Progresi of Foreign Science. 335 



present our readers with a view of the discussions on Light, 

 which now occupy so keenly the attention of some leading 

 members of the Academy of Sciences. . . . , 



Maqnetism. Coulomb determined the law of magnetic at- 

 tractions to be inversely as the square of the distance by 

 making a magnetic needle viljrate, at different distances from 

 the magnetic pole of a bar, and counting at each station, 

 the nun^ber of vibrations. Professor Hansteen of Christiana 

 has lately applied the same method, to discover if the absolute 

 magnetic'force of the earth be an uniform or variable quantity, 

 for any particular place on its surface. He suspends a mag- 

 netized cylindric rod, 2 inches and | long, and ^^ ot an 

 inch diameter, by a single thread of a silk cocoon, de- 

 pending from the top of a brass tube, fixed m the centre 

 of the lid of a brass box. On either side of this vertical 

 tube the lid of the box is formed of glass, to permit the gra- 

 duated arcs at the bottom to be seen. This mode of suspension 

 has also, we observe, been adopted by M. Arago, lor his very 

 delicate needle mounted on the Royal Observatory ot 1 aris ; 

 and, it is considered by him as infinitely superior to the 

 suspension on a point, whose form is liable continually to 

 change by the effect of friction. Professor Hansteen sets his 

 needle in movement by presenting to one of its poles a piece ot 

 iron, which is immediately withdrawn. He then counts, by a 

 chronometer, the period of every ten vibrations up to 360; and he 

 compares each equal portion of the whole to find the mean 

 period Thus he takes the difference between the end ot the 

 first, and the end of the 300th; the end of the second, and 

 that of the 310th; and so on, till that of the 60th and the 

 360th Did not the resistance of the air gradually diminish 

 the arc of the vibrations (till towards the close, it becomes only 

 two degrees), the above measures would be equal; but m 

 consequence of this resistance, the last measure of the time is 

 found to be about -r»ij of a second less than the first. Ihe 

 longest period of 300 oscillaUons observed by M. Hansteen 

 was 813.6"; this time he considers as corresponding to hiS 

 minimum or zero of magnetic force. In fact, this observation 

 was made during an aurora borealis. But the force of the 

 magnetic power is inversely proportional to the squares of the 

 times of similar viI)rations of the same magnetic needle ; and, if 

 * T T' be in two experiments, the observed times of equal vibra- 

 tions of the sauic ne.dle, and if I 1' denote the corresponding 

 forces of U,c»,ag„cUcpo«r; thea I : I' llj^^!^^^^ 



ifx this case I = /813.6y 



From numerous observations of the above kind, which the 



* 111 A Idler to Mr. R.iniker lately of Hamburg, of which an abstract 

 isgivtu by I'fufessor Cilbcrt, iu his Aunals for last July. 



