Balance or Vertical Force Magnetometer. xxxvii 



The amount of change in the time of vibration, for 1° of temperature can only be 

 determined from the changes within short periods, since, 



50. 3c?, The time of vibration diminishes with time. The balance needle was 

 adjusted, Jan, 27*^ 1844, the times of vibration after the adjustment were 



Feb. 1^ — 6^ 1844, Mean time of one vibration, 9s-24 Temperature of needle, 33°-9 



Feb. 191— 27-* 8«-65 32°-0 



Dec. 9<i— 20<i 6^-90 33°-7 



060.121— 1845,.:. 6^03 33°-5 



The temperature of the needle is nearly the same in these cases ; it appears, 

 therefore, that the time of vibration has diminished fully two seconds in the first ten 

 months. This diminution is altogether independent of any variation in the magnetic 

 moment of the needle, since the time of vibration in a horizontal plane remains nearly 

 constant. From the beginning of 1844, to the end of 1845, the mean position of the 

 needle Lad varied about 300 mici-ometer divisions. Since the position of the needle 

 also varies with temperature, it does not at first appear improbable that the variation 

 in the time of vibration is due to the varying position alone. Increasing tempera- 

 ture at the same time raises the north end of the needle and increases the time of 

 vibration ; from the beginning of 1844 till 1846, however, the north end of the 

 needle has been rising, while the time of vibration has been diminishing. It is pro- 

 bable from this, and certain from other observations, that the time of vibration is 

 nearly constant for any angle which the magnetic axis of the balance needle makes 

 with the horizontal. During a considerable magnetic disturbance, April 17'* 1844, 

 observations of vibration were obtained for positions of the balance needle varying 

 400 micrometer divisions, yet the observed time of vibration only varied four-tenths 

 of a second, and that not directly with the inclination of the needle, but from errors of 

 observation and variation of temperature. Such a variation of position, if due to 

 temperature alone, would have required a change of 50° Fahr., which would have 

 produced a change of about 3^'8, in the time of vibration.* 



51. In order to determine more distinctly whether change of inclination of the 

 magnetic axis affected the time of vibration to any considerable extent, the follow- 

 ing observations were made during an adjustment of the instrument. 



January 18, 1848. The balance needle with its magnetic axis in the magnetic 

 meridian, nearly horizontal, mean position + 160 mic. div. 



Arc of vibration at commencement, 32'. Time of one vibration, 8s-05. 



After this observation, turned out the horizontal screw one revolution, which changed 

 the reading from + 160 mic. div. to — 818 mic. div. 



Arc of vibration at commencement, 32'. Time of one vibration, 8s-12. 



* Transactions of the Royal Society, Edinburgh, vol. xvi,, p. 72, Table IV. 

 MAG. AND MET. OBS. 1845 AND 1846. k 



