Cassini had a very different agenda. Having married into a family of the noblesse de robe in 1711, he saw astronomy as a much loftier pursuit than geography, and was concerned to protect the reputation of his father and the family’s scientific lineage; perversely, he regarded the opportunity to restart the survey primarily as an opportunity to counter the Newtonians and conclusively prove Descartes’s theory of the earth’s shape once and for all. Work began again on the painstaking process of measuring base lines and triangulating distances in 1733. In an age when national map surveys are accepted as a normal and routine activity, it is difficult to imagine the momentous scale of Cassini II’s undertaking. Without the use of modern surveying instruments or transportation, and without the understanding of the local community, even the basic tasks involved would prove extremely arduous. The teams began with a reconnaissance of the area to be surveyed, establishing physical and man-made features, and deciding where to measure base lines and angular distances. This immediately presented problems. Unlike the earlier surveys conducted in built-up areas of reasonably hospitable terrain, the geometers now confronted a landscape that proved unyielding to the progress of scientific accuracy. They needed to survey regions that were often bereft of notable landmarks from which to triangulate distances, or mountainous regions where siting equipment would prove extremely perilous. Working in the Vosges Mountains in the summer of 1743, they were suspected of being Anabaptists, and accused of inciting a revolt with their secretive encampments and mystifying behaviour; in the early 1740s one was hacked to death by the villagers of Les Estables in the Mezenc region, who suspected his instruments of bewitching the local crops.26
The teams also encountered tiny villages, populated by people with little connection to the wider world and with no idea as to why a group of strangers were marching round pointing strange instruments at the landscape, asking awkward questions. Even as the surveyors began to reconnoitre, their equipment was stolen, they were denied horses and guides and many were pelted with stones. Local knowledge proved difficult to obtain, as even those who understood what was happening remained opposed to the work, convinced (quite rightly) that the results would only lead to the imposition of higher tithes, rents and taxes.
When (or if) the basic reconnaissance of an area was completed, preparations were made to measure a base line. Micrometers and quadrants were used to calculate exact latitude. The base line could now be constructed, using wooden rods each measuring 2 toises laid end to end over the course of at least 100 toises. Only when the base line was correctly laid and measured could the process of triangulation begin. Having verified the distance between two points on the base line, the surveyors could now choose a third point to create a triangle. But even this presented problems. The surveyors had no way of measuring the altitude of the terrain; all they could try to do was triangulate points from a specific man-made vantage point – usually a church bell-tower. Having established this position, the angle to the third point was measured using a quadrant or graphometer. Turning to their trigonometrical tables, the surveyors measured all three angles and calculated the two new sides of the triangle. Having ascertained the three angular distances, the team could then construct a second triangle, and so on until the entire region was surveyed according to a network of adjacent triangles. As each triangle was completed, a plane table was used to sketch the beginnings of what would eventually become the precise map of the region.
The sheer physical task of moving such cumbersome equipment from place to place, before then taking and rechecking measurements and calculations to ensure their accuracy, was exceptionally gruelling, and the margins for error were legion. Not surprisingly, work was painfully slow. The surviving manuscript maps made in the field give an idea of the countless observations, readings and calculations that were made. Hardly any physical details are shown, apart from towns, villages and rivers. Instead, the maps were criss-crossed with innumerable angular lines representing triangulated measurements, which dominated entire sheets. As the survey gradually built up its vast databank of measurements, the realization began to dawn on those assessing the fieldwork back in Paris that Picard’s original calculations were not as infallible as had been assumed. The survey had started by drawing triangles related to the original Paris meridian drawn by Picard. By 1740, having measured 400 triangles and 18 base lines, Cassini II and his young son, César-François, realized that Picard’s original measurement of the position of the meridian was out by 5 toises, or 10 metres. The error was small, but if multiplied over the entire country, it would compromise all the original calculations. A complete recalculation of the established measurements was required. When they were completed in 1738, the results were once again bad news for Cassini II: the recalculated latitudes confirmed Maupertuis’s Lapland measurements. Even the measurement of French soil now proved, once and for all, that Newton’s theories were correct.
The influence of the Cassini lineage might have ended there but for the increasing involvement in the survey of Jacques’s son, César-François Cassini de Thury, Cassini III – a better geographer than an astronomer, and a shrewd diplomat. He quietly accepted the triumph of Newtonianism, understood Orry’s requirements for the new survey, and throughout the 1730s and 1740s deftly steered the arduous survey not only to its completion but to printed publication. As his disillusioned and increasingly aloof father appeared out of step with the shift towards a more professional approach to geography, Cassini III began planning the public dissemination of the survey’s work.
