September 5, 1884.] 



SCIENCE. 



193 



But, substantially, we who meet here to-day are a 

 new generation, more numerous certainly, and in 

 some respects unquestionably better equipped for 

 our work, than our predecessors were; though we 

 might not care to challenge comparisons as regards 

 native ability, or clearness of insight, or lofty pur- 

 pose. 



And the face of science has greatly changed in the 

 mean time; as much, perhaps, as this great city and 

 the nation. One might almost say, that, since 1848, 

 'all things have become new' in the scientific world. 

 There is a new mathematics and a new astronomy, 

 a new chemistry and a new electricity, a new geology 

 and a new biology. Great voices have spoken, and 

 have transformed the world of thought and research 

 as much as the material products of science have 

 altered the aspects of external life. The telegraph 

 and dynamo-machine have not more changed the 

 conditions of business and industry than the specu- 

 lations of Darwin and Helmholtz and their compeers 

 have affected those of philosophy and science. 



But, although this return to our birthplace sug- 

 gests 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 for- 

 wards rather than backwards, and confining myself 

 mainly to topics which lie along the pathway of my 

 own line of work. 



The voyager upon the inland sea of Japan sees 

 continually rising before him new islands and moun- 

 tains of that fairyland. Some come out suddenly 

 from behind nearer rocks or islets, which long con- 

 cealed 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 great- 

 est of them all, are first seen as the minutest specks 

 upon the horizon, and grow slowly to their final gran- 

 deur. 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 practised eye of the skilful 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 re- 

 motely interesting in various degrees; and some, as 

 yet, are mere suggestions, almost too misty and in- 

 definite 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 interest- 

 ing, or likely to be fruitful, from a philosophic point 

 of view. 



Taking first those that lie nearest, we have the 

 questions which relate 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 produce 

 serious embarrassment in the reduction and compari- 

 son of certain lunar observations. The length of the 

 line joining, say, the Naval observatory at Washing- 

 ton with the Eoyal observatory at the Cape of Good 

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

 hundred feet, as commonly supposed, but the uncer- 

 tainty amounts to some thousands of feet, and may 

 possibly be a mile or more, probably not less than a 

 ten-thousandth of the whole distance; and the direc- 

 tion of the line is uncertain in about the same degree. 

 Of course, on those portions of either continent which 

 have been directly connected with each other by geo- 

 detic triangulations, no corresponding uncertainty 

 obtains ; and as time goes on, and these surveys are 

 extended, the form and dimensions of each continu- 

 ous land-surface will become more and more perf ectly 

 determined. But at present we have no satisfactory 

 means of obtaining the desired accuracy in the rela- 

 tive position of places separated by oceans, so that 

 they cannot be connected by chains of triangulation. 

 Astronomical determinations of latitude and longi- 

 tude do not meet the case ; since, in the last analysis, 

 they only give at any selected station the direction of 

 gravity relative to the axis of the earth, and some 

 fixed meridian plane, and do not furnish any linear 

 measurement or dimension. 



Of course, if the surface of the earth were an exact 

 spheroid, and if there were no irregular attractions 

 due to mountains and valleys and the varying density 

 of strata, the difficulty could be easily evaded; but, 

 as the matter stands, it looks as if nothing short of 

 a complete geodetic triangulation of the whole earth 

 would ever answer the purpose, — a triangulation 

 covering Asia and Africa, as well as Europe, and 

 brought into America by way of Siberia and Be- 

 ring Strait. 



It is indeed theoretically possible, and just con- 

 ceivable, that the problem may some day be reversed, 

 and that the geodesist may come to owe some of his 

 most important data to the observers of the lunar 

 motions. When the relative position of two or more 

 remote observatories shall have been precisely deter- 

 mined by triangulation (for instance, Greenwich, 

 Madras, and the Cape of Good Hope), and when, by 

 improved methods and observations made at these 

 fundamental stations, the moon's position and motion 

 relative to them shall have been determined with an 

 accuracy much exceeding any thing now attainable, 

 then by similar observations, made simultaneously 

 at any station in this hemisphere, it will be theoreti- 

 cally possible to determine the position of this sta- 

 tion, and so, by way of the moon, to bridge the 

 ocean, and ascertain how other stations are related 

 to those which were taken as primary. I do not, 

 of course, mean to imply, that, in the present state of 

 observational astronomy, any such procedure would 

 lead to results of much value ; but, before the Asiatic 



