504 



NA TURE 



[Sept. i8, 1884 



planet's proximity, or something peculiar to the region in which 

 the planet moves ; but as yet no satisfactory explanation has been 

 established. 



Speaking of unknown planets, we are rather reluctantly obliged 

 to admit that it is a part of our scientific duty as astronomers to 

 continue to search for the remaining asteroids ; at least, I suppose 

 so, although the family his already become embarrassingly large. 

 Still I think we are likely to learn as much ab jut the con-titution, 

 genesis, and history of the solar system from these little Hying 

 rocks as from their larger relatives ; and the theory of perturba- 

 tions will be forced to rapid growth in dealing with the effects of 

 Jupiter and Saturn upon their motions. 



Nor i> it unlikely that some clay the searcher for these in- 

 significant little vagabonds may be rewarded by the discovery of 

 some great world, as yet unknown, slaw moving in the outer 

 desolati in beyond the remotest of the present planetary family. 

 Some configu ations in certain cometary orbits, and some almost 

 evaie cent peculiarities in Neptune's motions, have been thought 

 to paint to the exi-tence of such a world; and there is n> 

 evidence, nor even a presumption, against it. 



Mercury as yet defies all our attempts to ascertain the length 

 of its day, and the character and condition of its surface. 

 Apparently the instruments and methods now at command are 

 insufficient to ope with the difficulties of the problem ; and it is 

 not easy to say how it can be successfully attacked. 



With Venus, the earth's twin sister, the state of things is a 

 little better : we do already know, with some degree of approxi- 

 mation, her period of rotation ; and the observations of the last 

 few months bid fair, if followed up, to determine the position of 

 her p pies, and p >ssibly to give us some knowledge of her moun- 

 tains, continents, and seas. 



It would be rash to say of Mais that we have reached the limit 

 of possible knowledge as regards a planet's surface ; but the 

 main facts are now determined, and we have a rather surprising 

 amount of supposed knowledge regarding its geography. By 

 "supposed " I mean merely to insinuate a modest doubt whether 

 some of the map-makers have not gone into a little mare elaborate 

 detail than the circumstances warrant. At any rate, while the 

 " areographies " agree very well with each other in respect to 

 the planet's more important features, they differ widely and 

 irreconcilably in min ir points. 



As regards the phy-ical features of the asteroids, we at present 

 know practically nothing : the field is absolutely open, w hether 

 it is worth anything may be a question ; and yet, if one 

 reach it, I a ,1 persuaded that a knowledge of the substance, 

 form, density, rotation, temperature, and other physical cha- 

 racteristics, of one of these little orphans would throw vivid light 

 on the nature and behaviour of inter-planetary space, and \i ould 

 be of great use in establishing the physical theory of the solar 

 system. 



The planet Jupiter, lordliest of them all, still, as fiom the first, 

 presents problems of the highest importance and interest. A 

 sort of connecting-link between suns and planets, it seems as if, 

 perhaps, we might find, in the beautiful and varied phenomena 

 he exhibits, a kind of halfway house between familiar terrestrial 

 facts and solar mysteries. It seems quite certain that no analogies 

 drawn from the earth and the earth's atmosphere alone will 

 explain the strange things seen upon his disk, some of which, 

 especially the anomalous differences observed between the rota- 

 tion periods derived from the observation of markings in different 

 latitudes, are very similar to what we find upon the sun. "The 

 great red spot" which has just disappeared, after challenging for 

 several years our best endeavours to understand and explain it, 

 still, I think, remains as much a mystery as ever, — a mystery 

 probably hiding within itself the master-key to the constitution 

 of the great orb of whose inmost nature it was an outward and 

 most characteristic expression. The same characteristics are also 

 probably manifested in other less conspicuous but equally curious 

 and interesting markings on the varied and ever-changing coun- 

 tenance of this planet ; so that, like the moon, it will well repay 

 the most minute and assiduous study. 



Its satellite system also deserves careful observation, especially 

 in respect to the eclipses which occur ; since we find in them a 

 measure of the time required for light to cross the orbit of the 

 earth, and so of the solar parallax, and also because, as has been 

 already mentioned, they furnish a test of the constancy of the 

 earth's rotation. The photometric method of observing these 

 eclipses, first instituted by Prof. Pickering at Cambridge in 

 1878, and since re-invented by Cornu in Paris, has already much 

 increased the precision of the results. 



