The first column requires no explanation. We see, however, at once that the day in Mars is nearly equal to our own, while in the large planets, Jupiter and Saturn, it is not half so long. Now the revolutions of these planets round the Sun are accomplished as follow:

We can therefore easily find the number of days according to the period of rotation of each planet, which go to make each planetary year: thus in Saturn's year there are 24,584 Saturnian days, or 67 times more days than in our own.

254. In the second column the inclination of the planets' axes of rotation is given. It will be recollected that the inclination of our own is 231°, and that it is on this inclination that our seasons depend. It will be seen at once therefore that Mars and Saturn are much like the Earth in this respect, and that Jupiter is a planet almost without seasons, for the inclination of its axis is only 30, while that of Uranus is 100°. The axis of rotation. of Uranus, in fact, lies almost in the plane of its orbit.

255. As in the case of the Earth, we find in many instances the axis of rotation, or polar diameter, of the other planets shorter than the equatorial diameter. The amount of polar compression,—that is, the amount of flattening, by which the polar diameter is less than the equatorial one,-measured in fractions of the latter, is as follows:

From this Table we learn that if the equatorial diameter of Mercury be taken as 29, the polar one is only 28 in the cases of Jupiter and Saturn, the diameters are as 17 to 16 and 9 to 8, respectively. In these two last the rotation is very rapid (Art. 253); and this great flattening is what we should expect from the reasoning in Art. 196

256. We now come to what we can glean of the physical structure of the planets as seen in the telescope when they are nearest the Earth. Let us begin with Mars. We give in Plate IX. two sketches, taken in the year 1862. Here at once we see that we have something strangely like the Earth. The shaded portions represent water, the lighter ones land, and the bright spot at the top of the drawings is probably snow lying round the south pole of the planet, which was then visible.

257. The two drawings represent the planet as seen in an astronomical telescope, which inverts objects so that the south pole of the planet is shown at top. The upper drawing was made on the 25th of September, the lower one on the 23d. In the upper one a sea is seen on the left, stretching down northwards; while, joined on to it, as the Mediterranean is joined on to the Atlantic, is a long narrow sea, which widens at its termination.

In the lower drawing this narrow sea is represented on the left. The coast-line on the right strangely reminds one of the Scandinavian peninsula, and the included Baltic Sea.

258. It will be now easy to understand how we have been able to determine the length of the day and the inclination of the axis. We have only to watch how long

it takes one of the spots near the equator of each planet to pass from one side to the other, and the direction it takes, to get at both these facts.

259. Mars not only has land and water and snow like us, but it has clouds and mists, and these have been watched at different times. The land is generally reddish when the planet's atmosphere is clear; this is due to the absorption of the atmosphere, as is the colour of the setting Sun with us. The water appears of a greenish tinge.

260. Now, if we are right in supposing that the bright portion surrounding the pole be ice and snow, we ought to see it rapidly decrease in the planet's summer. This is actually found to be the case, and the rate at which the thaw takes place is one of the most interesting facts to be gathered from a close study of the planet. In 1862 this decrease was very visible. The summer solstice of Mars occurred on the 30th of August, and the snow-zone was observed to be smallest on the 11th of October, or fortytwo of our days after the highest position of the Sun. This very rapid melting may be ascribed to the inclination of the axis, which is greater than with us; to the greater eccentricity of the planet's orbit; and to the fact that the summer time of the southern hemisphere occurs when the planet is near perihelion.

261. For a reason that will be easily understood when we come to deal with the effect of the Earth's revolution round the Sun on the apparent positions and aspects of the planets, we sometimes see the north pole, and sometimes the south pole of Mars, and sometimes both: when either pole only is visible, the features, which appear to pass across the planet's disc in about twelve hours that is, half the period of the planet's rotation-describe curves with the concave side towards the visible pole. When both poles are visible they describe straight lines, exactly as in the case of the Sun (Art. 106). These changes enable all