This subject, though in itself deeply interesting, and leading to conclusions of much importance for elucidating many obscure questions connected with the constitution of the Universe, has hitherto received but little attention. Bouguer attempted to fix the actual amount of diminution approximatively, by comparing the moon's light with that of wax candles at different ascertained elevations; and in this way obtained data, from which he inferred, that about a fifth part of the entire rays of the sun is absorbed in traversing the atmosphere vertically. Lambert concluded, that the loss sustained is much greater.