608 Foreign and Miscellaneous Intelliyence. 



on each side of it, that its true figure has been deduced. If we 

 could determine the inclination of the dipping needle at any place, 

 then we could, by a trigonometrical formula, ascertain the latitude of 

 that point where the dip would be nothing ; or, in other words, the 

 corresponding point in the magnetic equator. The formula which 

 we employ for the purpose is the following : The tangent of the 

 magnetic latitude is equal to half the tangent of the inclination of 

 the dipping needle. 



From these principles it is found that the node of the magentic 

 equator, or that point where it crosses the equator, is near the island 

 of St. Thomas, about 3 20' to the east of the meridian of Paris. It 

 then advances rapidly towards the north-east, across the continent 

 of Africa, and the Red Sea at the Straits of Babelmandel. It then 

 stretches almost parallel to the equator for a short distance, and 

 gradually declines, passing through the south of Hindostan, and 

 touching the northern extremity of the island of Ceylon. It then 

 stretches in an irregular line through Malacca, the northern point of 

 the island of Borneo, stretching onwards to the south of the Caro- 

 linas, and again crossing the equator at about 175 degrees east of 

 Paris. It then passes on, making a small angle with the equator, 

 till it reaches about 100 degrees west from Paris, when it begins 

 to deviate rapidly from the equator, and sweeps through South 

 America, at its greatest distance from the equator, being about 

 sixteen degrees. It then rises, in a very irregular line, through the 

 Atlantic Ocean, till it reaches the island of St. Thomas. 



By tracing this line on a map it will be seen, that the two points 

 at which it crosses the equator are almost diametrically opposite; 

 that in the ocean it declines very little from the equator, but where it 

 approaches islands it feel their influence, and its deviation increases ; 

 and when it reaches the massy continents of Africa and America, 

 their influence seems to be powerfully felt, and its deviation from the 

 equator becomes greatest. If the magnetic equator be the resultant 

 of electric currents, circulating perhaps at no very great depth below 

 the surface of the earth, then it is obvious that such a country as 

 South America, abounding with metalliferous veins, ought to have a 

 decided influence on the needle, and here we observe that its de- 

 viation is the greatest*. 



13. NEW DIPPING NEEDLE. 



In dipping needles formed for ascertaining the dip in different lati- 

 tudes, the axis must be made cylindrical. But in the one made by 

 M. Gambey, at Paris, to be used at St. Petersburgh, the axis is a 

 knife-edge, as in a fine hydrostatic balance. The edge is placed 

 exactly in the centre of gravity of the whole compound needle, and 

 so fixed, that when the needle dips 71 the edge rests perpendicularly 

 on two agate plates. It can, therefore, only be employed at those 

 places where the dip is but a little more or less than 71, and is 

 intended to ascertain minute variations of the inclination at the same 

 place f. 



* Annales de Chimie et de Physique. f Idem. 



