89 



core was removed altogether, then the induced force remained un- 

 changed, it being precisely the same whether the body were taken 

 hollow or solid. In accordance with this result, hollow cylindri- 

 cal magnets were found as susceptible of magnetic power as solid 

 masses of the same temperament and dimensions ; an unmagnetized 

 solid and tempered steel cylinder, placed within a hollow tempered 

 steel cylinder, does not become magnetic on touching the external 

 cylinder in the usual way. The magnetism, however, of a hollow 

 cvlindrical magnet is partially destroyed by placing within it a cy- 

 linder of soft iron, or the reverse poles of another magnet ; nor can a 

 hollow cylinder of tempered steel having a solid core of soft iron 

 be rendered magnetic by the usual methods of touch. These re- 

 sults, it is considered, supply the experiments thought by Mr. Barlow 

 so desirable to confirm his deductions relative to the action of iron 

 shells and balls on the compass needle, which he found to be as the 

 f power of the surface, whatever the weight and thickness of the 

 iron. 



The author now proceeds to notice the investigations of Hawks- 

 bee, Brook Taylor, Muschenbroek and others, and thinks the inqui- 

 ries of these philosophers have not been sufficiently considered or 

 appreciated ; that instead of the results exhibiting anomalies and 

 discrepancies, they are really necessary consequences of the more 

 elementary laws of induction, and perfectly explicable upon the fun- 

 damental principles of magnetism. He endeavours to show, that by 

 the changes in the law of the induction, as already stated, laws of 

 force will arise perfectly coincident with the results arrived at by 

 Hawksbee, Brook Taylor and others ; that is to say, the law of force 

 may appear to be as the f power of the distance inversely, as found 

 by Brook Taylor; or as the f power inversely, as found by Martin ; 

 or in the inverse duplicate ratio of the distance, as observed by 

 Lambert ; or as the simple distance inversely, as determined by Mus- 

 chenbroek in several cases; or it may be as the cubes of the di- 

 stances inversely, as stated by Newton. Examples are given in 

 which these several laws were found to obtain. 



In examining the laws of magnetic repulsion, similar results are 

 arrived at. The inductive forces here, however, are subversive of 

 the existing polar arrangements ; hence the apparent repulsion : so 

 long as the existing magnetic polarities remain unchanged, the law 

 of force will be generally as the second power of the distance in- 

 versely ; when the distances are small, it will be inversely as the 

 simple distance ; when the inductive actions subvert the existing 

 polarities, then the law of force appears irregular and subject to no 

 regular variation, as observed in all the early experiments with re- 

 pellent poles. 



The author is led to conclude, that the apparent law of attractive 

 force will be found to depend in certain cases on the distances at 

 which the force operates, as referred to the total distance or limit of 

 action. Taken between f ths and -fths of the limit of action, the 

 force may be inversely as the third powers or cubes of the distances ; 

 taken between fths and fths of the limit of action, it may be in the 



