Terrestrial Magnetic Intensity. 



65 



sity would appear too great (the magnetic energy in iron being 

 increased by cold and diminished by heat). 1 therefore en- 

 deavoured to compare the intensity of the needles employed 

 within the range of temperatures usually observed. The ap- 

 paratus employed was of this kind. The needle was first al- 

 lowed to take the temperature of a heated room and vibrated. 

 Then everything else remaining the same (and of course any 

 local attraction which might exist being unaltered) the appa- 

 ratus was placed in a cylindrical glass jar, with ice in the bot- 

 tom, placed in a dish of ice, and covered with a glass plate 

 also covered with ice. A steady temperature, but little above 

 the freezing point, was thus attained, and the oscillation again 

 observed. These experiments were repeated many times. 

 One series was undertaken at Geneva in October 1832, an- 

 other at Edinburgh on four different days of August 1834. 

 Those for the needle, No. 1, were conducted with the most 

 scrupulous care, nearly 5000 vibrations having been counted 

 for this purpose alone. One set was discarded as differing 

 too much from the others, and the remainder agreed very 

 closely, although made under such different circumstances, 

 and at such different times. The result adopted for needle 

 No. 1 gave an increase in time of '00045 for a diminution of 

 temperature of 1° Reaumur, and vice versa ; for the flat needle 

 (determined from two concordant series, both observed at 

 Geneva on different days,) '00030. From these results the 

 following tables were calculated, giving the reduction in each 

 case to 0° of Reaumur (which being the scale attached to the 

 instrument, was always observed in these experiments). This 

 seems preferable to referring to any other arbitrary tempera- 

 ture, upon which observers do not generally agree. 



Table III. 



Additive corrections applicable iojive place Logarithms of the 



Time, for the effect of Temperature. 



Third Scries. Vol. 11. No. 64. July 1837. 



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