mers named the new constellations, called the Camelopard, the Unicorn, the Fly, and the rivers Jordan, Euphrates, and Tigris. In the latter part of the seventeenth century, the rivers were rejected, and instead of them, and in some other vacant spots, were introduced the Hounds, Mount Menalus, Cerberus, the Fox and Goose, the Lizard, the Shield of Sobieski, the Lynx, the Little Lion, the Little Triangle, and the Sextant, and also the Bow and Arrow of Antinous.

Many other names of constellations were added as compliments to monarchs or patrons, or as commemorative of interesting events or distinguished men. These often replaced former constellations, and speedily disappeared from the maps, and many are not known at the present day.

§ 324. A natural feature in the heavens, more marked than any of the constellations, is the milky-way. This has not improbably presented the same appearance and kept the same position ever since the creation of our cluster. It traverses the constellations Cassiopeia, Perseus, Auriga, Orion, Gemini, Canis Major, and Argo, where it appears most brilliant. It then passes through the feet of the Centaur, the Cross, the Southern Triangle, and returns towards the north by the Altar, and the tail of the Scorpion, where it divides into two branches. One branch passes through the tail of Scorpio, the bow of Sagittarius, Aquila, Antinous, Sagitta, and the Swan. The other branch passes through the upper part of the tail of Scorpio, the side of Serpentarius, Taurus, Poniatowski, the Goose, and the neck of the Swan, where it again unites with the other branch, and passes on to the head of Cepheus. Here the branches unite, after remaining separate for the space of more than 100°. There is another small separation of the milky-way between Cassiopeia and Cygnus. In some parts this zone is ten or fifteen degrees broad, as in the southern parts of Scorpio, Ara, and the Cross; in others, as between Perseus and Auriga, it is not more than five degrees in width. Some parts of it are visible at all seasons of the year. In northern latitudes it is most conspicuous from July to November. It is most brilliant in the southern hemisphere.

Instead of a confused milky light, it is there more studded with brilliant stars.

§ 325. One would suppose that nearly eighty constellations were quite enough for all useful purposes. But in the eighteenth century twenty-six more were added to the number. This extravagant number of new constellations, some of which were formed of scarcely visible stars, by no means made the study of astronomy more easy, but on the contrary confused it, and rendered it difficult. Moreover the new constellations are unsuited to the others, and chosen without taste. Astronomical instruments have some claim to a place in the heavens; but figures like the Chemical Furnace, the Easel, the Air-pump, the Printing Press, and the Electrical machine have no natural relation to the sky.

It is desirable that the heavens should be freed from so tasteless and useless an accumulation. In doing this, uniformity must be secured for the maps. The same constellations must be retained in all, and the same stars should be placed in the same parts of the figures. The forms chosen should be beautiful and pleasing, their outlines should be definite, and when once adopted should remain unchanged. It would be well to avoid similarity of names in the constellations. We have now an Ursa Major and Minor, three Triangles, Pisces and Piscis, Telescopium repeated three times, &c.

§ 326. According to the present system, some constellations are so extensive that they exhaust three or more alphabets, and therefore it is necessary, beside the letter of the star, to give its right ascension and declination.

The largest stars of each constellation are named by the letters of the Greek alphabet, beginning with the brightest and proceeding in order. The stars next in brightness are numbered according to the Roman alphabet, and sometimes a third alphabet of Italian letters, or one numbered is required, as a2; or numbers alone are used.

The letters do not indicate the magnitude of the stars they represent, but merely the relative magnitude of those in the same constellation. Thus a Virginis is a star of the

first magnitude; a Libræ, a star of the second magnitude; and Aquarii, a star of the third magnitude.

Among the most conspicuous constellations in the northern hemisphere are the Lesser Bear, in the direction of which the north pole of our earth continually points, the Great Bear, which is more distant from the pole, Perseus, Cassiopeia, Lyra, Hercules, the Wagoner, Orion chasing the Pleiades and Hyades, while the Dog Star, though in the southern hemisphere, follows in the train. The southern constellations are more brilliant. The Southern Cross, the Argo, the Southern Triangle, the Centaur, and the Southern Crown, are among the most splendid.

A new system of arrangement and nomenclature has been proposed, in which the heavens should be covered by a net-work of imaginary circles crossing each other at regular intervals, so that each star could be referred to its exact place. Meanwhile much confusion must exist where 3,487 stars are to be formed into 94 figures whose outlines are imaginary and undefined, neither coincide with the position of the stars nor are definite in themselves, and seem made uncouth and perplexed purposely to baffle the ob

server.

The zodiacal constellations which we have described are 12 in number, and contain 1,016 stars. The northern constellations are 34 in number, and contain 1,444 stars. The southern constellations are 47 in number, and contain 1,027 stars. A large number of these are telescopic stars, but they are well known.

These constellations should be seen on a map or a concave celestial globe. Such a globe represents them as they really are, and a convex one reverses their appearance to us.

§ 326. In a celestial map the eastern part of the heavens is toward the left hand, the western part toward the right. If we stand on the earth with our face toward the north, we have the eastern part of the earth and of the heavens on our right hand. But if we face these heavens, and make a map of them, the eastern part must be depicted on the left hand. The eastern part of the

heavens is that you reach by travelling eastward, and when you face it it appears on the left hand.

In all drawings of the celestial motions in this book, the west is to the right hand, the east to the left hand. On maps of the earth, the east and west points are differently placed. In looking at the heavens we look at a concave sphere, in looking at the earth we have a convex sphere, and hence the maps which represent portions of each are differently made. For the concave sphere the west is on the right, and the east on the left hand. For the convex sphere the east is on the right, the west on the left hand.

We must remember that north and south are points, the imaginary ends of the pole of rotation, while east and west are directions. This difference exists both in the earth and the heavens. One definite part of the globe is called the northern point, but no point or part is called the eastern in this sense. If we travel north on the surface of the globe we come to this point, where we can no longer go north, but must turn and go south. But we can travel east till we return to our starting place, nay we might go round the globe again and again and yet always travel eastward. Thus in the heavens the earth moves always in an east direction. There is no part of the heavens which is called the eastern, none which is called the western part.

CHAPTER XIV.

LAWS OF SHAPE AND MOTION.

Attraction of Gravitation. Effect of Gravitation on the figures of the Sun and Planets. The Figure of the Earth that of Equilibrium. Illustration of the effect of Rotation on a Fluid Mass. Laws of Gravity. Centre of Gravity.

§ 328. In the preceding pages many individual facts have been stated. The form, relative masses, and orbits of the members of the solar system have been given. We would now study the laws which decide these forms, and govern these motions. Before doing this we must make ourselves familiar with a principle whose workings pervade all we yet know of the universe, a principle which influences the form and motions of all matter devoid of life,the principle of gravity. Of the nature of gravity we know nothing; we call it an attraction because in obedience to it bodies approach one another. But though we are ignorant of its nature and of its mode of action, we know with certainty the results to explain which we suppose its existence, and we know that they take place invariably.

Every particle of matter, as far as we know, attracts every other particle. If the particles are in a fluid or gaseous state, and are hindered by no other force, they rush together, and take the form which satisfies their mutual attraction. In this way we account for the form of the celestial bodies. If the bodies are solid, so that their particles cannot move easily, they are drawn to one another without losing their form. It is in this way that we explain the motions of the planets.

We will first consider what form would result from the gravitation of free particles toward each other, and then see how motion would modify this form. We will afterward inquire what motions gravity, acting in connection with a primitive impulse, would impress on these bodies.