George Biddell Airy to Faraday   15 May 1861

Royal Observatory, Greenwich, | London, S.E. | 1861 May 15

My dear Sir

A matter has come out in the examination of the Greenwich Magnetical Observations (which examination was one of the occupations that made me unfaithful to my lecture-engagement)¹, on which I greatly want your opinion.

The diurnal changes of the forces acting on the magnet (including all forces that act in the plane of the Greenwich horizon) follow a very singular law, which I can explain only by supposing that the radiation of the Sun upon the sea, as distinguished from the land, produces an attraction of the north or marked end of a magnet at Greenwich: and that it is this excitement of the North Atlantic Ocean which mainly produces the diurnal inequalities of magnetic force. The radiation upon the land of Africa seems to produce little or no effect².

Now I want to ask you whether this harmonizes with other known laws of the mediate effect of solar radiation?

I do not doubt, from all that I have seen of atmospheric electricity, that any cause which promotes the absorption of aqueous vapour into the air will produce electrical effect the electricity being of the nature of frictional electricity. But this is so totally different from magnetism or galvanism in all its modes of development, and the exhibitions of accidental atmospheric electricity and accidental terrestrial galvanism are so totally unconnected, that I hardly expect any explanation to follow from that consideration.

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The examination of the luno-diurnal inequality shews a tidal throb of magnetism occurring in every lunar day twice towards and twice from the Hudson’s Bay direction. Can you suggest any explanation of this?

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I am, my dear Sir, | Yours most truly | G.B. Airy

Professor Faraday