with powerful telescopes; all around it there is a shallow depression, occupying a region about as large as the whole crater had been before. It seems impossible to doubt that a great change has taken place here, and the question arises whether the change has been produced by volcanic activity or otherwise. Sir John Herschel pronounced somewhat confidently in favor of the former hypothesis. "The most plausible conjecture," said he, "as to the cause of this disappearance, seems to be the filling up of the crater from beneath, by an effusion of viscous lava, which, overflowing the rim on all sides, may have so flowed down the outer slope as to efface its ruggedness, and convert it into a gradual declivity casting no stray shadows." "But how tremendous the volcanic energy," we note in the passage referred to, "required to fill with lava a crater nearly seven miles in diameter, and more than half a mile deep! The volcanic hypothesis seems on this account utterly incredible, for if such energy resided in the moon's interior we should find her whole surface continually changing. Far more probable seems the idea that the wall of this crater has simply fallen in, scattering its fragments over what had once been the floor of the crater. The forces at work on the moon are quite competent to throw down steep crater-walls like those which seem formerly to have girt about this deep cavity."[1]
That the kind of vitality evidenced by such changes as these still exists in the moon's frame, is not merely probable but certain. Other changes, however, which were once supposed to have been observed, must be dismissed as having had no real existence. The effects of various kinds of illusion have to be taken into account in considering such phenomena. Thus the theory that a process of monthly change, due perhaps to vegetation, affects the floor of the large lunar crater Plato (called by Hevelius the greater Black Lake), is now rejected, because the supposed change has been shown to be a death. mere effect of contrast. The apparent change is of this nature: — As the sun first begins to rise above the floor of the crater — or, in other words, as the light of the filling moon gradually flows over the crater — the floor appears bright, getting brighter and brighter as the sun rises higher and higher, up to a certain point. But afterwards the floor darkens, becoming darkest towards lunar midday. Lastly, as the lunar afternoon progresses, the floor of Plato gets gradually lighter again. The midday darkening was attributed to some process of vegetation or else to chemical changes. It has no real existence, however, but is due simply to the effect of contrast with the great brightness of the crater-wall all around, which is formed of some very white substance, and looks peculiarly bright and lustrous at the time of lunar midday, so that contrasted with it the floor looks peculiarly dark. On the other hand, during the morning and evening hours, the black shadow of the crater wall is thrown across the floor, which by contrast looks brighter than it really is. This explanation has indeed been denied very confidently by some who formerly advocated the theory that lunar vegetation causes the darkening of the floor but there can be no doubt of its justice, for no one (not prejudiced in favor of a theory) who has tested the matter experimentally, eliminating the effects of contrast, has failed to find that there is no real darkening of the floor of Plato.
It seems as certain as any matter not admitting of actual demonstration can be that the moon is, to all intents and purposes, dead. Her frame is indeed still undergoing processes of material change, but these afford no more evidence of real planetary life than the changes affecting a dead body are signs of still lingering vitality. Again, it seems certain that the processes through which the moon has passed in her progress towards planetary death, must be passed through in turn by all the members of the solar system, and finally by the sun himself. Every one of these orbs is constantly radiating its heat into space, not indeed to be actually lost, but still in such sort as to reduce all to the same dead level of temperature, whereas vitality depends on differences of temperature. Every orb in space, then, is tending steadily onwards towards cosmical death. And, so far as our power of understanding or even of conceiving the universe is concerned, it seems as though this tendency of every individual body in the universe towards death involved the tendency towards death of the universe itself. It may indeed be said that since the universe is of necessity infinite, whereas we are finite, we cannot reason in this way from what we can understand, or conceive, to conclusions respecting the universe, which we cannot even conceive, far less understand. Still it must be admitted,
- ↑ The present writer, in the Spectator for June 24, 1876.
