There is a greater mystery still. The stars have their own individual motions, and in the case of a few these are of the most amazing swiftness. The earth in its motion round the sun travels nearly nineteen miles in a second, say one thousand times faster than the quickest rush of an express train. The sun's rate of motion is probably not quite so swift, but Arcturus, a sun far larger than our own, has a pace some twenty times as swift as the orbital motion of the earth. This is not a motion that we can conceive of as being brought about by gravitation, for if there were some unseen body so vast as to draw Arcturus with this swiftness, other stars too would be hurtling across the sky as quickly. Such 'runaway stars' afford a problem to which we have as yet no key, and, like Job of old, we are speechless when the question comes to us from heaven, 'Canst thou guide Arcturus and his sons?' It will be seen then that, fundamentally, Greenwich Observatory was founded and has been maintained for distinctly practical purposes, chiefly for the improvement of the eminently practical science of navigation. Other inquiries relating to navigation, as, for instance, terrestrial magnetism and meteorology, have been added since. The pursuit of these objects has of necessity meant that the Observatory was equipped with powerful and accurate instruments, and the possession of these again has led to their use in fields which lay outside the domain of the purely utilitarian, fields from which the only harvest that could be reaped was that of the increase of our knowledge. So we have been led step by step from the mere desire to help the mariner to find his way across the trackless ocean, to the establishment of the secret law which rules the movements of every body of the universe, till at length we stand face to face with the mysteries of vast systems in the making, with the intimate structure of the stellar universe, with the apparently aimless, causeless wanderings of vast suns in lightning flight; with problems that we cannot solve, nor hope to solve, yet cannot cease from attempting, problems to which the only answer we can give is the confession of the magicians of Egypt--'This is the finger of God.' INDEX Aberration of light, 79 Adams, John C., his discovery of Neptune, 217 Adhara, 183 Airy, George Biddell, seventh Astronomer Royal, his early life, 102; his work at Cambridge, 105; comes to Greenwich, 105; his relations with the Visitors, 106; his autobiography, 108; his character, 111; his labours, 113; attacks on, 114; his distinctions, 118; his resignation, 119; his death, 120; anecdote of, 142; his conduct _re_ Adams, 217; his water telescope, 304 Alderamin, 183 _Almagest_, 185 Almanac making, 29 Alpha Aquilæ, telescope for, 303 ---- Cygni, telescope for, 303 Altazimuth the, 114; description and work of, 207, _et seq._