Faraday to Edward Sabine   7 January 1853

Royal Institution | 7 Jany 1853

My dear Sabine

I have read your remarks¹ with great interest & am much obliged for them. I was puzzled a little at first, for from habit when thinking of a free needle in each hemisphere, I always compare their upper ends, and forget continually that you were comparing the north ends i.e. the upper end of one & the lower end of the other. I seem to have had the variation, which you so properly distinguish as an annual variation, in my mind before but then I had had it as part of a larger effect i.e. the increase of the whole extent of the daily variations at Hobarton² or Toronto³ as the Sun approaches either one or the other; or as the solstice comes on. The eastern effect at Toronto in its summer morning appears to me to be half of a large result of which the other half is the western effect in the evening - and both these diminish together as the Sun goes South & as the correspondent effect at Hobarton increases. Whatever the physical nature of the suns action may be, as he comes on from the East, at the time of the equinox, we may suppose him to act equally on the needles at Hobarton & Toronto. But as he goes north towards the June Solstice the daily variation increases in extent at Toronto & diminishes at Hobarton. This natural action will produce precisely the variation which you describe i.e. the northern end of the needle at Toronto will at 7AM go most east in June or thereabouts because of an increase in the extent of daily variation and the North end of the needle at Hobarton will at the same time go least west or as you put it (if I am not mistaken) most east for the evening hour because the extent of variation is the least at that time. When the Sun goes South the other part of the change occurs in its due order[.]

Perhaps I have not caught the whole of your meaning and I think that is more likely because I have only given above the views which I had in 1850⁴ and the endeavour I then made to shew that the annual increase & diminution of the extent of the daily variation coincided with the action of the cause which I then assigned as a probable one. That the sun is the cause I do not doubt; that he acts in the way I supposed may be true only for a part and even a very small part of the whole effect[.] But the other part of the action if large must be in the same direction as my hypothetical action; for otherwise the effect you refer to would not and could not as I believe occur. Your deductions therefore confirm my expectations and I am also very glad to find that your plate II⁵ agrees with my plate of curves in the 1850 paper⁶. The amount of daily variation & its annual change are noted in paragraphs 2948⁷, 3009, 3027⁸ & elsewhere. I do not intend to refer to this subject in my evening but mean to keep myself in the latter part of it entirely to your magneto-solar coincidences⁹. I find that my last letter to Wolf in which I referred to your publication was written 13 Novr¹⁰. If he had it in time he ought to have referred to it in his pamphlet¹¹[.]

Ever My dear Sabine | Very Truly Yours | M. Faraday