direction, and so do the satellites, with one exception. This exception is found in the motion of the satellites of the planet Uranus, which move from cast to west.

139. Let us next inquire into the various distances of the planets from the Sun, bearing in mind, that as the orbits are elliptical, the planets are sometimes nearer to the Sun than at other times. This will be explained by and by; in the meantime we may say, that the average or mean distances are as follow; the times of revolution are also given :

140. Let us next see what are the sizes of the different Their diameters are as follow:

planets.

Venus

EARTH

Mars

Jupiter.

Saturn

Uranus

Neptune

Diameter in Miles.

2,962

7,510

7,901

4,000

141. We have before attempted to give an idea of the comparative sizes of the Earth and Sun, and of the distance between them; let us now complete the picture. Still

taking a globe some two feet in diameter to represent the Sun, Mercury would now be proportionately represented by a grain of mustard-seed, revolving in a circle 164 feet in diameter; Venus, a pea, in a circle of 284 feet in diameter; the Earth also a pea, at a distance of 215 feet; Mars, a rather large pin's head, in a circle of 654 feet; the smaller planets by grains of sand, in orbits of from 1,000 to 1,200 feet; Jupiter, a moderate sized orange, in a circle nearly half a mile across; Saturn, a small orange, in a circle of four-fifths of a mile; Uranus, a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half; and Neptune, a goodsized plum, in a circle about two miles and a half in diameter.*

142. As the planets revolve round the Sun at vastly different distances, so do the satellites revolve round their primaries. Our solitary Moon courses round the Earth at a distance of 240,000 miles, and its journey is performed in a month. The first satellite of the planet Saturn is only about one-third of this distance, and its journey is performed in less than a day. The first satellite of. Uranus is about equally near, and requires about two and a half days. The first satellite of Jupiter is about the same distance from that planet as our Moon is from us, and its revolution is accomplished in one and threequarters of our days. The only satellite which takes a longer time to revolve round its primary than our Moon, is Japetus, the eighth satellite of Saturn. We have seen above (Art. 140), that the diameter of the smallest planet -leaving the asteroids out of the question—is 2,962 miles. We find that among the satellites we have three bodiesthe third and fourth satellites of Jupiter, and the sixth unoon of Saturn-of greater dimensions than one of the

Sir John Herschel.

large planets, Mercury, and nearly as large as another, Mars.

It is not necessary in this place to give more details concerning the distances and sizes of the planets and satellites. A complete statement will be found in Tables II. and III. of the Appendix.

143. The relative distances of the planets from the Sun was known long before their absolute distances—in the same way as we might know that one place was twice or three times as far away as another without knowing the exact distance of either. When once the distance of the Earth from the Sun was known, astronomers could easily find the distance of all the rest from the Sun, and therefore from the Earth. Their sizes were next determined, for we need only to know the distance of a body and its apparent size, or the angle under which we see it, to determine its real dimensions.

144. In the case of a planet accompanied by satellites we can at once determine its weight, or mass, for a reason we shall state by and by (Chap. IX.); and when we have got its weight, having already obtained its size or volume, we can compare the density of the materials of which the planet is composed with those we are familiar with here; having first also obtained experimentally the density of our own Earth.

145. Let us see what this word density means. Το do this, let us compare platinum, the heaviest metal, with hydrogen, the lightest gas. The gas is, to speak roughly, a quarter of a million times lighter than the metal; the gas is therefore the same number of times less dense: and if we had two planets of exactly the same size, one composed of platinum and the other of hydrogen, the latter would be a quarter of a million times less dense than the former. Now, if it seems absurd to talk of a hydrogen planet, we must remember that if the materials

of which our system, including the Sun, is composed, once existed as a great nebulous mass extending far beyond the orbit of Neptune, as there is reason to believe, the mass must have been more than 200,000,000 times less dense than hydrogen!

146. Philosophers have found that the mean density of the Earth is a little more than five and a half times that of water, that is to say, our Earth is five and a half times heavier than it would be if it were made up of water. If we now compare the density of the other planets with it, we find that they almost regularly increase in density as we approach the Sun; Mercury being the most dense; Venus, the Earth, and Mars, having densities nearly alike, but less than that of Mercury; while Saturn and Uranus are the least dense.

147. Here is a Table showing the volumes, masses, and densities of the planets; those of the Earth being taken as 100:

148. To sum up, then, our first general survey of the Solar System, we find it composed of planets, satellites, comets, and several rings or masses of meteoric bodies; the planets, both large and small, revolving round the Sun in the same direction, the satellites revolving in a similar manner round the planets. We have learned the mean distances of the planets from the Sun, and we have

compared the distances and times of revolution of some of the satellites. We have also seen that the volumes, masses, and densities of the various planets have been determined. There is still much more to be learnt, both about the system generally, and the planets particularly; but it will be best, before we proceed with our general examination, to inquire somewhat minutely into the movements and structure of the Earth on which we dwell.

POLES.

LESSON XI. — THE EARTH. ITS SHAPE. EQUATOR. LATITUDE AND LONGITUDE. DIAMETER.

149. As we took the Sun as a specimen of the stars, because it was the nearest star to us, and we could therefore study it best, so now let us take our Earth, with which we should be familiar, as a specimen of the planets.

150. In the first place, we have learned that it is round. Had we no proof, we might have guessed this, because both Sun and Moon, and the planets observable in our telescopes, are round. But we have proof. The Moon, when eclipsed, enters the shadow thrown by the Earth; and it is easy to see on such occasions, when the edge of the shadow is thrown on the bright Moon, that the shadow is circular.

151. Moreover, if we watch the ships putting out to sea, we lose first the hull, then the lower sails, until at last the highest parts of the masts disappear. Similarly the sailor, when he sights land, first catches the tops of mountains, or other high objects, before he sees the beach or port. If the surface of the Earth were an extended plain, this would not happen; we should see the nearest things and the biggest things best : but as it is, every point