Scientific Newst Accounts ef Books, &c. $li 



SCIENTIFIC NEWS, ACCOUNTS OF BOOKS, 8[c. 



Method of afcertalnlng the Inclination of the Magnetic Needle. . By ClT> CoULOMM. 



HOUGH the inftrument which is ufually employed to meafure the- inclination of the. 

 magnetic needle is very fimple in its conftrudion, it is never thelefs liable to great errors, 

 which in general arife from the almoft abfolute impofiibility of placing the needle in all the 

 pofitions it can take in equilibrium, with regard to thfe efFeft of gravitation, that is to fay, 

 fo that its centre of gravity may always exa£lly agree with the point on which it turns.- 

 When the dimenfions are confiderable, a new inconvenience arifes from a degree of 

 flexure which, though fcarcely fenfible, is nevenhelefs produftive of very great efFeds from 

 the flighted difplacement of the centre of gravity producing a combination of the power of 

 gravitation with that of magnetifm. 



To obviate thefe difficulties, Cit. Coulomb, inftead of endeavouring to afcertain im- 

 mediately, as has been hitherto done, the direction of the magnetic needle ,in the vertical 

 plane which paffes through the magnetic pole, conceives the force of this pole to be de— 

 compofed or refolved into two others in the fame plane, the one a6ling in a horizontal,, 

 and the other in a vertical direftion. He determines feparately the intenfity of each of 

 thefe lafl: forces, and the refult gives the direftion in which the magnetic force a£ls, and. 

 which a needle governed Cngly by this force would take. 



Cit. Coulomb has proved, in the Memoirs of the Academy of Sciences for the year- 

 1789, that the magnetic needle fufpended by its centre of gravity is inceflantly brought 

 back to its true direflion by a confliant force at the fame place and time. It thence follows 

 that by obferving the number of ofcillations made in a givea time by a needle horizontally 

 fufpended, the ratio of the hgrizontal component part of the magnetic power with gravity 

 may be obtained. As to the vertical component part, it is meafured by determining with 

 care the weight neceflary to be added to the fouthern part of the magnetic needle,, to 

 maintain it in a perfe£tly horizontal pofition. That being done, if A and B reprefent the 

 refpe£live naeafures of the horizontal and vertical component parts of the magnetic power 



-^ will be the tangent of the angle made by their refult with the horizontal force, and,, 



confequently, it will be the inclination of the magnetic needle. 



In the experiments made by Cit. Coulomb the needle had the form of a right angled 

 parallelipipedon, very thin in proportion to its breadth, and always fufpended, fo that its 

 breadth was kept in a vertical plane. Let P reprefent the weight of the needle, / the half 

 of its length, ^ the length of a pendulum that performs its ofcillations in the fame time as 

 the needle when it; obeys the magnetic power in an horizontal plane. Cit. CoulomB then 



Vol. IV.— February 1801.. 3X. givea> 



