Oscillations of the Horizontal Needle. 13 



the magnetic pendulum, those induced in the magnetism of the 

 bar itself, from temperature and other causes, may be considered 

 the most important. The effects of heat in the magnetic tension 

 seem to be extremely uncertain, since it tends more freely to a 

 state of neutrality in some magnets than in others. It is therefore 

 requisite also to examine experimentally, in the way above stated, 

 the influence of change of temperature in each individual bar, so 

 as to discover whether it has an invariable magnetic state or not. 

 The influence of heat also, evidently decreases more rapidly than 

 the magnetic intensity, so that bars of weak intensity seem inva- 

 riable under certain low ranges of temperature. I have hence 

 found it desirable to work with bars of low tension, treated in the 

 way above mentioned (16). The interesting and profound re- 

 searches of Mr Christie*, however, would lead us to infer that 

 certain variable states may occur in magnetic intensity from heat, 

 without a final loss of force, and for which he has endeavoured 

 to deduce adequate expressions. Although such authority must 

 necessarily be highly appreciated, yet the imperfect state of our 

 knowledge in the sciences of electricity and magnetism requires 

 a very extensive examination of exceptions, deducible by expe- 

 riment from any particular instance. 



24. After a careful inquiry into the effects of temperature on 

 certain small magnetic bars employed to determine terrestrial in- 

 tensities, I have not been enabled to discover, by the method of 

 vibration, variable changes in their magnetic state, such, that the 

 tension, being weakened by heat, it could again be restored by 

 cold ; nor have I, in the particular instances which came under 

 my notice, been enabled ever to increase the magnetic energy of 

 a bar by reducing its temperature. On the contrary, the exposure 

 of small bars to very low temperature, seemed, as tested by the 

 method of vibrations, to be attended by a decreased energy. 

 Thus, a small bar of moderate force was observed to make in va- 



* Transactions of the Royal Society for 1825. 



