ON SUNSPOTS AND TERRESTRIAL TIIENOMENA. 



451 



from a, memoir by Fritz, will repay a careful study. While especially in 

 curve 4, wliicli takes in the largest number of observations, wo can traco 

 the rise at each spot maximum, it will be seen that the amplitude of tho 

 oscilliition is very irregular, but suggests the existence of a secular period. 

 The length of that secular period can bo approximately estimated by 

 taking account of some older observations. In this way Fritz has 

 fixed its period at fifty-five years, but this is a point on which it is safe 

 to speak witli caution. 



Curve 1 of the plato represents tho sunspot frequency. In curve 2 

 tlic European observations of auroras between latitudes 4(i° and 55° were 

 taken into account. Curve 3 rojiresents tho frequency of aurora3 observed 

 hi America between latitudes O'' and 60°. Finally, curve 4 takes in all 

 European observations extending to the arctic circle. 



General Disciissiun. 



A very remarkable connection between sunspots and terrestrial 



plicuomona is thus established, and the question naturally arises whether 



we can form any idea as to the nature of tho connection. Though this 



may seem hopi'less at present, wc can at least form an opinion as to how 



the connection cannot be explained. In the first place, we must not think 



of a direct raay^netic effect between sun and eirJi. The sun is too hot 



to be bodily magnetised ; and even were it made of hard steel magnetised 



to saturation, it would not sensibly affect the magnetic needles of tho 



earth. It has hi'cn suggested that the sun may be electrified to a high 



liotentiiil, and by its induction affect the magnetic elements of the earth. 



It lias also bet' II maintained that the space between tho sun and earth ia 



;i 1,'ood conductor of electricity, and must thus bring their electric states 



into conne(!tiim. I reserve for another opportunity tho discussion of 



tlie possibility or probability of either of theso hypotheses ; at present it 



will be sufUt'icnr to say tliat I am unable to conceive how, even granting 



tliuir truth, tli-.y can possibly explain the increase in the diurnal variation 



at times of numv' spots. 



We do not, at present know what the daily variation of the magnetic 



needle is diH> to, and therefore it seems premature to consider what 



causes may niddify the efl'ect. The solar influence bears on it the stamp 



(if a temporafuro eflbct, while tho lunar influence seems tidal. It is 



difficult to siiy. without very careful consideration, what the thermo- 



cloctric effiMMs in the earth's crust might be. The electro-motive force 



J no doubt is very small, but then the resistance is small also, and the 



^ tiuvrcnt might be quite apj'jreciable. Tides in the earth's intei'ior, like those 



assumed by Prnicssor W. G. Adams,' do not appar'cntly exist to any appre- 



1 ciablu degree, 'ut there must be a tidal stress, and though very small 



,jj the conseqaen strain might produce a difl'erence of resistance in diflerent 



If directions, iiiu' tl)^.^ alter tho channels of the electric currents. The 



; superposition > ' sueh a thermal and tidal effect seems to t .e to be the most 



„iliopeful hyp t' V sis at present, but I have not followc " out sufficiently 



Ito be able to do more than to throw it outas asuggestio oseand other 



^considerations, however, lead us on to inquire whether liieso magnetic 



i»' icctric cff'T's are not perhaps only secondary, and that the sun acts 



Royal Institution, June o, 1881. 



G G 2 



