b'Bouguer and the shape of the EarthFeatureFigure 3.A map of the Audiencia of Quito (mostly now Ecuador) around the time of the expedition showing Manta, where Bouguer first landed and Mt. Chimborazo (just north of Riobamba). From Ferreiro 2011, also crediting Eliecer Vilchez Ortega.to be accurate) but his assumption of the topography was toois of the detail surrounding Quito at around the time of the uniform. He lacked the knowledge of the detailed topographicExpedition, shows the location of Mt. Chimborazo9. Bouguer variations that only became available over 200 years later fromwasnt to know that the summit of this mountain is the point on Digital Terrain Models8. Earth that is furthest from the Earths centre.During another one of the lulls in progress with determiningThe survey was performed in December 1738 with the the shape of the Earth, Bouguer found time to try anotherassistance of his companion, La Condamine. Because of the experiment that he had always hoped to do one day. That wasrough conditions the base of the mountain could only be to practically validate Newtons theory of universal gravitationalreached on foot, and with bad weather and problems with attraction of discrete masses, something that had so far notthe instruments the deflection at around seven seconds was been done even though it was proposed by Newton (1687)not as much as estimated. However, it was anomalous and over 50 years earlier. Newton had theoretically calculated thathad proved the concept. As revealed by Ferreiro (2011), this a hemispherical mountain with a radius of 3 miles (4.83 km)feat was not repeated by Bouguer nor by anyone else in the and the same density as the Earth would have enough mass to20 more years of his lifetime. In 1774 a British astronomer, cause a deflection in the local vertical of almost two minutesMaskelyne, used a technique similar to Bouguers on a of arc. At the time Newton was sceptical that it could be evermountain in Scotland10.measured to this level of accuracy (see Milsom 2018, 146). Of course, the assumption was being made at the time that However, Bouguer knew that he was capable of detecting athe Earths crust was uniform in density surrounding the site of deviation of less than one minute. He had tried this before inmeasurement (the Bouguer plate). It wasnt until the early 1800s France but none of the mountains in Europe were large enoughthat the variability of the density of the crust was appreciated to produce an effect that he could measure. In Ecuador he wasand the anomalies it produced, after standard corrections are surrounded by the Andes, which were thought, at the time,applied, were the Bouguer anomalies now ascribed to near-to be the biggest mountains in the world. As it happened,final gravity results and used in geological mapping. Nor was just 25 kilometres from Pierres hacienda was Mt. Chimborazo, the tallest of them all at 6263 m. Its height had already been measured, and it had the regular shape of the extinct volcano,9Also shown in Figure 3 is Manta, on the western coast where Bouguer so he decided to try and use it. The map in Figure 3, whichmeasured gravity on his first arrival to Ecuador.10In the early 1800s the Survey of India conducted largely by the Surveyor General, George Everest, was fully aware of the deviation of their verticals 8More on this is in Milsom (2018, 1335). caused by the Himalayas.OCTOBER 2020 PREVIEW 42'