yond 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 Neptune are the least dense. 147. Here is a Table showing the volumes, masses, and densities, of the planets; those of the Earth being taken 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. LESSON XI. The Earth. Its Shape. Poles. 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 of the Earth's surface is the top, as it were, of a flattened dome; such a dome therefore is interposed between us and every distant object. The inequalities of the land render this fact much less obvious on terra firma than on the surface of the sea. 152. On all sides of us we see a circle of land, or sea, or both, on which the sky seems to rest: this is called the sensible horizon. If we observe it from a little boat on the sea, or from a plain, this circle is small; but if we look out from the top of a ship's mast or from a hill, we find it largely increased-in fact, the higher we go the more is the horizon extended, always however retaining its circular form. Now, the sphere is the only figure which, looked at from any external point, is bounded by a circle; and as the horizons of all places are circular, the Earth is a sphere, or at all events nearly so. 153. The Earth is not only round, but it rotates, or turns round on an axis, as a top does when it is spinning; and the names of north pole and south pole are given to those points on the Earth where the axis would come to the surface if it were a great iron rod instead of a mathematical line. Half-way between these two poles, there is an imaginary line running round the Earth, called the equator or equinoctial line. The line through the Earth's centre from pole to pole is called the polar diameter; the line through the Earth's centre, from any point in the equator to the opposite point, is called the equatorial diameter, and one of these, as we shall see, is longer than the other. 154. We owe to the ingenuity of a French philosopher, M. Léon Foucault, two experiments which render the Earth's rotation visible to the eye. For although, as we shall presently see, it is made evident by the apparent motion of the heavenly bodies and the consequent succession of day and night, we must not forget that these effects might be, and for long ages were thought to be, produced by a real motion of the Sun and stars round the Earth. The first method consists in allowing a heavy weight, suspended by a fine wire, to swing backwards and forwards like the pendulum of a clock. Now, if we move the beam or other object to which such a pendulum is suspended, we shall not alter the direction in which the pendulum swings, as it is more easy for the thread or wire, which supports the weight, to twist than for the heavy weight itself to alter its course or swing when once in motion in any particular direction. Therefore, in the experiment, if the Earth were at rest, the swing of the pendulum would always be in the same direction with regard to the support and the surrounding objects, but would vary if the earth were in motion. 155. M. Foucault's pendulum was suspended from the dome of the Panthéon in Paris, and a fine point at the bottom of the weight was made to leave a mark in sand at each swing. The marks successively made in the sand showed that the plane of oscillation varied with regard to the building. Here, then, was a proof that the building, and therefore the Earth, moved. 156. Such a pendulum swinging at either pole would make a complete revolution in 24 hours, and would serve the purpose of a clock were a dial placed below it with the hours marked. As the Earth rotates at the north pole from west to east, the dial would appear to a spectator, carried like it round by the Earth, to move under the pendulum from west to east, while at the south pole the Earth and dial would travel from east to west; midway between the poles, that is, at the equator, this effect, of course, is not noticed, as there the two motions in opposite directions meet. 157. The second method is based upon the fact, that when a body turns on a perfectly true and symmetrical axis, and is left to itself in such a manner that gravity is not brought into play, the axis maintains an invariable position; so that if it be made to point to a star, which is a thing outside the Earth and not supposed to move, it will continue to point to it. A gyroscope is an instrument so made that a heavy wheel set into very rapid motion shall be able to rotate for a long period, and that all disturbing influences, the action of gravity among them, are prevented. 158. Now, if the Earth were at rest, there would be no apparent change in the position of the axis, however long the wheel might continue to turn; but if the Earth moves and the axis remains at rest, there should be some difference. Experiment proves that there is a difference, and just such a difference as is accounted for by the Earth's rotation. In fact, if we so arrange the gyroscope that the axis of its rotation points to a star, it will remain at rest with regard to the star, while it varies with regard to the Earth. This is proof positive that it is the Earth which rotates on its axis, and not the stars which revolve round it; for if this were the case the axis of the gyroscope would remain invariable with regard to the Earth, and change its direction with regard to the star. 159. If we look at a terrestrial globe, we find that the equator is not the only line marked upon it. There are other lines parallel to the equator,—that is, lines which are at the same distance from the equator all round,—and other lines passing through both poles, and dividing the equator into so many equal parts. These lines are for the purpose of determining the exact position of a place upon the globe, and they are based upon the fact, that all circles are divided into 360 degrees (marked ©), each degree into 60 minutes ('), and each minute into 60 seconds ("). 160. We have first the equator midway between the poles, so that from any part of the equator to either pole is one quarter round the Earth, or 90 degrees. On either side of the equator there are circles parallel to it; that is to say, at the same distance from it all round, dividing the distance to the poles into equal parts. Now, it is necessary to give this distance from the equator some name. The term latitude has been chosen: north latitude from the equator towards the north pole; south latitude from the equator towards the south pole. 161. This, however, is not sufficient to define the exact position of a place, it only defines the distance from the equator. This difficulty has been got over by fixing upon Greenwich, our principal astronomical observatory, and supposing a circle passing through the two poles and that place, and then reckoning east and west from the circle |