nately close to its inner side and to its outer` side, at points dividing the ring nearly into twenty-five equal parts.

We see, then, that so far as the moon's path is concerned, she may be regarded as a planet. Nor is she markedly inferior in bulk to some of the other planets forming the sun's inner family, consisting, as we know, of Mercury, Venus, the Earth and Moon, and Mars. She is certainly the smallest of this family, but compared with Mercury, she is not so small by far as Mercury is compared with the earth, and she is not much smaller compared with Mars than Mars is compared with the earth. This will be easily seen from the following numbers, which represent the volumes of the five planets which circle nearest to the sun, arranged in order of magnitude :—

The

And it must be remembered also that the absolute dimensions of the moon are by no means insignificant moon's diameter is about 2,160 miles in length; she has a surface of 14,600,000 miles, and a solid content of about 10,000 millions of cubic miles. If we consider her surface as the feature by which she is most readily brought into comparison with our earth, then it is to be noted

that the earth's surface only exceeds the moon's about 13 times, and the moon's surface is fully as large as Africa and Australia together, or nearly as large as North and South America without their islands.

In mass or quantity of matter the moon is somewhat more markedly inferior to the other four planets, as the following list of numbers, showing the relative mass of the five planets, sufficiently indicates:

But still, it will be seen, the earth exceeds Mercury, in mass as in volume, to a greater degree than Mercury exceeds the moon; and Mars holds much the same relative position between the earth and the moon in this list as in the list of numbers representing the volume of the five planets. Moreover, the quantity of matter in the moon cannot be looked upon as absolutely insignificant, when we consider that the average density of the moon is more than three times as great as the density of water, and that she contains, as above mentioned, about 10,000 millions of cubic miles of matter. If the moon could be weighed against a quantity of water-say in some vast balance placed on the surface of the sun or of some other very large and massive orb-it would be found that about

34,500 cubic miles of water would be required to counterbalance the moon's weight.

There is one circumstance, however, in which the moon shows a sort of dependence upon the earth, producing a very striking distinction between her and the other planets. She turns round on her axis in such a way as always to turn the same or nearly the same face towards the earth. As she turns uniformly, but does not travel at a quite uniform rate, and as she also turns on a slightly inclined axis, she sometimes shows a little more of her eastern and western sides, or again of her northern and southern sides, than at other times; but her average rate of turning is absolutely identical with her average period of motion round the earth, and accordingly she never sways more than a certain portion of her surface into view or out of view by these libratory or balancing motions.

Now this rate of rotation is exceedingly slow. For we know that the moon takes about a month in circling round the earth; and therefore she takes about a month in turning upon her axis. In other words, the moon's day lasts about four of our weeks; and if we suppose it divided as we divide our day, into twenty-four equal parts, then each of these parts lasts more than one of our days— in fact, a lunar hour lasts nearly 29 of our hours. Daytime lasts on the average rather more than a fortnight of terrestrial time; and night lasts as long. Here, then, there is a very singular contrast between the state of matters on the moon and on our earth.

The contrast is rendered even more striking by the circumstance that the lunar year is shorter than our year. This will seem strange at first sight, because I have said that the moon travels round the sun on a path almost identical with the earth's, and of course in the same time. Now, we all know that our year is the time occupied by the earth in going once round the sun and it might seem that the lunar year must necessarily be the period occupied by the moon in going once round the sun, this period being our common year. But a peculiarity which very slightly affects our own year of seasons, making it in reality more than twenty minutes shorter than the year of circling round the sun, affects the moon's year in a much greater degree, insomuch that while the lunar year of circling round the sun lasts, like ours, 365 days, six hours, and nine minutes, the lunar year of seasons lasts only 346 days, fourteen hours, and thirty-four minutes.

It follows that there are not quite twelve lunar days in a lunar year. Each of the four seasons lasts rather less than three days.

But the seasons are also very slightly distinguished one from the other. Lunar winter differs from lunar summer no more than on our earth the 16th of March

The peculiarity is in fact a swaying of the earth's axis like the slow reeling of a mighty top, each reel occupying about 25,866 years. The cor responding motion of the moon's axis is completed in about 18 years and 7 months.

differs from the 26th, or than the 27th of September differs from the 19th.

If the contrast between winter and summer is slight, however, the contrast between day and night is very remarkable. In order clearly to understand this, we must not only consider the great length of the lunar day, but the condition of the moon as respects those circumstances which on our own earth temper the mid-day heat and the cold of midnight. In the telescope the moon appears to be a perfectly waterless globe, her arid surface being covered with ring-shaped mountains, mountain-ranges, peaks, fissures, and rocks, except in certain regions, called seas, where the surface (really solid) is apparently quite smooth. It was formerly supposed that these smooth regions, which are rather darker than the rest of her surface, are seas; and, by a singular perversity, astronomers who have been but too ready to introduce new names among the constellations have continued to call these regions 'seas,' long after it has been demonstrated that they are land-surfaces. It is certain, then (at least as respects the side of the moon. turned earthwards), that none of those beneficial effects which result on earth from the presence of extensive water-covered regions can be produced on the moon. No clouds can temper the heat of the lunar day, or at night prevent the too rapid escape of the heat which had been garnered up, so to speak, in the daytime; nor can any of those more subtle processes take place which result from the presence in our air of the unseen vapour of water.