Some writers, however, suppose that possibly we may trace indications of lunar inhabitants in their works, and that they may in like manner recognise the existence of the inhabitants of our planet. An author, who has reflected much on subjects of this kind, reasons as follows: "A navigator who approaches within a certain distance of a small island, although he perceives no human being upon it, can judge with certainty that it is inhabited, if he perceives human habitations, villages, corn-fields, or other traces of cultivation. In like manner, if we could perceive changes or operations in the moon, which could be traced to the agency of intelligent beings, we should then obtain satisfactory evidence that such beings exist on that planet; and it is thought possible that such operations may be traced. A telescope which magnifies twelve hundred times will enable us to perceive, as a visible point on the surface of the moon, an object whose diameter is only about three hundred feet. Such an object is not larger than many of our public edifices; and therefore, were any such edifices rearing in the moon, or were a town or city extending its boundaries, or were operations of this description carrying on, in a district where no such edifices had previously been erected, such objects and operations might probably be detected by a minute inspection. Were a multitude of living creatures moving from place to place, in a body, or were they even encamping in an extensive plain, like a large army, or like a tribe of Arabs in the desert, and afterwards removing, it is possible such changes might be traced by the difference of shade or color, which such movements would produce. In order to detect such minute objects and operations, it would be requisite that the surface of the moon should be distributed among at least a hundred astronomers, each having a spot or two allotted to him, as the object of his more particular investigation, and that the observations be continued for a period of at least thirty or forty years, during which time certain changes would probably be perceived, arising either from physical causes, or from the operations of living agents."[9] FOOTNOTE: [8] Dick's 'Celestial Scenery,' Chapter IV LETTER XVI. THE MOON.--PHASES.--HARVEST MOON.--LIBRATIONS. "First to the neighboring Moon this mighty key Of nature he applied. Behold! it turned The secret wards, it opened wide the course And various aspects of the queen of night: Whether she wanes into a scanty orb, Or, waxing broad, with her pale shadowy light, In a soft deluge overflows the sky."--_Thomson's Elegy._ LET us now inquire into the revolutions of the moon around the earth, and the various changes she undergoes every month, called her _phases_, which depend on the different positions she assumes, with respect to the earth and the sun, in the course of her revolution.