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LVII. On certain hregularitics in the Vibrations of the 

 Magnetic Needle produced by partial Warmth; and some 

 Remarks on the Electro-Magnetism of the Earth. By 

 Robert Were Fox*. 



TN prosecuting some inquiries relative to the intensity of the 

 -*■ terrestrial magnetism, I have been not a little perplexed 

 by the anomalous results which the vibrating needle aiForded, 

 especially when it was removed from one station to another; 

 and in order to ascertain the cause of these discrepancies, I 

 instituted a series of experiments, some of which I shall ven- 

 ture briefly to mention. 



I had a box made of sheet copper, leaving one side open, 

 which was afterwards covered with glass for the purpose of 

 observing the vibrations of a magnetic needle delicately sus- 

 pended in it by unspun silk. This box, the glass side ex- 

 cepted, was inclosed in a copper case of much larger dimen- 

 sions, with sufficient space between them to admit of my sur- 

 rounding the former with water at any given tempei'ature. 



The needle I employed was six inches long, and vibrated 

 as follows : — With water at the temperature of 



130°, it made 40 vibra.in 163". Commenced with an arc of 90°, 



and ended with one of 28° 

 So ... do. 163 do. 90° do. 30-5° 



54- ... do. 163 do. 90 do. 31° 



These results appeared to me to be at variance with the 

 prevailing opinions of the influence of temperature on the vi- 

 brations of a magnetic needle, and induced me to enter into 

 further investigations of this subject. 



I next placed a wooden box, containing a needle ten inches 

 long, on a heated block of granite, a thermometer having been 

 put into the box to ascertain the temperature. At the tem- 

 perature of 



95°, it made 80 vib. in 510"-5. Arc at first 90°, & ended with24;° 

 72 ... do. 509-5 ... 90 ... 24 



55 ... do. 509 ... 90 ... 26-5 



A box of slate containing a light needle, ten inches long, upon 

 heated granite also. At the temperature of 



85°, it made 30 vib. in 176"-5. Arc at first 40°, & ended with 9° 

 60 ... do. 177 do. ... 13 



A box of slate containing a heavier needle, nine inches and 

 a half long, under similar circumstances. At the tempera- 

 ture of 



* Communicated by the Author. 



