Edward Sabine to Faraday   8 January 1853

11, Old Burleigh S. | January 8, ‘53

Dear Faraday

I think that the facts point to a more distinct recognition of distinct causes than appears in your note¹: that we must separate the effects which are occasioned by the earths revolution in its orbit, from those which are occasioned by its revolution on its axis: the one producing a variation in the magnetic direction which is properly called “annual” because its period is a year, and the other “diurnal” because its period is a day. Now with respect to the first, or annual variation, & confining ourselves to our element, viz the Declination: and (the better to fix our ideas) to one hour, viz 8 am, (as that is one of the hours followed out in the dates which I sent to you). Now, as far as we have yet experience, if the declination be observed at the same hour of 8 am in any part of the world, i.e. in any meridian, or in any part of any meridian, whether north or south of the Equator, or on the Equator, the declination so observed will shew an annual variation, the direction and amount of which will be every where the same: i.e. at the northern solstice the needle will be at its eastern extreme, at the southern solstice at its western extreme, and it will pass through its mean position at the two equinoxes: in the one case when proceeding from east to west, & in the other when returning from west to east. The direction of the motion is every where the same, i.e. of the north end of the needle from east to west from June 21 to December 21, and from west to East from December 21 to June 21, without reference to whether the one end of the needle be directed upwards (by reason of the Dip) or the other end of the needle be directed upwards, or whether the needle be horizontal:- and the amount is also every where the same, or very nearly so, viz five minutes of Arc.

Now, having found & measured this annual variation, we can eliminate it; and when it is eliminated, the needle will have its mean place for 8 am all the year round, so far as the annual variation is concerned. Now let the same thing be done for each of the 24 hours separately, (assuming that you have hourly observations to deal with), and let it be done for three places, one in Northern Latitude, a second in Southern Latitude, and a third on the Equator; (the annual variation so eliminated will have been the same amount, & have had the same direction at each of the three locations as already stated). Now then the comparison of the direction of the needle at the different hours of the day will give us the diurnal variation; and, as the result of the diurnal variation, we find that at 8 am the north end of the needle is considerably to the East of its mean position in the 24 hours at the station in North Latitude, - considerably to the West of its mean position at the station in South Latitude; whilst at the Equator the diurnal variation is null; the needle at 8 am pointing the same as it does on the mean of the 24 hours. The character of the diurnal variation at 8 am then is that the north end of the needle is affected in opposite senses in opposite hemispheres, passing thro’ a Zero on the Equator - and that its amount varies from 0 at the Equator to several minutes in the high Latitudes being continually variable according to some law into which we need not now enter - whereas the Character of the annual variation is that the same end of the needle is affected in the same sense, and to the same amount, equally whether the locality be in the North or South hemisphere or on the Equator.

There is another feature in the Annual Variation which is a very remarkable one, which is shown I think well by the Plate I sent you. I have said that the annual variation passes thro’ its mean position at the Equinoxes; it not only does so on the very day of the Equinox, but in one week after the Equinox is passed, it has reached in amount the full extent (or very nearly so indeed) of the half amplitude of the whole movement; in the same way that a week before the Equinox the position of the needle depending on the annual variation is almost if not quite as distant from the mean position in the other direction. This phenomena is most markedly the same at St. Helena & the Cape of Good Hope - whilst at St. Helena the dip is 21˚ & at the Cape 54’; the Lat of the one 17˚, & <<o>>f the other 34˚.5.

You can lay bye this note till you have more leisure to look into the subject, and then I shall be very glad to receive your remarks[.]

Very truly yours | Edward Sabine