One of these sash-like belts of desert runs through southern California, Arizona, New Mexico, the Sahara, Arabia Petræa and the Desert of Gobi; the other traverses Peru, the South African veldt, and Western Australia. They are desert because in them rain is rare; and even clouds seldom form. In a twofold way they conduce to astronomic ends. Absence of rain makes primarily for clear skies and secondarily for steady air; and the one of these conditions is no less vital to sight than the other. Water vapor is a great upsetter of atmospheric equilibrium and commotion in the air the spoiler of definition. Thus from the cloudlessness of their skies man finds in them most chance of uninterrupted communion with the stars, while by suitably choosing his spot he here obtains as well that prime desideratum for planetary work, as near a heavenly equanimity in the air currents over his head as is practically possible. From the fact that these regions are desert they are less frequented of man, and the observer is thus perforce isolated by the nature of the case, the regions best adapted to mankind being the least suited to astronomic observations. In addition to what nature has thus done in the matter, humanity has further differentiated the two classes of sights by processes of its own contriving. Not only is civilized man actively engaged in defacing such part of the Earth’s surface as he comes in contact with, he is equally busy blotting out his sky. In the latter uncommendable pursuit he has in the last quarter of a century made surprising progress. With a success only too undesirable his habitat has gradually become canopied by a welkin of his own fashioning, which has rendered it largely unfit for the more delicate kinds of astronomic work. Smoke from multiplying factories by rising into the air and forming the nucleus about which cloud collects has joined with electric lighting to help put out the stars. These concomitants of advancing civilization have succeeded above the dreams of the most earth-centred in shutting off sight of the beyond so that today few city-bred children have any conception of the glories of the heavens which made of the Chaldean shepherds astronomers in spite of themselves.