September 5, 1884.1 



SCIENCE. 



197 



motest of the present planetary family. Some config- 

 urations in certain cometary orbits, and some almost 

 evanescent peculiarities in Neptune's motions, have 

 been thought to point to the existence of such a 

 world ; and there is no evidence, nor even a presump- 

 tion, against it. 



Mercury as yet defies all our attempts to ascertain 

 the length of its day, and the character and condi- 

 tion of its surface. Apparently the instruments and 

 methods now at command are insufficient to cope 

 with the difficulties of the problem ; and it is not easy 

 to say how it can be successfully attacked. 



With Yenus, the earth's twin-sister, the state of 

 things is a little better : we do already know, with 

 some degree of approximation, her period of rotation ; 

 and the observations of the last few months bid fair, 

 if followed up, to determine the position of her poles, 

 and possibly to give us some knowledge of her moun- 

 tains, continents, and seas. 



It would be rash to say of Mars that we have 

 reached the limit of possible knowledge as regards a 

 planet's surface; but the main facts are now deter- 

 mined, and we have a rather surprising amount of 

 supposed knowledge regarding his geography. By 

 ' supposed ' I mean merely to insinuate a modest 

 doubt whether some of the map-makers have not 

 gone into a little more elaborate detail than the cir- 

 cumstances warrant. At any rate, while the ' areog- 

 raphies ' agree very well with each other in respect 

 to the planet's more important features, they differ 

 widely and irreconcilably in minor points. 



As regards the physical features of the asteroids, 

 we at present know practically nothing: the field 

 is absolutely open. Whether it is worth any thing 

 may be a question ; and yet, if one could reach it, I 

 am persuaded that a knowledge of the substance, 

 form, density, rotation, temperature, and other 

 physical characteristics, of one of these little orphans, 

 would throw vivid light on the nature and behavior 

 of interplanetary space, and would be of great use in 

 establishing the physical theory of the solar system. 



The planet Jupiter, lordliest of them all, still, as 

 from the first, presents problems of the highest im- 

 portance 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 ro- 

 tation periods derived from the observation of mark- ■ 

 ings 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 endeavors 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 character- 

 istic expression. The same characteristics are also 

 probably manifested in other less conspicuous but 



equally curious and interesting markings on the 

 varied and ever-changing countenance of this planet; 

 so that, like the moon, it will well repay the most 

 minute and assiduous study. 



Its satellite system also deserves careful observa- 

 tion, 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 Professor 

 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 neces- 

 sary care and prolixity ; but a new and more com- 

 pendious method is most desirable. 



The problems of Saturn are much the same as 

 those of Jupiter, excepting that the surface and at- 

 mospheric phenomena are less striking, and more dif- 

 ficult of observation. But we have, in addition, the 

 wonderful rings, unique in the heavens, the loveliest 

 of all telescopic objects, the type and pattern, I sup- 

 pose, of world-making, in actual progress before our 

 eyes. 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 different 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 care- 

 fully, and set down exactly what he sees. It may 

 well be that even a few decades will develop most 

 important and instructive phenomena 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 recognized 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 sur- 

 face and physical characteristics. Their own motions 

 and those of their satellites, have been well worked 

 out; but it remains to discuss their rotation, topog- 

 raphy, 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 



