b'Bouguer and the shape of the EarthFeatureFigure 2.A map of South America as it is thought to be around the time of the expedition. Quito is near the equator on the western side and Cayenne is on the eastern side. From Ferreiro 2011, also crediting Eliecer Vilchez Ortega.only air in the space between the two levels. For this he could1991). It is achieved by assuming an infinite plate of uniform use Newtons inverse square law, and from this Bouguer deduceddensity and thickness equal to the difference in measurement that the decrease in measured gravity was proportional to heightlevels, called the Bouguer plate (see Sheriff 1991).above sea level. This is now known as the free -air effect and a correction involving height can be made for it. (see Sheriff 1991). Bouguer was thus developing, for the first time, the corrections to measured gravity due to latitude, elevation and topography.Next, Bouguer also realised that there would be an additional effect on the measured value due to the attraction of the massHe then made an attempt to determine the overall density of between the measurement level and sea level. Milsom (2018)the Andes by comparing the ratio of gravity at two heights to suggests that Bouguer may well be the first person to realisethe ratio of the measurement elevation to the radius of the this effect on measured gravity. Suitably then, the allowanceEarth. His results were incorrect, not because his measurements for this effect is now called the Bouguer correction (see Sheriffwere not sufficiently accurate (as it transpired, they were found 41 PREVIEW OCTOBER 2020'