In 1836, I communicated to this Society the results of an extensive series of observations on Terrestrial Magnetic Intensity, made with the Hansteen Apparatus, which is the property of the Society. Some results with a small Dipping Needle, belonging to myself, were also given, but without great confidence in their accuracy.

I held then, however, the opinion which I still do, and to which the remarkable geometrical researches of Professor Gauss of Göttingen on Terrestrial Magnetism have given additional weight, that the element of horizontal intensity ought to be determined, and its laws of variation, in the first place, ascertained, independent of any other. Even should the deduction of total intensity be the sole ultimate object, I hold that an observer with only portable, and consequently imperfect instruments at his command, would do well to separate completely his investigations as to horizontal intensity from those upon dip, and then, by the skilful grouping of each set, having obtained a law of variation of each element according to the co-ordinates of Latitude and Longitude, the two partial results may be combined into the general one of total intensity , whilst either series may be used to check future observations, or be combined with any single future series in which one of the elements should be better determined.