For men like Fra Mauro (see last post), the exploration of the external world led to an understanding of the inner landscape of men. So too, from the triangulation of the world (see post here) came a greater understanding of the internal shape of the earth.

I mentioned previously that in order to determine the shape of the world, a French survey team went to Ecuador and Peru at the same time as a team went to Lapland. Unfortunately for the Peruvian team, the Lapland team discovered the answer to the scientific question long before the Peruvian team ever finished their survey. While in South America, however, the Peruvian team struggled over mountains and through jungles and noted different gravity readings as they took their survey measurements along their route. They surmised that the differences in the readings might reflect the varying landscape and theorized that mountains might be made of less dense material than the lowlands. A good theory, but it took over a hundred years for the matter to be more fully explored and the discovery was made far from South America. It took the British Raj’s need to survey India to bring understanding to what the mass of mountains might mean.

In the mid nineteenth century, the horrendous task of surveying India through triangulation was almost undone by the discovery of an error of some 150 meters between the distance the triangulation measurements computed, and an actual direct surface measurement. To determine what had led to the error, the cartographers involved had to reexamine their assumptions – in this case they had assumed that the mass of the Himalayan Mountains would greatly influence the gravity measurements of their survey instruments. What they found was that they had overestimated the lower mass of the mountains.

This led to a theory of the earth’s crust that still exists today, namely that every (theoretical) column of the earth (from core to outer surface following a theoretical plum line) should have approximately the same mass. Given that tall mountains have a lower mass, they must have an equally large (but low mass) protuberance at the bottom of the earth’s crust to achieve the same mass as denser areas of the earth’s crust. Conversely, under the oceans where the ocean basins are very dense, the earth’s crust would be relatively thin. The theoretical result would be that inside the earth there would be a mirroring of the plains, mountains and ocean valleys we see on the surface, much like a tall iceberg has a large underwater presence to balance it out. Modern science has supported this, by obtaining crust measurements off the coast of South America that show the Andes have roots as deep as 75 kilometers.

Think of this the next time you are crossing the mountains, staring up at those tall, snow-covered peaks, for they are the frosting, the outer conceit of an iceberg of stone that conceals the deeper reality of the roots of the mountains.

Much like Fra Mauro saw the illusions of men obscure the truth of his map of the world.