« Six acres of farmland and a one acre pond with a mill and a small house, situated beyond the fausse Porte Saint Jacques in the place known as the "grand Regard "[between] the "Grand Chemin" extending from Paris to Orléans [and the] road from Paris to Chevreuse. »
The grand Regard refers to the house of the hydraulic engineer, 27th and last access point of the Medici aqueduct.
An unobscured view of the horizon...
This land satisfies completely the desiderata of the Academy of Sciences astronomers : to the South, free of the smoke from the Parisian chimneys, and a splendid horizon not just to the South but also to the the East and West. The site is is elevated since it was once a fortress.
The terrain was bought in the name of the King for the sum of 6604 livres tournois, and was soon enclosed by a wall, which actually cost more than the ground itself.
A part of the original wall is still in place.
Until the start of the XIXth century, one reached the Observatory via the Faubourg-Saint-Jacques and the site of this entrance can still be seen.