With reference to the mathematical theory of the motion of 



these satellites, the same remarks apply as to the planetary 

 theory. As yet nothing appears in the problem to be beyond the 

 power and scope of existing methods, when carried out with 

 the necessary care and prolixity ; but a new and more compendious 

 method is most desirable. 



The problems of Saturn are much the ame as those of Jupiter, 

 excepting that the surface and atmospheric phenomena are less 

 striking, and more difficult of observation. But we have, in 

 addi'ion, the wonderful rings, unique in the heavens, the loveliest 

 of all telescopic objects, the type and pattern, I suppose, of 

 world-making, in actual progress before our eye-. There seems 

 to be continually accumulating evidence from the observations of 

 Struve, Dawes, Henry, and others, that these whirling clouds 

 are changing in their dimensions and in the density of their dif- 

 ferent parts ; and it is certainly the duty of every one who has 

 a good telescope, a sharp eye, and a chastened imagination, to 

 watch them carefully, and set down exactly what he sees. It 

 may well be that even a few decades will develop most important 

 and instructive phen imena in this gauzy girdle of old Chronos. 

 Great care, however, is needed in order not to mistake fancies 

 and illusions for solid facts. Not a few anomalous appearances 

 have been described and commented on, which failed to be 

 recognised by more cautious observers with less vivid imaginations, 

 more trustworthy eyes, and better telescopes. 



The outer planets, Uranus and Neptune, until recently, have 

 defied all attempts to study their surface and physical character- 

 istics. Their own motions and those of their satellites, have 

 been well worked out ; but it remains to discus; their rotation, 

 topography, and atmospheric peculiarities. So remote are they, 

 and so faintly illuminated, that the task seems almost hopeless : 

 and yet, within the last year or two, some of our great telescopes 

 have revealed faint and evanescent markings upon Uranus, which 

 may in time lead to some further knowledge of that far-off 

 relative. It may, perhaps, be that some great telescope of the 

 future will give us some such views of Neptune as we now get 

 of Jupiter. 



There is a special reason for attempts to determine the rotation 

 periods of the planets, in the fact that there is very possibly 

 some connection between these periods on the one hand, and. 

 on the other, the planets' distances from the sun, their diameters 

 and masses More than thirty years ago, Prof. Kirkwood 

 supposed that he had discovered the relation in the analogy 

 which bears his name. The materials for testing and establishing 

 it were then, however, insufficient, and still remain so, leaving 

 far too many of the data uncertain and arbitrary. Could such a 

 relation lie discovered, it could hardly fail to have a most import- 

 ant significance with respect to theories of the origin and 

 development of the planetary system. 



The great problem of the absolute dimensions of our system 

 is, of course, commanded by the special problem of the solar 

 parallax ; and this remains a problem still. Constant errors of 

 one kind or another, the origin of which is still obscure, seem to 

 affect the different methods of solution. Thus, while experi- 

 ments upon the velocity of light and heliometric measurements 

 of the displacements of Mars among the stars agree remarkably 

 in assigning a smaller parallax (and greater distance of the sun) 

 than seems to be indicated by the observations of the late 

 transits of Venus, and by methods founded on the lunar motions, 

 on the other hand, the meridian observations of Mars all point 

 to a larger parallax and smaller distance. While still disposed 

 to put more confidence in the methods first named, I, for one, 

 must admit that the margin of probable error seems to me to 

 have been rather increased than diminished by the latest published 

 results deduced from the transits. I do not feel so confident of 

 the correctness of the value 8" "80 for the solar parallax as I did 

 three years ago. In its very nature, this problem is one, how- 

 ever, that astronomers can never have done with. So funda- 

 mental is it, that the time will never come when they can 

 properly give up the attempt to increase the precision of their 

 determination, and to test the received value by every new 

 method that may be found. 



The problems presented by the sun alone might themselves 

 well occupy more than the time at our disposal this evening. Its 

 mass, dimensions, and motions, as a whole, are, indeed, pretty 

 well determined and understood ; but when we come to questions 

 relating to its constitution, the cause and nature of the appear- 

 ances presented upon its surface, the periodicity of its spots, its 

 temperature, and the maintenance of its heat, the extent of its 

 atmosphere, and the nature of the corona, we find the most 

 radical differences of opinion. 



The difficulties of all solar problems are, of course, greatly 



