TEANSACTIONS OF THE SECTIONS. 



Section A.— MATHEMATICAL AND PHYSICAL SCIENCE. 



PllESIDENT OF THE SECTION — Pl'ofesSOr OLIVER J. LODUE., D.SC, LL.D., F.E.S. 



THURSDAY, AUGUST 20. 

 The President delivered the following Address : — 



During the past year three or four events call for special mention in an annual 

 deliverance of this kind by a physicist. 



One is the Faraday centenary, which was kept in a happy and simple manner 

 by a cosmopolitan gathering in the place so long associated with his work, and by 

 discourses calling attention to the modern development of discoveries made by him. 



Another is the decease of the veteran Wilhelm Weber, one of the originators 

 of that absolute system of measurement which, though still hardly grasped in its 

 simplicity and completeness by the majority of men engaged in practice, nor even, 

 I fear, wholly understood by some of those engaged in University teaching, has yet 

 done so much, and is destined to do still more, for the unification of physical 

 science, and for a thorough comprehension of its range and its limitations. 



A third event of importance during the year is the discovery in America of a 

 binary system of stars, revolving round each other with grotesque haste, and with 

 a proximity to each other such as to render their ordinary optical separation quite 

 impossible. Ideas concerning the future of such systems, if, as seems probable, 

 their revolution period is shorter than their axial period, will readily suggest them- 

 selves, in accordance with the princi])les elaborated by Prof. George Darwin. The 

 subject more properly belongs to our President, but I may parenthetically exclaim 

 at the singular absurdity of the notion which was once propounded by a philoso- 

 pher, that motion of stars in our line of sight must for ever remain unknown to us ; 

 whereas the mere time of revolution of a satellite, compared with its distance from 

 its central body, is theoretically sufficient to give us information on this head. As 

 a matter of pedatrogy it is convenient to observe that the principle called Doppler's, 

 which is generally Imowu to apply to the periodic disturbances called Light and 

 Sound, applies equally to all periodic occurrences; and that the explanation of 

 anomalies of Jupiter's first satellite by Rr emer may be regarded as an instance of 

 J'oppler's principle.^ Any discrepancy between the observed and the calculated 

 times of revolution of stars round each other can possibly be explained by a 

 relative motion between us and the pair of bodies along the line of sight. 



If our text-books clearly recognised this, we should not so often lind exami- 

 nation candidates asserting that the apparent time of revolution of a satellite of 

 Jupiter depends on the distance of the earth from that planet, instead of on the speed. 



' Dr. Huo^gins has just pointed out to me a perfectly clear statement to the 

 above effect in Professor Tait's little book on Light. 



