PENDING PROBLEMS OF ASTRONOMY. 55 



have not gone into a little more elaborate detail than the circum- 

 stances 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 minor points. 



Of the asteroids there is but little to be said. We are rather re- 

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

 astronomers to continue to search for the remaining members of the 

 group, 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 hunter for 

 this small game may be rewarded by the discovery of some great 

 world, as yet unknown, slow moving in the outer desolation beyond 

 the remotest of the present planetary family. Some configurations 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 presumption, 

 against it. 



As regards the physical features of the asteroids we at present 

 know practically nothing : the field is absolutely open. Whether it is 

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

 am persuaded that a knowledge of the substance, form, density, rota- 

 tion, temperature, and other physical characteristics, of one of these 

 little vagabonds 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 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 half-way house between familiar terrestrial facts and solar mys- 

 teries. 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 ob- 

 Berved between the rotation 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 chal- 

 lenging for several years our best endeavors to understand and explain 

 it, still, I think, remains as much a mystery as ever — a mystery proba- 

 bly 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 mark- 

 ings on the varied and ever-changing countenance of this planet ; so 



