In 1825, Captain Duperrey had accurately laid down the point of intersection of the magnetic, with the terrestrial equator. In 1837, we learned from Sabine's investigations of magnetic inclination near the Island of St. Thomas (on the Equator, adjoining the above portion of the coast of Africa), that this point of intersection had already shifted four degrees to the westward. A period of twenty years having elapsed since Sabine's expedition for determining observations with the pendulum, it would be most desirable that fresh investigations should be made in that neighbourhood, for the purpose of verifying the secular changes of all magnetic curves, especially with regard to their variation. In 1840, the line of no declination in America began 9° 30' E. of South Georgia, whence it ran to the S.E. coast of Brazil, near Cape Frio, thus traversing the mainland of South America only between the latter point and the parallel of 0° 36' S., when it leaves the continent a little to the east of Gran Parà, near Cape Tigioca, cutting the terrestrial equator again, but in 50° 6' W. According to Bache's Map of Equal Magnetic Declination, it reaches the coast of North America near Cape Fear, to the south-west of Cape Lookout. This line, along which the magnetic declination is _nil_, extends to a point in Lake Erie, 2° 40' W. of Toronto, where the declination is already 1° 27' W.[2] [Footnote 2: Wherever, in this paper, it is not precisely expressed to the contrary, the scale of the Centigrade Thermometer, the longitude from the Meridian of Paris, the French foot (_pied du roi_=12·79 inches English), and the geographical mile, 15 to a degree of the Equator, measuring 3807 "toises," are meant.] It is evident from the observations of Captains Beechey and Findley, and still more particularly from those of the French Captain Kerhallet, that the remarkable subdivision of the main equinoctial current, flowing from east to west into two branches, one directed to the N.W., the other to the S.S.W., commences at a considerable distance from the Capes of St. Roque and St. Augustin. This bifurcation has always, and with good reason, been ascribed to the protruding convexity of the South American continent at these two promontories. It would be an important step gained in verifying the theory of currents, could the precise distance be ascertained by chronometer. It is apparently like an "_actio in distans_," probably a phenomenon of what is known as "packing." As the frigate, on leaving Rio de Janeiro is to make for the Cape of Good Hope, the opportunity will present, should she steer sufficiently southerly, for many interesting observations with respect to the _connecting current_ W.N.W. and E.S.E. which encounters that from Madagascar and Mozambique, close to the Cape, more especially with regard to the temperature of the sea.