either by astronomical observation or by terrestrial measurement. In practice, however, the vast majority of the latitudes and longitudes which he gives are the result of nothing more than dead reckoning.2 None the less, new light can be shed on the use of instruments in calculating the size of the earth and the heights of mountains. Astronomers and terrestrial surveyors shared the dioptra, which was one of the parents of that important and long-lived device the plane astrolabe; but otherwise astronomical instruments have little direct relevance to terrestrial surveying and will receive short shrift here. Part I of this book therefore concentrates on instruments for military and especially for engineering surveying. Since it would be non- sensical to discuss the instruments and not their application, Part II includes a number of examples of how different surveying tasks might have been undertaken. The subject, too, is one that cries out for experimental archaeology, and the results are presented here of trials with versions of the dioptra and libra which have never, as far as I am aware, been reconstructed before.

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