1825.] Philosophical Transactions for' iS25, Part L 461- 



the magnet. This is in direct contradiction to the notion which 

 has been entertained of destroying the magnetism of the needle 

 by the application of intense cold. 



" 2. With a certain increment of temperature, the decrement 

 of intensity is not constant at all temperatures, but increases 

 as the temperature increases. 



" 3. From a temperature of about 80° the intensity decreases 

 very rapidly as the temperature increases : so that, if up to this 

 temperature, the differences of the decrements are nearly con- 

 stant, to ascertain which requires a precision in the experiments 

 that perhaps their nature does not admit of, beyond this temper- 

 ature, the differences of the decrements also increase. 



" 4. Beyond the temperature of 100°, a portion of the power of 

 the magnet is permanently destroyed. 



'* 5. On a change of teinperature, the most considerable por- 

 tion of the effect, on the intensity of the magnet, is produced 

 instantaneously ; showing that the magnetic power resides on 

 or very near the surface. This is more particularly observable 

 when the temperature of the magnet is increased, little change 

 of intensity taking place after the first effect is produced ; on 

 the contrary, when the temperature of the magnet is diminished, 

 although nearly the whole effect is produced instantly, yet the 

 magnet appears to continue to gain a small power for some 

 time. 



' " 6. The effects produced on unpolarized iron by changes of 

 temperature are directly the reverse of those produced on a 

 magnet ; an increase of temperature causing an increase in the 

 magnetic power of the iron, the limits between which 1 observed 

 being 50° and 100°. That the effect on iron of an increase of 

 temperature should be the reverse of that produced on a magnet, 

 is, 1 think, a strong argument against the hypothesis, that the 

 action of iron upon the needle arises from the polarity which is 

 communicated to it from the earth. 



'^ It may be objected to the method which I have adopted 

 for determining the diurnal changes in the terrestrial magnetic 

 intensity, that, after the observations have been made, they 

 require a correction for temperature, which can only be deter- 

 mined by experiments previously made on the magnets and 

 needle employed. The same objection may, however, be made 

 against the method of determining the intensity by the vibra- 

 tions of a needle. As such a correction has not in the latter 

 case been hitherto applied, the results which have been obtained 

 relative either to the diurnal changes of intensity, or the intensi- 

 ties in different parts of the earth, by means of observations on 

 the vibrations of a needle, will be so far incorrect as the needle 

 may happen to have been affected by differences in the temper- 

 ature. The method I have described, however, possesses 

 advantages over the other : a very considerable one is, that 



