270 Mr. Abraham on Magnetic and Electric Influence, Sfc. 



have long anticipated, — ahiiost an identity of these two extra- 

 ordinary fluids. Every natural and artificial magnet is sur- 

 rounded by a magnetic atmosphere ; consequently the " great 

 globe itself" must be encompassed by an attractive power, 

 which decreases in an unknown ratio, as its extent from the 

 earth's surface increases. If the particles composing the mag- 

 netic atmosphere of an artificial magnet be sufficiently subtile 

 to penetrate a block of marble or a stone wall of half a yard 

 in thickness, so as to disturb the repose of a magnetic needle 

 on the opposite side of the block or wall (which is easily 

 proved to be the case, by a bar magnet and a delicate needle), 

 and on removing the interposing body, if the needle be af- 

 fected by the atmosphere of the same magnet at a greater 

 distance (which is a fact), it proves that free space is necessary 

 for the magnetic body to act with full force upon any object 

 within the sphere of its attraction. 



If an atmosphere or influencing power extend to the di- 

 stance of 50 or 60 inches from the poles of a small bar of 

 steel rendered powerfully magnetical by artificial means, to 

 what distance must the polar influence of the terrestrial mag- 

 net extend ? 



Were we in the possession of the ratio in which the magnetic 

 power decreases in either a natural or an artificial magnet of 

 a certain pffwer, we ought likewise to have a knowledge of 

 that force or power in the magnet submitted to experiment, 

 to enable us to calculate its action upon other substances at 

 any given distance from its point of greatest force of attraction 

 or repulsion, and likewise the properties of the bod}' experi- 

 mented upon. 



In the works which I have read upon this interesting sub- 

 ject, I do not recollect having met with a theory which would 

 enable us to ascertain the extent of the magnetic influence of 

 our globe in any given latitude. Professor Poisson remarks, 

 that " the magnetic power of the earth, like that of all other 

 magnets, is the product of two factors ; one of which depends 

 upon the distribution of the two fluids, the boreal and the 

 austral in its interior ; and the other, common to all substances 

 capable of magnetization, expresses the intensity of attraction 

 and repulsion at a unity of distance, and between quantities 

 of fluid also taken as unity. It may therefore vary for two 

 different reasons; because the particular magnetic state of the 

 terrestrial splieroid has changed, or because the mutual action 

 of the particles of the magnetic fluid weakens or strengthens 

 in all substances capable of retaining magnetism." 



A knowledge of the ratio and extent of the magnetic at- 

 mosphere of our earth in any latitude, appears to me so ne- 

 cessary 



