III. Magnetical Observations at Port Bowen, &c. A. D. 1824-25, comprehending observations on the diurnal variation and diurnal intensity of the horizontal needle; also on the Dip of the magnetic needle at Woolwich, and at different stations, within the arctic circle.

The daily variation of the horizontal needle was first observed in London by Mr. Graham, in the year 1722; but in consequence of its small amount, it has always been considered amongst the minor phenomena which the magnetic needle presents. Mr. Barlow , however, having explained a method of encreasing the amount of this daily change very considerably, in any latitude, this phenomenon began to assume a different character, and seemed to furnish a subject of highly interesting, if not of useful, enquiry.