39. The constellations visible below the zodiacal ones, called the southern constellations, are:

40. The whole heavens, then, being portioned out into these constellations, the next thing to be done was to invent some method of referring to each particular star. The method finally adopted and now in use is to arrange all the stars in each constellation in the order of brightness, and to attach to them in that order the letters of the Greek alphabet, using after the letters the genitive of the Latin name of the constellation. Thus Alpha (a) Lyrœ denotes the brightest star in the Lyre; a Ursa Minoris, the brightest star in the Little Bear. Some of the brightest stars are still called by the Arabian or other names they were known by in former times, thus, a Lyræ is known also as Vega, a Boötis as Arcturus, ß Orionis as Rigel, a Ursa Minoris as Polaris (the Pole star), &c.

41. All the constellations, and the positions of the prin cipal stars, have been accurately laid down in Star-Maps and on Celestial Globes. With one or other of these the reader should at once make himself familiar. In starmaps the stars are laid down as we actually see them in the heavens, looking at them from the earth; but in globes their positions are reversed, as the earth, on which the spectator is placed, is supposed to occupy the centre of the globe, while we really look at the globe from the outside. Consequently the positions of the stars are reversed. So if we suppose two stars, the brighter one of them to the right in the heavens, the brighter one will be shown to the right of the other on a star-map, but to the left of it on a globe.

42. The twenty brightest stars in the heavens, or first magnitude stars, are as follow: they are given in the order of brightness, and should be found on a map of globe.

43. Now, although the stars, and the various constellations, retain the same relative positions as they did in ancient times, all the stars are, nevertheless, in motion; and in some of them nearest to us, this motion, called proper motion, is very apparent, and it has been measured. Thus Arcturus is travelling at the rate of at least fifty-four miles a second, or three times faster than our Earth travels round the sun, which is one hundred times faster than an ordinary railway train.

44. Nor is our Sun, which be it remembered is a star, an exception; it is approaching the constellation Hercules at the rate of four miles in a second, carrying its system of planets, including our Earth, with it, Here, then, we have an additional cause for a gradual change in the positions of the stars, for a reason we shall readily understand, if, when we walk along a gas-lit street, we notice the distant lamps. We shall find that the lamps we leave behind close up, and those in front of us open out as we approach them: in fact, the stars which our system is approaching are slowly opening out, while those we are quitting are closing up, as our distance from them is increasing.

45. The real motions of the stars,-called, as we have seen, their proper motions,—and the one we have just pointed out, however, are to be gathered only from the most careful observation, made with the most accurate instruments. There are apparent motions, which may be detected in half an hour by the most careless observer.

46. These apparent motions are caused, as we

shall fully explain by and by (Chap. IV.), by the two real motions of the Earth, first round its own axis, and secondly round the Sun,

LESSON III.-DOUBLE AND MULTIPLE STARS.
VARIABLE STARS.

47. A careful examination of the stars with powerful telescopes, reveals to us the most startling and beautiful

Fig. 1.-Orbit of a Double Star.

appearances. Stars which appear single to the unassisted eye, appear double, triple, and quadruple, and in some instances the number of stars revolving round a centre common to all is even greater. Because our Sun is an isolated star, and because the planets are now dark bodies, instead

of shining, like the Sun, by their own light, as they once must have done, it is difficult, at first, to realize such phenomena, but they are among the most firmly-established facts of modern astronomy. A beautiful star in the constellation of the Lyra will at once give an idea of such a system, and of the use of the telescope in these inquiries. The star in question is (e) Lyræ, and to the naked eye appears as a faint single star. A sinall telescope, or opera-glass even, suffices to show it double, and a powerful instrument reveals the fact that each star composing this double is itself double; hence it is known as "the Double-double." Here, then, we have a system of four suns, each pair, considered by itself, revolving round

a point situated between them; while the two pais

considered as two single

stars, perform a much larger journey round a point situated between them.

It may be stated roundly that the wider pair will complete a revolution in 2,000 years; the closer one in half that time; and possibly both double systems may revolve. round the point lying between them in something less than a million of years.*

48. More than 6,000 double stars are now known,

and of these motion has already been detected in nearly 700, the motion in soine cases being very rapid. In some cases the brilliancy of the component stars is nearly equal, but in others the light is very unequal. For instance, a first magnitude star may have a companion of the fourteenth magnitude. Sirius has, at least, one such companion. Here is a list of some double stars, showing the time in which a complete revolution is effected: