DEFECT IN THE THEORIES OF MAGNETISM. 159 



the whole depth of the iron, and in some degree pro- 

 portional to that depth. The results derived in Article 

 68 from the suppositions in Article 63, and the imme- 

 diate results of Poisson's theory, are therefore both 

 erroneous, in regard to the magnitude of the coeffi- 

 cients of the terms representing compass-disturbance. 

 And we are left in complete doubt whether there is 

 or is not a relation or system of relations between 

 the coefficients in the general equations of Article 70. 

 If, however, the inductional effects of external magnetic 

 forces acting in the direction of different co-ordinates 

 admit of being combined by the same law as the super- 

 positions of small displacements, it would appear that 

 formulae similar to Poisson's will hold (with the doubt 

 on relation of coefficients above mentioned), and that 

 the law of quadrantal disturbance in Article 65 and of 

 other disturbances in Article 68, will be unchanged. 



These remarks lead us to a consideration of the 

 really important defect in the present theory of magnet- 

 ism of steel and iron. We possess no information, and 

 no plausible theory, on the permanent distribution of 

 magnetism in a steel magnet, or on the temporary dis- 

 tribution of magnetism in an iron bar affected by 

 external magnetic action. It seems not unlikely that 

 it may be subject to laws something like those of 

 induced electricity. Many of the most important 

 deductions can, however, be securely established with- 

 out that knowledge ; but, till it is obtained, we cannot 

 regard magnetism as possessing the highest claims to 

 regard as a Physical Science. 