In 1744, the survey was finally completed. Its geometers had completed an extraordinary 800 principal triangles and nineteen base lines. Cassini III had always envisaged printing regional maps as they were produced, and by 1744 the map was published in eighteen sheets. Its new map of France, on an appropriately small scale of 1:1,800,000, shows the country represented as a network of triangles, with virtually no expression of the land’s physical contours, and with large areas such as the Pyrenees, the Jura and the Alps left blank. It was a geometrical skeleton, a series of points, lines and triangles following coasts, valleys and plains in connecting the key locations from which observations were carried out. Over it all lay the triangle, the new, immutable symbol of rational, verifiable scientific method.27 On Cassini III’s map the triangle almost takes on its own physical reality, a sign of the triumph of the immutable laws of geometry and mathematics over the vast, messy chaos of the terrestrial world. The Babylonians and the Greeks had revered the circle; the Chinese celebrated the square; the French now showed that it was the application of the triangle that would ultimately conquer the earth.
The publication of the 1744 survey represented the fulfilment of Colbert and Orry’s original plans. It was not in modern terms a national survey based on comprehensive topographical detail, but a geodetic survey which produced a positional illustration of places significant to the requirements of state planning. Cassini III admitted as much when he explained that his surveying teams ‘didn’t go off into each village, into each hamlet in order to survey the plan. We haven’t visited each farm or followed and measured the course of every river . . . such detail is required only of plans for some seigniorial land; the reasonable size at which one ought to fix the map of a country doesn’t allow one to be able to mark so many things without great confusion.’28 The logistics of completing not one but two surveys were simply too great; to complete a further, third survey of the entire topography of the country would have required a level of money, manpower and technical precision that Cassini III clearly regarded as unrealistic. As far as he was concerned, his work, and that of his family, was finished. Public and private individuals and organizations could now fill in the topographical blanks that were clear for all to see on his 1744 map. The commercial map trade in Paris had already produced a distinguished collection of mapmakers, including Alexis-Hubert Jaillot (1632–1712) and Guillaume Delisle (1675–1726), but a new generation, including Jean Baptiste Bourgignon d�
��Anville and Didier Robert Vaugondy (1723–86), now emerged to produce maps and atlases capitalizing on the cartographic opportunity presented by Cassini III.29
Though nobody, least of all Cassini III, regarded the survey of 1733–44 as a preliminary study for an even greater description of the country,30 this is nevertheless precisely what happened and Cassini was asked to embark on yet another survey in the service of the Bourbon monarchy’s dynastic and military ambitions. Just as his father had led France into one costly war over the dynastic succession to the throne of Spain, so Louis XV intervened in a similar dispute in 1740. This time it revolved around the Habsburg territories claimed by Austria on France’s northern and eastern borders. The Austrian War of Succession (1740–48) led Louis into a series of bloody and expensive campaigns, which by the spring of 1746 saw his armies fighting in the Austrian Netherlands. Cassini III was invited to advise the French engineers on measuring base lines along the River Scheldt, and in October 1746 assisted in drawing up the topographic plans for the Battle of Rocoux outside Liège.
Fig. 28 César-François Cassini de Thury, ‘New Map of France’, 1744.
Following the French victory, Louis XV visited the region and compared its terrain with Cassini’s maps. His comments would provide a turning point in the future of the national survey. ‘The King,’ recalled Cassini III, ‘map in hand, found the country and the disposition of his troops so well represented that he had no question for his generals nor for the guides; and he did me the honour of saying to me: “I want the map of my kingdom to be done in the same way, and I charge you with doing it, inform [Jean-Baptiste de] Machault [the controller-general] of this”.’ After the labours of the previous eighty years, both king and cartographer knew that this would be no easy undertaking, but ‘he did me the honour’, Cassini continued, ‘of asking me several times if the work would be easy of execution, and how much time it would require to bring it to perfection’.31
The pragmatic Cassini III soon had an answer to Louis’s question. Despite his concerns about the feasibility of such a gigantic project, the chance to conduct yet another survey, this time covering every topographical feature in the country, from its rivers to its hamlets and villages, was too good to miss, and scientific immortality beckoned. He calculated that the survey would take eighteen years to complete. One hundred and eighty regional maps on a uniform scale of 1:86,400 would be required to cover the entire country, with ten maps produced every year, each costing 4,000 livres, which included the costs of equipment, surveying and printing. The annual budget of 40,000 livres would pay for ten teams of two engineers to measure and record the relevant information on the ground, which would then be sent back to the Paris Observatory where it would be checked prior to engraving and publication. Each printed map sheet would be sold for 4 livres on an estimated print run of 2,500 each. If all 180 sheets were sold at this figure, the project would raise revenue of 1,800,000 livres – an impressive return on a projected state investment of just 720,000 livres. Considering that a skilled worker could earn up to 1,000 livres a year, and that the king’s cabinet-maker could charge 938,000 livres for ten years’ work, Controller-General Machault appreciated that in purely financial terms Cassini’s survey looked like an excellent investment.32 He had been shocked at the depletion of the kingdom’s coffers following the Austrian Wars of Succession, and was keen to reform the state’s outdated tithe system by introducing a universal flat rate of taxation, much to the consternation of the nobility and clergy who had benefited from the old feudal arrangements. The new survey promised to assist him in his new scheme, and benefit many of those people who had so aggressively opposed its predecessors.
