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SCIENCE. 



[Vol. IV., No. 83. 



astronomer on a hundred different roads ; and, until 

 it is overcome, progress in these directions must be 

 slow and painful. One could not truly say, perhaps, 

 that the lunar theory must, in the mean while, remain 

 quite at a standstill : labor expended in the old ways, 

 upon the extension and development of existing 

 methods, may not be fruitless, and may, perhaps, 

 after a while, effect the reconcilement of prediction 

 and observation far beyond the present limits of ac- 

 curacy. But if we only had the mathematical powers 

 we long for, then progress would be as by wings: we 

 should fly, where now we crawl. 



As to the physical problems presented by the moon, 

 the questions relating to the light and heat — the 

 radiant energy — it sends us, and to its temperature, 

 seem to be the most attractive at present, especially 

 for the reason that the results of the most recent in- 

 vestigators seem partially to contradict those obtained 

 by their predecessors some years ago. It now looks 

 as if we should have to admit that nearly all we re- 

 ceive from the moon is simply reflected sun-light and 

 sun-heat, and that the temperature of the lunar sur- 

 face nowhere rises as high as the freezing-point of 

 water, or even of mercury. At the same time, some 

 astronomers of reputation are not disposed to admit 

 such an upsetting of long-received ideas; and it is 

 quite certain, that, in the course of the next few years, 

 the subject will be carefully and variously investigated. 



Closely connected with this is the problem of a 

 lunar atmosphere — if, indeed, she has any. 



Then there is the very interesting discussion con- 

 cerning changes upon the moon's surface. Consider- 

 ing the difference between our modern telescopes and 

 those employed fifty or a hundred years ago, I think 

 it still far from certain that the differences between 

 the representations of earlier and later observers 

 necessarily imply any real alterations. But they, no 

 doubt, render it considerably probable that such alter- 

 ations have occurred, and are still in progress; and 

 they justify a persistent, careful, minute, and thor- 

 ough study of the details of the lunar surface with 

 powerful instruments: especially do they inculcate 

 the value of large-scale photographs, which can be 

 preserved for future comparison as unimpeachable 

 witnesses. 



I will not leave the moon without a word in respect 

 to the remarkable speculations of Professor George 

 Darwin concerning the tidal evolution of our satellite. 

 Without necessarily admitting all the numerical re- 

 sults as to her age and her past and future history, 

 one may certainly say that he has given a most plau- 

 sible and satisfactory explanation of the manner in 

 which the present state of things might have come 

 about through the operation of causes known and 

 recognized, has opened a new field of research, and 

 shown the way to new dominions. The introduction 

 of the doctrine of the conservation of energy, as a 

 means of establishing the conditions of motion and 

 configuration in an astronomical system, is a very 

 important step. 



In the planetary system we meet, in the main, the 

 same problems as those that relate to the moon, with 

 a few cases of special interest. 



For the most part, the accordance between theory 

 and observation in the motions of the larger planets 

 is as close as could be expected. The labors of Le- 

 verrier, Hill, Newcomb, and others, have so nearly 

 cleared the field, that it seems likely that several 

 decades will be needed to develop discrepancies suf- 

 ficient to furnish any important corrections to our 

 present tables. Leverrier himself, however, indi- 

 cated one striking and significant exception to the 

 general tractableness of the planets. Mercury, the 

 nearest to the sun, and the one, therefore, which 

 ought to be the best behaved of all, is rebellious to a 

 certain extent: the perihelion of its orbit moves 

 around the sun more rapidly than can be explained 

 by the action of the other known planets. The evi- 

 dence to this effect has been continually accumu- 

 lating ever since Leverrier first announced the fact, 

 some thirty years ago; and the recent investigation 

 by Professor Newcomb, of the whole series of ob- 

 served transits, puts the thing beyond question. Le- 

 verrier' s own belief (in which he died) was, that the 

 effect is due to an unknown planet or planets be- 

 tween Mercury and the sun; but, as things now 

 stand, we think that any candid investigator must 

 admit that the probability of the existence of any 

 such body or bodies of considerable dimensions is 

 vanishingly small. We do not forget the numerous 

 instances of round spots seen on the solar disk, nor 

 the eclipse-stars of Watson, Swift, Trouvelot, and 

 others ; but the demonstrated possibility of error or 

 mistake in all these cases, and the tremendous array 

 of negative evidence from the most trustworthy ob- 

 servers, with the best equipment and opportunity, 

 makes it little short of certain that there is no Vul- 

 can in the planetary system. 



A ring of meteoric matter between the planet and 

 the sun might account for the motion of the peri- 

 helion; but, as Newcomb has suggested, such a ring 

 would also disturb the nodes of Mercury's orbit. 



It has been surmised that the cause may be some- 

 thing in the distribution of matter within the solar 

 globe, or some variation in gravitation from the ex- 

 act law of the inverse square, or some supplementary 

 electric or magnetic action of the sun, or some 

 special effect of the solar radiation, sensible on ac- 

 count of the planet's proximity, or something pecul- 

 iar to the region in which the planet moves ; but as 

 yet no satisfactory explanation has been established. 



Speaking of unknown planets, we are rather reluc- 

 tantly obliged to admit that it is a part of our scien- 

 tific duty as astronomers to continue to search for the 

 remaining asteroids ; at least, I suppose so, although 

 the family has already become embarrassingly large. 

 Still I think we are likely to learn as much about 

 the constitution, genesis, and history of the solar 

 system from these little flying rocks as from their 

 larger relatives ; and the theory of perturbations will 

 be forced to rapid growth in dealing with the effects 

 of Jupiter and Saturn upon their motions. 



Nor is it unlikely that some day the searcher for 

 these insignificant little vagabonds may be rewarded 

 by the discovery of some great world, as yet unknown, 

 slow moving in the outer desolation beyond the re- 



