ON THE NUMERICAL RELATIONS OF GRAVITY AND MAGNETISM. 



135 



N O T E. 



The following extracts show that Secchi's hypothesis of solar magnetic induction was 

 only provisional, and that a fuller report than has yet been published of the details of his 

 investigations is greatly to be desired. 



" We, too, in the same spirit, and merely for the purpose, if possible, of combining facts, 

 have supposed the sun to act as a great magnet. The explanations hitherto proposed may 

 be reduced either to thermo-electric currents induced by the sun in the different strata of 

 the earth, or to the electricity developed in the meteorological changes of which the sun 

 is the principal cause. A single reflection seems to exclude these from being principal 

 causes of the magnetic diurnal period. The characteristic fact, as we have already noticed, 

 is that the magnetic elements have a double period, diurnal and nocturnal 



" To this proof in support of the solar magnetic theory, may be added another, already 

 noticed by General Sabine, and worked out by us in § 1 of Part II of this memoir, viz., 

 the opposite action of the sun according to its declination, the inversion occurring exactly 

 at the epoch of the equinoxes ; and here another difference will be seen between the effects 

 of thermical and meteorological causes and the magnetic effect of the sun. The former do 

 not reach their extremes for a considerable time after the corresponding astronomical 

 phases, while the latter have an almost exact coincidence with them 



" We do not pretend, however, that there arc not considerable difficulties in the way of 

 this hypothesis ; and although it explains very well certain very singular facts, — as, for 

 example, the interval of six hours between the diurnal maxima and minima, a fact the 

 explanation of which has never, as far as I am aware, been even attempted on any other 

 hypothesis, and which yet is so marked in all the magnetic variations in the mean lati- 

 tudes ; also, the singular exception which it suffers at the equator, becoming simple for the 

 horizontal and for the vertical components, and various other points, — yet we must confess 

 that there are some irregularities which our formula) do not explain. Of this nature is 

 the fact, that at St. Helena, and generally under the equator, the period for the declina- 

 tion of the needle appears to be rather eight hours than twelve, so that it presents some- 

 times three maxima 



" The fact mentioned above, that the maxima of the perturbations at Hobarton succeed 

 each other with the same retardation -as the other magnetic phases, is one which cannot 

 be explained either by the retardation of the effect of temperatures, or by the condensa- 

 tion of vapor. We cannot conceive how these should account for the general retardation 

 of one hour. It is then a purely magnetic fact, the explanation of which depends on that 

 of the physical cause of solar and terrestrial magnetism. The same may be said of the 



