( 97 ) • J 



On the Uniform Perrneahility of' aU knmon Substances to the 

 Magnetic Injiitence^ and tlie Application of the fact in En- 

 gineering and Minings for the Determination of the Thick- 

 ness of Solid Substances not otherwise Measurable. By the 

 Rev. William Scoresby, F. R. S. Lond. & Edin., Corres- 

 pondent of the Institute of France, &c. &c. Communicated 

 by the Author. Concluded from p. 334 of precrding Vol. 



Investigation op the law of the directive power of bar- 

 magnets, AND the extent TO WHICH THE MAGNETIC IN- 

 FLUENCE MAY BE CARRIED FOR MEASURING DISTANCES. 



2. X HE laiv of the directive poioer of har-magnets^ at different 

 distances, was the next subject of investigation. 



Coulomb, I believe, was the first to establish, by the test of 

 satisfactory and consistent experiments, the previously assumed 

 law, that the force of magnetic attraction and repulsion is in 

 the inverse ratio of the squares of the distance. The applica- 

 tion of this law to the investigation in hand served at once to 

 verify the law, and to render the results of my experiments of 

 general application. In regard to the comparison of distances, 

 it appeared to me to be of considerable advantage to estimate all 

 distances in lengths of the bar made use of, by which the re- 

 sults for any one bar became applicable to all other bars of a 

 proportional form and quality, and state of magnetic energy. 

 And such, therefore, with a certain modification, afterwards 

 found to be necessary, was the measure constantly adopted. 



Placing now the magnet in the direction of the east or west 

 point of the compass, or at right angles to the magnetic meri- 

 dian, I proceeded to ascertain experimentally the deviations pro- 

 duced, either by the same pole constantly, or by the mean of 

 the action of each pole alternately, first at the distance of one 

 length, and then successively at other distances to the extent of 

 ten lengths of the bar. 



Preparatory, however, to a general application of the results 

 thus obtained, it will be useful to ascertain by calculation the 

 actual force exerted by any magnet on a compass at different 

 distances, according to the above law of attractions. 



VOL. XIII. NO. XXV. JULY 1882. G 



