III. On the degree of uncertainty which local attraction, if not allowed for, occasions in the map of a country, and in the mean figure of the Earth as determined by geodesy: a method of obtaining the mean figure free from ambiguity, from a comparison of the Anglo-Gallic, Russian, and Indian arcs: and speculations on the constitution of the Earth’s crust

After referring to a former paper in which he had shown that, in the Great Indian Arc of meridian, deflections of the plumb-line amounting to as much as 20" or 30" would be produced if there were no sources of compensation in variations of density beneath the surface of the earth, and after alluding to a remarkable local deflection which M. Otto Struve had discovered in the neighbourhood of Moscow, the author proceeds to consider, in the first instance, the effect of local attraction in mapping a country according to the method followed by geodesists, in which differences of latitude and longitude are determined by means of the measured lengths of arcs, by substituting these lengths and the observed middle latitudes in the known trigonometrical formulae, using the mean figure of the earth, although the actual level surface may differ from that belonging to the mean figure in consequence of local attraction.