enough to become visible to the naked eye). Now, the actual region of space traversed by the earth in four hours is a cylinder 260,000 miles long and having a cross section nearly 8,000 miles in diameter. Such a cylinder would have a volume of 12,500,000,000,000 cubic miles, and to each meteor of the 10,000,000 there would therefore correspond an average space of 1,250,000 cubic miles; that is, a space corresponding to a cube nearly 108 miles in length and breadth and height. Since such meteors as were seen on the night of November 27 have been estimated at less than an ounce in weight (in many cases only a few grains), it follows that their dimensions are inconsiderable. The largest can scarcely, when solid, be an inch in diameter. It will be conceived, therefore, how small must be the prospect of seeing a flight of bodies so exceedingly minute compared with the average space occupied by each. It is, indeed, easy to estimate the luminosity of such a flight regarded as a whole, if of given depth and at a given distance from the sun. Thus, suppose the flight of a million miles in depth, and at the earth's distance from the sun. Along a range of a million miles there would be less than 10,000 meteors. Now, granting each to have a disc one square inch in real area, we should have a total area of 10,000 square inches (that is, 8 feet square). And the ratio of this area to 108 miles square gives the ratio of the luminosity of the meteor-cloud (when of the given depth) to the luminosity of a surface illuminated by the sun at the earth's distance-(say, the moon for example). Now 108 miles square, or 11,664 square miles contain 11,664 ×

(1,760)2 × (36)2 square inches, or roughly (for great nicety would be useless in such a problem) 10,000 × (2,000)2 × (35)2 square inches; so that the ratio we require is 1 to (70,000)2 or 1 to 4,900,000,000. That is, the luminosity of the meteor-cloud would be one-4,900,000,000th only of the moon's, and necessarily the meteor-cloud would be quite undiscernible. Hence we may be assured that if the object seen by Pogson was connected at all with the meteor-cloud through which the earth passed on November 27, he saw a very much denser part of the meteor cloud; and there is no reason why this dense portion or nucleus of the meteor-cloud may not have been at a considerable distance from the earth on the 27th of last November. This consideration would serve to remove some of the more perplexing circumstances of the recent observations.

BO

JUPITER.

THE planet Jupiter has passed lately (this was written in 1873) through a singular process of change. The planet has not, indeed, assumed a new appearance, but has gradually resumed its normal aspect after four or five years, during which the mid-zone of Jupiter has been aglow with a peculiar ruddy light. The zone is now of a creamy-white colour, its ordinary hue. We have, in fact, reached the close of a period of disturbance, and have received a definite answer to questions which had arisen as to the reality of the change described by observers. Many astronomers of repute were disposed to believe that the peculiarities recently observed were merely due to the instruments with which the planet has been observed-not, indeed, to any fault in those instruments, but, in fact, to their good qualities in showing colour. A considerable number of the earlier accounts of Jupiter's change of aspect came from observers who used the comparatively modern form of telescope known as the silvered-glass reflectors, and it is well known that these instruments are particularly well suited for the study of colour-changes. Nevertheless, observations made with the ordinary refracting telescope were not wanting; and it had begun to be recognised that Jupiter really had

altered remarkably in appearance, even before that gradual process of change which, by restoring his usual aspect, enabled every telescopist to assure himself that there had been no illusion in the earlier observations.

I propose now to discuss certain considerations which appear to me to indicate the nature and probable meaning of the phenomena which have recently been observed in Jupiter. It seems to me that these phenomena are full of interest, whether considered in themselves or in connection with those circumstances on which I had been led to base the theory that Jupiter is a planet altogether unlike our earth in condition, and certainly unfit to be the abode of living creatures.

I would first direct special attention to the facts which have been ascertained respecting the atmosphere of Jupiter.

It does not appear to have been noticed, as a remarkable circumstance, that Jupiter should have an atmosphere recognisable from our distant station. Yet, in reality, this circumstance is not only most remarkable, but is positively inexplicable on any theory by which Jupiter is regarded as a world resembling our own. It is certain that, except by the effects produced when clouds form and dissipate, our terrestrial atmosphere could not be recognised at Jupiter's distance with any telescopic power yet applied. But no one who has studied Jupiter with adequate means can for a moment fail to recognise the fact that the signs of an atmosphere indicate much more than the mere formation and dissipation of clouds. I speak

opposition, with a very fine reflecting telescope by Browning, very generously placed at my disposal by Lord Lindsay; and I feel satisfied that no one can study Jupiter for many hours (on a single night) without becoming convinced that the cloud-masses seen on his disc have a depth comparable with their length and bread. Now the depth of terrestrial cloud-masses would at Jupiter's distance be an absolutely evanescent quantity. The span of his disc represents about 84,000 miles, and his satellites, which look little more than points in ordinary telescopes, are all more than 2,000 miles in diameter. I am satisfied that anyone who has carefully studied the behaviour of Jupiter's cloud-belts will find it difficult to believe that their depth is less than the twentieth part of the diameter of the least satellite. Conceive, however, what the depth of an atmosphere would be in which cloud-masses a hundred miles deep were floating!

It may be asked, however, in what sense such an atmosphere would be inexplicable, or, at least, irreconcilable with the theory that Jupiter is a world like our earth. Such an atmosphere would be in strict proportion, it might be urged, to the giant bulk of the planet, and such relative agreement seems more natural than would be a perfect correspondence between the depth of the atmosphere on Jupiter and the depth of our earth's atmosphere.

But it must not be forgotten that the atmosphere of Jupiter is attracted by the mass of the planet; and some rather remarkable consequences follow when we pay