begun to shorten again, so that it would appear to float upon the surface of currents of variable speed rather than to be an established landmark of the globe itself. The rotation period derived from it was, in 1902, 9 hours 55 minutes 39·3 seconds. The mention of the changing period of rotation of the red spot lends emphasis to the fact that no single period of rotation can be assigned to Jupiter as a whole. It is impossible to say of the great planet that he rotates in such and such a period: the utmost that can be said is that certain spots upon his surface have certain rotation periods; but these periods are almost all different from one another, and even the period of an individual marking is subject, as already seen, to variation. In fact, as Mr. Stanley Williams has shown, no fewer than nine different periods of rotation are found to coexist upon the surface; and though the differences in the periods seem small when expressed in time, amounting in the extreme cases only to eight and half minutes, yet their significance is very great indeed. In the case of Mr. Williams's Zones II. and III., the difference in speed of these two surface currents amounts to 400 miles per hour. Certain bright spots near the equator have been found to move so much more rapidly than the great red spot as to pass it at a speed of 260 miles an hour, and to 'lap' it in forty-four and a half days, completing in that time one whole rotation more than their more imposing neighbour. It cannot, therefore, be said that Jupiter's rotation period is known; but the average period of his surface markings is somewhere about nine hours fifty-two minutes. Thus the rotation period adds its evidence to that already afforded by the variations in colour and in form of the planet's markings that here we are dealing with a body in a very different condition from that of any of the other members of our system hitherto met with. We have here no globe whose actual surface we can scrutinize, as we can in the case of Mars and the moon, but one whose solid nucleus, if it has such a thing, is perpetually veiled from us by a mantle which seems more akin to the photosphere of the sun than to anything else that we are acquainted with. The obvious resemblances may, and very probably do, mask quite as important differences. The mere difference in scale between the two bodies concerned must be a very important factor, to say nothing of other causes which may be operative in producing unlikeness. Still, there is a considerable and suggestive general resemblance.