1892.] President's Address. 303 



testing. The report of the results of the whole inquiry contains a 

 large mass of most interesting matter, and the Committee's work ends 

 in a set of practical recommendations, from which we may hope that 

 much benefit will come, in the prevention of inconvenience and 

 disaster liable to be produced by mistake of colour signals, both at 

 sea and on railways. 



Mr. Ellis's communication* to the Royal Society of last May, and 

 Professor Grylls Adams' communicationf of June J 891, both on the 

 subject of simultaneous magnetic disturbances found by observations 

 at magnetic observatories in different parts of the world ; the award 

 of a Royal medal two years ago to Hertz, for his splendid experi- 

 mental work on electro-magnetic waves and vibrations ; and Professor 

 Schuster's communication J to the Royal Society, of June, 1889, on the 

 " Diurnal Variations of Terrestrial Magnetism ;" justify me in saying 

 a few words on the present occasion regarding terrestrial magnetic 

 storms, and the hypothesis that they are due to magnetic waves 

 emanating from the sun. 



Guided by Maxwell's " electro-magnetic theory of light," and the 

 undulatory theory of propagation of magnetic force which it includes, 

 we might hope to perfectly overcome a fifty years outstanding diffi- 

 culty in the way of believing the sun to be the direct cause of mag- 

 netic storms in the earth, though hitherto every effort in this direction 

 has been disappointing. This difficulty is clearly stated by Professor 

 W. G. Adams, in the following sentences, which I quote from his 

 Report to the British Association of 1881 (p. 469), " On Magnetic 

 Disturbances and Earth Currents" : " Thus we see that the magnetic 

 changes which take place at various points of the earth's surface at 

 the same instant are so large as to be quite comparable with the 

 earth's total magnetic force ; and in order that any cause may be a 

 true and sufficient one, it must be capable of producing these changes 

 rapidly." 



The primary difficulty, in fact, is to imagine the sun a variable 

 magnet or electro-magnet, powerful enough to produce at the earth's 

 distance changes of magnetic force amounting, in extreme cases, to 

 as much as 1/20 or 1/30, and frequently, in ordinary magnetic storms, 

 to as much as 1/400 of the undisturbed terrestrial magnetic force. 



The earth's distance from the sun is 228 times the sun's radius, 

 and the cube of this number is about 12,000,000. Hence, if the sun 

 were, as Gilbert found the earth to be, a globular magnet, and if it 

 were of the same average intensity of magnetisation as the earth, we 

 see, according to the known law of magnetic force at a distance, that 

 the magnetic force due to the sun at the earth's distance from it, in 



* ' Roy. Soc. Proc.,' November, 1822, vol. 52, p. 191. 

 f ' Phil. Trans.,' vol. 183, 1891-92, p. 131. 

 I ' Phil. Trans.,' vol. 180, 1889, p. 467. 



