388 Royal Suciedj : — 



Extract of a Letter from Professor Langberg of Christiania to 

 Colonel Sabine, dated June 10, 1S55. 



" Of all the important results from the discussions of the British 

 Colonial Observatory, the discovery of the direct action of the sun 

 on the magnetism of the earth is certainly a fact of the highest in- 

 terest, in opening quite a new field for investigation; and few modern 

 discoveries in this branch of science have interested me more than 

 yours of the annual variation of the diurnal variation of declination. 

 It seems that M. Secchi of Rome has nearly touched at the same 

 discovery, and I am indeed glad that the enormous quantity of cal- 

 culations, which you are superintending, did not prevent you from 

 publishing your results before the ripening fruit was plucked by an- 

 other. The first beautiful result of this annual variation is the ex- 

 planation of the fact, which you have deduced from the observations 

 at St. Helena and the Cape of Good Hope, that the horary variation 

 of declination docs not vanish in passing from the northern to the 

 southern magnetic hemisphere, but only changes signs at the equi- 

 noxes. I think every physicist will agree with 3'ou, that no thermic 

 hypothesis will be able to explain this annual variation, but as you 

 say (Toronto, ii. p. xix), it is 'obviously connected with, and de- 

 pendent on, the earth's position in its orbit relatively to the sun, 

 around which it revolves, as the diurnal variation is connected with 

 the rotation of the earth on its axis.' But you have given no hint 

 how this different position in its orbit can affect the magnetic condi- 

 tion of the earth, except so far as you suppose that the excentrinity 

 of the orbit is the reason that the total magnetic force is about y^o 

 greater at the perihelion than at the aphelion (page xciii) ; but even 

 granted that this variation is the effect of the excentricity, it cannot 

 be the cause of the annual variation of the declination, as this is of 

 contrary signs in the two semiannual periods. 



" I have thought that this annual variation might possibly be ex- 

 plained by tlie following considerations, which I (although with 

 extreme diffidence) shall venture to lay before you. 



" As the recent magnetic observations have proved without doubt 

 the direct magnetic action of the sun, or that the sun itself is a 

 magnet, the sun must accordingly have magnetic polarity or mag- 

 netic poles. Now in our ignorance of the causes of the magnetic 

 condition of the heavenly bodies, I think it reasonable to connect it 

 in some way with their rotation on their axes, and to assume that 

 generally their magnetic axis will nearly coincide with the axis of 

 rotation ; at all events, if these do not coincide, but include a small 

 acute angle, the sum of the magnetic influences on a distant mag- 

 netic body during a whole rotation, will be nearly the same as if the 

 magnetic jjoles were placed in the axis of rotation. If we supjiose 

 a mao'net E revolving about another S, the magnetic axis of E re- 

 maining parallel with itself, but not parallel with the axis of S (as 

 the earth around the sun), then the magnetic induction of S on E 

 will depend on the relative position of both magnetic axes. More- 

 over, if we only regard the mean of the magnetic induction of the sun 

 on the earth during several rotations of both on their axes, we may 