Cassini III grasped the opportunity to implement a new survey that would transform geographical understanding of France, but his methods would also change the entire practice of geography. He proposed to standardize the maps created from the survey by adopting what is now called a transverse equirectangular projection, which treats the globe as a cylinder projected onto a rectangle. The globe is rotated so that any given meridian acts just like the equator, ensuring that scale along this line, and anywhere at right angles to it, remains correct. The inevitable distortions at the northern and southern limits of such a projection were negligible for Cassini’s purposes, as the regional areas to be surveyed were too small to be seriously affected. Otherwise he pointed out that, unlike in the first two surveys, no new scientific innovations would be required. Having established a geodetic framework, Cassini III now introduced a method that would allow its topographic detail to be filled in. In line with Orry’s plan to educate a new generation of geographers in standard methods of surveying and mapmaking, Cassini III proposed to train his teams of engineers from scratch in the techniques of measurement and observation required to complete the survey. Each engineer would keep two logbooks. One would record topographical information, the location of villages, rivers, churches and other physical features, which would be verified by local priests and gentry. The other would record geodetic data on the measurement of triangulation in relation to established base lines and principal triangles, which would be sent to Paris and checked by members of the Observatory. Accuracy, uniformity and verifiability were established as central to the survey’s political and financial success. Under Cassini III’s guidelines, geography would now become a routine and continuous activity sanctioned by the state, its practitioners operating within strict guidelines determined by the authorities. The age of the learned savants uniting the arcane wisdom of astronomy, astrology and cosmography in the creation of their maps was coming to an end. Geographers were slowly but surely turning into civil servants.
Shortly after the conclusion of the Austrian War of Succession with the signing of the Peace of Aix-la-Chapelle in October 1748, the first instalment of money was paid to Cassini III to begin his new survey. Yet again, teams of engineers fanned out across the country, preparing to survey what Cassini III referred to as ‘this innumerable quantity of cities, towns, villages, hamlets and other objects scattered across the whole extent of the kingdom’.33 As usual, work started from the environs of Paris, following the tributaries of the Seine. Topography now preceded geometry, as Cassini’s engineers endeavoured to put geographical flesh onto the triangulated skeleton of the first two surveys. The work was less specialized, but it would bring about an unprecedented portrayal of the impact of human settlement on the earth.
Cassini III had already established his reputation for tact and diplomacy. To this he now added an obsessive attention to detail and accuracy, characterized by his tireless micromanagement of every aspect of the survey, from personal involvement in the fieldwork, to overseeing the engraving of the plates for publication. Nothing was left to chance, as his account of a surveyor’s average day in the field revealed:
Located on the highest part of the bell-tower and accompanied by either the parish priest or syndic or other person able to provide knowledge of the country and to indicate to them the names of the objects they see, they had to spend part of the day becoming sufficiently familiar with the area to be able to represent it on the map, checking the condition of their instruments and the parallelism of the telescopes and taking and several times retaking the angles between the principal points, checking whether the angles taken in circling the horizon did not exceed 360 degrees, as good a proof of the precision of the angles composing the tour d’horizon as the observation of the third angle of a triangle. The work of the day would be followed by the work of the study: having acquired an idea of the layout of the area, it would be necessary to roughly draw the heights, the valleys, the direction of the roads, the course of the rivers, the nature of the terrain; to draw up, in fact, the map of the area while they were there and able to check that it was accurate, and to correct it if it were erroneous.34
Of equal importance to the work in the field was the paper trail it created; from the land to the study, Cassini’s engineers were instructed to write
up their observations and translate them into handdrawn sketch maps, correct them where necessary and then dispatch everything to Paris for another round of verification. Cassini III insisted that when the map was drafted, it was returned to the local dignitaries involved in initially checking the relevant topographical data. ‘The geometrical part belongs to us,’ proclaimed Cassini; ‘the expression of the terrain and the spelling of the names are the work of the lords and priests; the engineers present the maps to them, profit from the information they provide, working under their orders, making in their presence the corrections to the map, which we publish only when it is accompanied by certificates’ confirming the veracity of the information recorded.35 It was an essential element in ensuring accuracy, but it had another consequence too: however reluctant the provincial nobility might have been to verify the observations made by the unwelcome engineers, they were now becoming part of the fabric of a national survey. Up to this point, local knowledge had been ignored in favour of the pure geometry of the triangulated framework of the survey; Cassini III now ensured that the visualization of the imagined community of France included the knowledge of those who lived and worked within it.
A History of the World in 12 Maps Page 39