680 MR J. A. BROUN ON THE DIURNAL VARIATION OF THE 



variation. This is a fact that must be considered in any theory which pretends 

 to account for the variations of the declination magnet.* 



It may be supposed that the variable movements in the month of March also 

 are, to some extent, due to the lunar action which shifts its epochs from day to day 

 with the hour for which the moon is on the meridian; and this is no doubt the case; 

 but a first approximation to eliminate the lunar effect from the solar curves has 

 shown me that the irregularity remains after the elimination nearly as before. I 

 shall endeavour hereafter to present the solar curves nearly free from the lunar effect. 



I have stated elsewhere, that the lunar law also passes through a period of 

 inversion, near the equator, like the solar law, when the sun passes from one 

 hemisphere to another. f This dependence of the law of lunar action upon the 

 position of the sun complicates the phenomena, and an effort for their separation 

 will be best made when the lunar laws have been investigated. 



Diurnal Variation at different Latitudes. 

 In the preceding pages we have examined the change of law of the diurnal 

 variation of magnetic declination, with the period of the year at a single station 

 near the magnetic equator. It will be desirable now to investigate the mode in 



* I have been obliged here to enter upon laws which will be the subject of another paper, in 

 order to explain certain apparent irregularities of solar action. I cannot leave the subject at 

 present without suggesting that the variations of magnetic declination are probably due to currents 

 in an electro-sphere, which, it appears to me, must surround each heavenly body. I have suggested 

 elsewhere that the solar spots are due to disruptions of these currents within the solar atmosphere 

 (below the photosphere, or forming part of the photosphere itself), produced by the planets, and 

 depending for their number and magnitude on the position (latitude and distance) of the latter rela- 

 tively to the plane of the solar equator. (See a letter addressed to Sir David Brewster, December 

 21st 1857, published in the " Philosophical Magazine," July 1858.) If this idea has any basis, 

 we may suppose it probable that the moon exercises some similar action upon the earth's electro- 

 sphere, an action depending for its amount on the electric tension of the spheres, these again de- 

 pending upon that of the sun. Some such idea is necessary to explain the fact, that the lunar effect 

 is very variable in its amount, varying from a movement of the free needle of about 5', in some 

 days, of one lunation, to a tenth part of this movement in succeeding lunations. 



f See " Proceedings of the Royal Society of London," vol. x. p. 1482, 861. I may note here 

 that, as I showed in the " Makerstoun Observations for 1844" (Trans. Roy. Soc. Edin., vol. xviii. 

 p. 354), the difference of the law of solar diurnal variation of declination in Europe for summer 

 and winter was of the same kind as that betwixt the laws for the same periods near the equator, 

 where the movement is inverted ; so it followed from the inversion of the lunar law near the equator, 

 with the sun in the northern and southern hemispheres, that in high latitudes, the lunar law should 

 present a greater range in summer than in winter, and that the mean law in high north latitudes should 

 be nearly the opposite of that for high south latitudes. The latter fact should have been evident 

 from preceding observations in mean latitudes of the two hemispheres, and on the discovery of the 

 inversal of the lunar variation with the sun's passage of the equator, I examined these observations 

 for this end. Unfortunately the discussion of the Toronto Observations by General Sabine gave a 

 result nearly the inverse of those derived from the observations at Makerstoun and Prague in the 

 same hemisphere, as I pointed out in the note cited above. General Sabine then discovered that 

 west had been substituted for east in the discussion of the Toronto Observations ; and this correction 

 made, the approximate opposition of the laws for the two hemispheres was at once evident. Of the 

 former fact I satisfied myself by a rediscussion of the " Makerstoun Observations" rejecting the 

 large disturbances ; and it has been verified also by the late Mr Bache, from observations at Phila- 

 delphia, though only two years after the result deduced by me of the inversion of the law at the 

 equator had made the conclusion, if not certain, at least extremely probable. 



