pbesideiNt's address. 1? 



every yeal'. Over live Imndi'ed arc now kiiowu. One of [these — ' 

 Eros — (No. 433) is particulai-ly interesting, since it is nearer to the sun 

 than is Mars, and gives a splendid opportunity for fixing with increased 

 accuracy the sun's distance from the earth. Two new satellites to Saturn 

 and two to Jupiter have been discovered by photography (besides one to 

 Jupiter in 1892 by the visual telescope of the Lick Observatory). One of 

 the new satellites of Saturn goes round that planet the xcn'owj way, thus 

 calling for a fundamental revision of our ideas of the origin of the solar 

 system. 



The introduction of photography has made an immense difference in 

 spectroscopic work. The spectra of the stars have been readily mapped out 

 and classified, and now the motions in the line of sight of faint stars can 

 be determined. This ' motion in the line of sight,' which was discernible 

 but scarcely measurable with accuracy before, now provides one of the 

 most refined methods in astronomy for ascertaining the dimensions and 

 motions of the universe. It gives us velocities in miles per second 

 instead of in an angular unit to be interpreted by a very imperfect know- 

 ledge of the star's distance. The method, initiated joi'-i-ctically by 

 Huggins thirteen years before, was in 18S1 regarded by many astro- 

 nomers as a curiosity. Visual observations wei-e begun at Greenwich in 

 1875, but were found to be affected by instrumental errors. The intro- 

 duction of dry plates, and their application by Yogel in 1887, was the 

 beginning of general use of the method, and line-of-sight work is now a 

 vast department of astronomical industry. Among other by-products 

 of the method are the ' spectroscopic doubles,' stars which we know to be 

 double, and of which we can determine the period of revolution, though 

 we cannot separate them visually by the greatest telescope. 



Work on the sun has been entirely revolutionised by the use of photo- 

 graphy. The last decade has seen the invention of the spectro-heliograph 

 — which simply means that astronomers can now study in detail portions 

 of the sun of which they could previously only get a bare indication. 



More of the same story could be related, but enough has been said to 

 show how full of life and progress is this most ancient and imposing of all 

 sciences. 



A minor though very important influence in the progress of astronomy 

 has been the provision, by the expenditure of great wealth in America, of 

 great telescopes and equipments. 



In 1877 my distinguished predecessor in the presidency of tlie 

 British Association started a line of mathematical research which 

 has been very fruitful and is of great future promise for astronomy. 

 He was able himself last year to give some account of this reseai'ch to 

 the Association. On the present occasion I may mention that as recently 

 as last April, at the Royal Astronomical Society, two important papers 

 were read — one by Mr. Cowell and the other by Mr. Stratton — which 

 have their roots in Sir George Darwin's work. The former was led to 

 suggest that the day is lengthening ten times as rapidly as had been 



1906. "^ • 



