The Arizona sentinel., 1888-07-07

nearly circular, or it may be very long
 and narrow. The elliptical orbits O.
 the returning comets vary greatly il this respect. Some are so long thai
 one end is very near to the sun, and the other stretches to the uttermost
 limits of our system. In such cases the comet occupies long time in mak- ing each journey. The comet of 1911, of which 1 have already spoken, is ex- pected to return thirty centuries hence. Other comets have short periods of only few years.
 Very few of the known comets are visible to the naked eye. During the first half of 1887 five comets have been carefully watched, but all required telescopic aid.
 -s7T7u0;5rh- soao--ef5as called s.comet sweepers" are used for finding them With one of these the astronomer sweeps the heavens by starting at a certain point, then moving the teles- cope along in one direction as far as he can; then he shifts it by the breadth of its field of view, and sweeps back again, then shifts again and sweeps forward, and so on until he has thus traversed the heavens or that part in which he is searching.
 He may continue this on every clear night, week after week, month after month, and pick up nothing but the well-known stars. At last he may be fortunate enough to see a hay speck in place occupied by no known star. He notes its exact position among the stars, then observes it again on the next night, and finds that it has moved. Therefore, it is not a star. He returns to it again. It has moved farther, is a little brighter or little fainter, and shows other peculiarities that prove it to be comet. What are these peculiarities? You will perhaps answer: ''It has tail"
 This, however, need not be the case. More comets are thus seen without tails than with them. Many are quite round, and only differ from stars in appearance, by being less bright and by being capable of being magnifed by the telescope. The telescope can only make a star appear brighter, not bigger. The stars are mere points of light so small that though magnified hundred times they still appear inn measurably small.
 Not so the planets, not so the comets. These are near enough to appear sensi- ply bigger according to the power of the telescope through which they are viewed. When Herschel first discov- cred the planet Uranus he thought it was comet, as it moved, and the tel escope magnifed it. Further examina- tion proved that he had found another world.
 The faint, luminous point usually presented by a comet when first seen, enlarges if it is coming nearer to us and the sun. Ih course of time it presents a bright speck in the center called the nucleus, surrounded by cloudy or nebulous light. This is called the coma.
 In many cases this is all; it comes and goes as mere luminous, misty spot invisible to the naked eye. In other instances an internal disturbance as it approaches the sun; the coma increases, then stretches out in one or more directions and forms a tail or tails.
 Generally speaking, the nearer a comet approaches the sun the greater is the tail, and the tail is always turned away from the sun as though produced by some repulsive force exerted by the sun. This tail-forming action has been watched. Jets of light or luminous matter have been seen first to shoot out from the head or nucleus towards the sun, then to turn round and outwards 1n every side and drift behind as though feeding the growing tail with fresh material.
 Nobody can tell what this material is. I doubt whether it is material at all. It may be, and 1 think it is, only an illumination by electrical dis charges or otherwise of material all ready existing in space. If the so- called tail were behind, that is, in the track from which the comet has come, where proper tail should be, it might be trail, or something that the comet had left behind, but this is not the case, the comets tail is at first behind, then, as the head approaches the sun, 1t whisks round with enormous veloci- y, and finally, as the comet is going away from the sun, the tail is fore most. is ahead of the head.
 The tail of the great comet of 1880 was above one hundred and forty mill lsz7gakiRiles long, and is said to have
 4 aed in two days. The tail 1f VI dexelIs) was above )ne hii11e Billions of miles long, and swept round the sun in semicircle n two hours, the outer part of the tail jhus moving at the rate of one hunred md fifty millions of miles per hour, md all the other parts with varying Velocities.
 l can not conceive any substance do- ng this, but beam of light thrown rom mirror turns round in this man. ler when we turn the mirror. This beam of light is an action, not a sub- stance. It appears to me that the tail )f comet must be similar.
 
 On the 30th of June, 1861, we passed through the tail of a comet. Such is he conclusion of Mir. Hind, a high au- 5hority on the subject. l remember 5he day. It was foggy in London, but