LETTER LXVI.

_Bureau des Longitudes_--Is on a more extensive scale than the Board of Longitude in England--National Observatory--Subterraneous quarries that have furnished the stone with which most of the houses in Paris are constructed--Measures taken to prevent the buildings in Paris from being swallowed up in these extensive labyrinths--Present state of the Observatory--_Lalande_, _Méchain_, and _Bouvard_--_Carroché_, and _Lenoir_--_Lavoisier_, and _Borda_--_Delambre_, _Laplace_, _Burckhardt_, _Vidal_, _Biot_, and _Puisson_--New French weights and measures--Concise account of the operations employed in measuring an arc of the terrestrial meridian--Table of the new French measures and weights--Their correspondence with the old, and also with those of England.