Aberration, or wandering, is the name given to this phenomenon. The term is not strictly accurate, as the apparent movements thus denominated are not irregular, but uniform. To discover the physical cause became an object of intense interest to Bradley, but it long baffled his researches and reasonings, and was at length developed by an accidental circumstance. He was accompanying a pleasure-party in a sail on the river Thames. The boat in which they were was provided with a mast which had a vane on the top of it; it blew a moderate wind, and the party sailed up and down the river for a considerable time. Bradley remarked, that every time the boat put about, the vane at the top of the mast shifted a little, as if there had been a slight change in the direction of the wind. He observed this three or four times without speaking; at last he mentioned it to the sailors, and expressed his surprise that the wind should shift so regularly every time they put about. The sailors told him that the wind had not shifted, but that the apparent change was owing to the change in the direction of the boat, and assured him that the same thing invariably happened in all cases. From that moment he conjectured that all the phenomena of aberration he had observed, arose from the progressive motion of light combined with the earth’s motion in its orbit. This sagacious conjecture satisfactorily explains the apparent movement of the stars. Suppose a body to pass from A to B in the same time that a ray of light passes from C to B. Owing to the two motions, the impression of the ray of light meeting the eye of a spectator at B will be exactly similar to what it would have been if the [Illustration]