an entirely different stage of planetary existence.
It is singular how slowly astronomers allowed other theories which they had learned to regard as practically demonstrated to exercise their just effect on our views respecting the planets. We do not refer here to official astronomers, who have, for the most part, limited their attention to questions of astronomical time and position, regarding the planets and the moon as convenient sky-marks and time-measurers, without caring much what their true nature may be. But even astronomers more correctly so called, those who look beyond the mere motion of the celestial bodies, and inquire into their significance, have not been careful to inquire whether, when they came to admit the general uniformity of planetary development, they were not bound to admit also that some planets must be much more fully developed than others, — that some planets must as yet be scarce formed, some must be young, some in mid-life, others passing on towards old age, and yet others decrepit, if not dead. This was an inevitable consequence of the doctrine astronomers had admitted, yet it was not mentioned till nearly half a century later, and when first mentioned was regarded as too wild to be entertained. The arguments in its favor were, however, quietly urged; and it was shown that this theory alone possessed the quality, which every true theory must possess, of according well with all the known facts. This theory made the larger planets of necessity the younger planets — not, of course, in years, but in development. According to it, the giant planets Jupiter and Saturn would as yet scarce have passed through the process of planetary growth; at the utmost, would be still in a condition of intense heat. Enwrapped in enormous atmospheric envelopes loaded with deep cloud-masses, they would not show their true surface, and the astronomer measuring from the outer cloud-layers would attribute to these planets a far greater bulk, and therefore a far smaller density, than they really possess. Expanded by heat, the deep atmosphere would not be compressed by the planets' mighty attraction, as it would be but for that heat. The movements of the atmosphere and of the vast cloud-masses thrown into it from the heated surface below, would be much more active than the movements of air and clouds under the weak rays of the sun which shines in the heaven either of Jupiter or Saturn. In all these respects, and in many others, the aspect and changes of aspect of the two planets correspond well with the theory that these orbs are still passing through the stage of planetary youth, and very ill with the theory which had been in vogue before.
Other circumstances had appeared not merely difficult to explain, but actually inexplicable according to the older theory, and were certainly not less so according to Whewell's ice-and-water theory. It had appeared from many observations that Saturn changes from time to time in shape. His shadow on his ring-system had presented equally inexplicable peculiarities, being at times not at all like the shadow of a solid globe. Jupiter had not been so notably changed at any time, no doubt because of the comparative uniformity of the conditions under which the prince of planets travels. Yet observers had occasionally suspected even Jupiter of changing in shape, and on one occasion, when a satellite happened to be crossing his outline, they had caught him in the act. For the outline, extending at first beyond the satellite (in appearance, be it understood) had shrunk so as to leave the satellite outside. We describe, indeed, rather what must have happened than what was observed, which was simply this, — a satellite, whose transit or passage across the face had just begun, was seen five minutes or so later outside the planet, so that moving onwards it began transit a second time. But as no one believes that Jupiter's moon had stopped in its course, retreated, and then advanced again, it is to all intents and purposes certain that the outline of the planet had changed. One may compare what had happened to the case of a bird passing at a great distance between the eye and a still more remote balloon. The bird's flight would at last cause it to be apparently projected upon the dark body of the balloon. Now, if a few moments later, the bird was again seen outside the balloon's disc, not having passed athwart, but reappearing where it had disappeared, we should be certain that either the bird had stopped, and even gone back a little on its former course, or else that the balloon had shrunk just at that part where the outline had been crossed by the bird. If we knew for certain that the bird had not stopped, we should be sure that the balloon had shrunk. Astronomers might have been equally sure that on the occasion referred to the outline of Jupiter had changed. Most of them preferred, however, to wait for more observations, many students of science considering that due respect for observations requires us
