Edward Sabine to Faraday   24 July 1858¹

Llanfair, Llandovery, | S. Wales | July 24. 58

Dear Faraday,

In one of your friday evening lectures at the Institution 2 or 3 years ago you spoke of the possibility of the lines of force (on which you were lecturing) affording by & bye a more precise means of measuring the magnetic variations than those which we have now in use². I wished then, and I have often wished since to make better known to you the capabilities of our present means; and a description which I have just completed of the lunar diurnal variation of the horizontal magnetic force at the Cape of Good Hope gives me I think a good opportunity of doing so. The observations are those hourly ones made between 1843 & 1846 which were published in the 1st Vol. of the Cape Magnetic Obsn. some years ago³ - The discussion will be in a volume now in the press⁴. The result is that twice in every lunar day, the horizl. magc. force of the earth is encreased by one two-hundred-thousandth of its whole amount; and twice in each lunar day the force is diminished by an equal amount the maxima are at intervals of 12 lunar hours from each other, as are the minima; & the minima six hours from the maxima. And not only are these wonderfully minute differences of the force cognisable & satisfactorily determined, but each progression from maximum to minimum & from minimum to maximum is determined by strictly independent observations at each of the intermediate hours with a very remarkable approach to regularity. Thus in every six lunar hours there is a variation of the force amounting to two, two-hundred thousandth of the whole force, which takes place by a regular progression cognisable by our present instrumental means by independent observations at every hour: These are the mean numerical values derived from 3 years of hourly observation. It really appears to me that greater precision than this need not even be desired in the present state of the science. If & when you show your audience a needle suspended by a silk thread under a glass cover directing itself into the m. meridian by the earths force you told them that by the instrument employed and the systems practised in magc. observatories, a variation of a hundred-thousandth part of that