56 THE POPULAR SCIENCE MONTHLY. 



that, like the Moon, it will well repay the most minute and assiduous 

 study. 



Its satellite system 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 photo- 

 metric method of observing these eclipses, first instituted by Professor 

 Pickering at Cambridge in 1878, and since reinvented 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 ajiply as to the planetary theory. As yet nothing 

 appears in the problem to be beyond the power and scope of exist- 

 ing 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 same as those of Jupiter, ex- 

 cepting that the surface and atmospheric phenomena are less striking, 

 and more difficult of observation. But we have, in addition, the won- 

 derful rings, unique in the heavens, the loveliest of all telescopic ob- 

 jects, the type and pattern, I suppose, of world-making, in actual prog- 

 ress 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 imagi- 

 nation, 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 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 bet- 

 ter telescopes. 



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

 all attempts to study their surface and physical characteristics. Their 

 own motions and those of their satellites have been well worked out ; 

 but it remains to discuss 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 further knowledge of that 

 far-off relative. Perhaps the 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 pe- 

 riods of the planets, in the fact that there is very possibly some con- 

 nection between these periods, on the one hand, and, on the other, the 



