made up of the latter; granite, a common igneous rock, composed of quartz, feldspar, and mica, is nearly half made up of oxygen in a state of combination in those substances. 208. The chemical composition, by weight, of 100 parts of the atmosphere at present is as follows : Carbonic acid quantity variable with the locality. Aqueous vapour. quantity variable with the temperature and humidity. Ammonia a trace. We said at present, because, when the Earth was molten, the atmosphere must have been very different. We had, let us imagine, close to the still glowing crustcomposed perhaps of acid silicates- —a dense vapour, made up of compounds of the materials of the crust which were volatile only at a high temperature; the vapour of chloride of sodium, or common salt, would be in large quantity; above this, a zone of carbonic acid gas; above this again, a zone of aqueous vapour, in the form of steam; and lastly, the nitrogen and oxygen.* As the cooling went on, the lowest zone, composed of the vapour of salt, and other chlorides, would be condensed on the crust, covering it with a layer of these substances in a solid state. Then it would be the turn of the steam to condense too, and form water; it would fall on the layer of salt, which it would dissolve, and in time the oceans and seas would be formed, which would consequently be salt from the first moment of their appearance. Then, in addition to the oxygen and nitrogen which still remain, we should have the carbonic acid, which, in the * David Forbes, in the Geological Magazine, vol. iv. p. 429. course of long ages, was used up by its carbon going to form the luxuriant vegetation, the pressed remains of which is the coal which warms us, and does nearly all our work. 209. Now it is the presence of vapour in our lower atmosphere which renders life possible. When the surface of the Earth was hot enough to prevent the formation of the seas, as the water would be turned into steam again the instant it touched the surface, there could be no life. Again, if ever the surface of the Earth be cold enough to freeze all the water and all the gaseous vapour in the atmosphere, life—as we have it would be equally impossible. If this be true, all the Earth's history with which we are acquainted, from the dawn of life indicated in what geologists call the oldest rocks, down to our own time, and perhaps onwards for tens of thousands of years, is only the history of the Earth between the time at which its surface had got cold enough to allow steam to turn into water, and that at which its whole mass will be so cold that all the water on the surface, and all the vapour of water in the atmosphere, will be turned into ice. 210. The nebular hypothesis comes in here and shows us how, prior to the Earth being in a fluid state, it existed dissolved in a vast nebula, the parent of the Solar System; how this nebula gradually contracted and condensed, throwing off the planets one by one; and how the central portion of the nebula, condensed perhaps to the fluid state, exists at present as the glorious heat-giving Sun. Although, therefore, we know that stars give out light because they are white-hot bodies, and that planets are not self-luminous because they are comparatively cold bodies, we must not suppose that planets were always cold bodies, or that stars will always be white-hot bodies. Indeed, as we have shown, there is good reason for supposing that all the planets were once white-hot, and gave out light as the Sun does now. LESSON XVI.-The Moon: its Size, Orbit, and 211. The Moon, as we have already seen, is one of the satellites, or secondary bodies; and although it appears to us at night to be so infinitely larger than the fixed stars and planets, it is a little body of 2,160 miles in diameter; so small is it, that 49 moons would be required to make one earth, 1,300,000 earths being required, as we have seen, to make one sun! 212. Its apparent size, then, must be due to its nearness. This we find to be the case. The Moon revolves round the Earth in an elliptic orbit, as the Earth revolves round the Sun, at an average distance of only 238,793 miles, which is equal to about 10 times round our planet. As the Moon's orbit is elliptical, she is sometimes nearer to us than at others. Her greatest and least distances are 251,947, and 225,719 miles the difference is 26,228. When nearest us, of course she appears larger than at other times, and is said to be in perigee (πepí near, and y the Earth); when most distant, she is said to be in apogee (anó from, and yŋ). 213. The Moon travels round the Earth in a period of 27d. 7h. 43m. 11s. As we shall see presently, she requires more time to complete a revolution with respect to the Sun, which is called a lunar month, lunation, or synodic period. 214. The Moon, like the planets and the Sun, rotates on an axis; but there is this peculiarity in the case of the Moon, namely, that her rotation and her revolution round the Earth are performed in equal times, that is, in 27d. 7h. 43m. Hence we only see one side of our satellite. But as the Moon's axis is inclined 1° 32′ to the plane of its orbit, we sometimes see the region round one pole, and sometimes the region round the other. This There are also a is termed the libration in latitude. libration in longitude, arising from the fact, that though its rotation is uniform, its rate of motion round the Earth varies, so that we sometimes see more of the western edge and sometimes more of the eastern one; and a daily libration, due to the Earth's rotation carrying the observer to the right and left of a line joining the centres of the Earth and Moon. When on the right of this line, we see more of the right edge of the Moon; when on the left, we see more of the left edge. 215. The plane in which the Moon performs her journey round the Earth is inclined 5° to the plane of the ecliptic, or the plane in which the Earth performs her journey round the Sun (Art. 105). The two points in which the Moon's orbit, or the orbit of any other celestial body, intersects the Earth's orbit, are called the nodes. The line joining these two points is called the line of nodes. The node at which the body passes to the north of the ecliptic is called the ascending node, the other the descending node. 216. The motions of the Moon, as we shall see by and by, are very complicated. We may get an idea of its path round the Sun if we imagine a wheel going along a road to have a pencil fixed to one of its spokes, so as to leave a trace on a wall: such a trace would consist of a series of curves with their concave sides downwards, and such is the Moon's path with regard to the Sun. 66 217. Besides the bright portion lit up by the Sun, we sometimes see, in the phases which immediately precede and follow the New Moon, that the obscure part is faintly visible. This appearance is called the Earth shine" (Lumen incinerosum, Lat.; Lumière cendrée, Fr.), and is due to that portion of the Moon reflecting to us the light it receives from the Earth. When this faint light is visible-when the "Old Moon" is seen in the "New Moon's arms"-the portion lit up by the Sun seems to belong to a larger moon than the other. This is an effect H Be of what is called irradiation, and is explained by the fact that a bright object makes a stronger impression on the eye than a dim one, and appears larger the brighter it is. 218. The average of four estimations gives the Moon's light as 547513 of that of the Sun, so we should want 547,513 full moons to give as much light as the Sun does ; and as there would not be room to place such a large number in the one-half of the sky which is visible to us, since the Moon covers 240000 of it, it follows that the light from a sky full of moons would not be so bright as sunshine. 219. At rising or setting, the Moon sometimes appears to be larger than it does when high up in the sky. This is a delusion, and the reverse of the fact; for, as the Earth is a sphere, we are really nearer the Moon by half the Earth's diameter when the part of the Earth on which we stand is underneath it; as at moonrise or moonset we are situated, as it were, on the side of the Earth, half-way between the two points nearest to and most distant from the Moon. Let the reader draw a diagram, and reason this out. 220. Now a powerful telescope will magnify an object 1,000 times; that is to say, it will enable us to see it as if it were a thousand times nearer than it is: if the Moon were 1,000 times nearer, it would be 240 miles off, consequently astronomers can see the Moon as if it were situated at a little less than that distance, since it is measured from centre to centre, and we look from surface to surface. In consequence of this comparative nearness, the whole of the surface of our satellite turned towards us has been studied and mapped with considerable accuracy. 221. With the naked eye we see that some parts of the Moon are much brighter than others; there are dark patches, which, before large telescopes were in use, were thought to be, and were named, Oceans, Gulfs, and so on. The telescope shows us that these dark markings are smooth plains, and that the bright ones are ranges of mountains and hill country broken up in the most tremen |