PENDING PROBLEMS OF ASTRONOMY. 47 



not more changed the conditions of business and industry than the 

 speculations of Darwin and Helmholtz, and their compeers, have af- 

 fected those of philosophy and science. 



But, although this return to our birthplace suggests retrospections 

 and comparisons which might profitably occupy our attention for even 

 a much longer time than this evening's session, I prefer, on the whole, 

 to take a different course ; looking forward rather than backward, and 

 confining myself mainly to topics which lie along my own line of 



work. 



The voyager upon the inland Sea of Japan sees continually rising 

 before him new islands and mountains of that fairy-land. Some come 

 out suddenly from behind nearer rocks or islets, which long concealed 

 the greater things beyond ; and some are veiled in clouds which give 

 no hint of what they hide, until a breeze rolls back the curtain ; some, 

 and the greatest of them all, are first seen as minute specks upon the 

 horizon, and grow slowly to their final grandeur. Even before they 

 reach the horizon-line, while yet invisible, they sometimes intimate 

 their presence by signs in sky and air ; so slight, indeed, that only 

 the practiced eye of the skillful sailor can detect them, though quite 

 obvious to him. 



Somewhat so, as we look forward into the future of a science, we 

 see new problems and great subjects presenting themselves. Some are 

 imminent and in the way — they must be dealt with at once, before 

 further progress can be made ; others are more remotely interesting 

 in various degrees ; and some as yet are mere suggestions, almost too 

 misty and indefinite for steady contemplation. 



With your permission, I propose this evening to consider some of 

 the pending problems of astronomy — those which seem to be most 

 pressing, and most urgently require solution as a condition of advance ; 

 and those which appear in themselves most interesting or likely to be 

 fruitful from a philosophic point of view. 



Taking first those that lie nearest, we have the questions which re- 

 late to the dimensions and figure of the earth, the uniformity of its 

 diurnal rotation, and the constancy of its poles and axis. I think the 

 impression prevails that we already know the earth's dimensions with 

 an accuracy even greater than that required by any astronomical de- 

 mands. I certainly had that impression myself not long ago, and was 

 a little startled on being told by the superintendent of our " Nautical 

 Almanac " that the remaining uncertainty was still sufficient to pro- 

 duce serious embarrassment in the reduction and comparison of cer- 

 tain lunar observations. The length of the line joining, say, the ISTaval 

 Observatory at Washington with the Royal Observatory at the Cape 

 of Good Hope is doubtful, not to the extent of only a few hundred 

 feet, as commonly supposed, but the uncertainty amounts to some thou- 

 sands of feet, and may possibly be a mile or more — probably not less 

 than a ten-thousandth of the whole distance ; and the direction of the 



