the alterations of dip occur, is also worthy of notice. At two hundred and eighty miles north of the magnetic equator, the dip was 9° 36′, showing about 2.05 minutes of change for every mile of latitude; at two hundred and ninety-two miles to the south, the dip was 9° 52′, or about 2.03 minutes for every mile of latitude. It is to be remembered that this large amount of change is limited to the region of the magnetic equator; near the poles, it requires an approach of about two miles to produce an alteration of a single minute of dip.
The geographical position of the magnetic equator where we crossed it was lat. 13° 45′ S, long. 30° 41′ W. Here we again felt the influence of a westerly current of nearly a mile an hour.
The next matter of "great and especial interest in a magnetic point of view," to which my attention was directed in the instructions drawn up for my guidance, at the request of the Lords Commissioners of the Admiralty, by the Committee of Physics of the Royal Society, was that of the situation of the point of minimum total intensity, or that point where the intensity is the least which occurs over the whole surface of the globe. It may be proper first to explain that, in passing from the north to the south magnetic hemisphere, there is upon every meridian a point at which the intensity, after having gradually diminished, again begins to increase as you advance to the higher magnetic latitudes.
These several points united form a circle round