Balance or Vertical Force Magnetometer. xxxix 



48. 1st, The first conclusion that may be obtained from this Table is, that after 

 the needle has been vibrated by any means through a large arc, its time of vibra- 

 tion has been increased ; this will be apparent from the observations of vibration 

 before and after April 30 d 7 h , July 24 d 18 h , July 26 d 3 h , November 5 d , and Novem- 

 ber 8 d 21 h . On all these occasions the needle was vibrated through large arcs, 

 either from the accidental approach of iron, or from the removal of the box for a 

 short period.* 



49. 2d, It is at once obvious, that the time of vibration depends upon the 

 temperature of the needle, a change of + 1° of temperature causing a change of from 

 + s- 05 to S '10 in the time of one vibration. 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. 3d, The time of vibration diminishes with time. The balance needle was 

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



Feb. l d — 6 d Mean time of one vibration, 9 S, 24 Temperature of needle, 33 0, 9 



Feb. 19 d — 27 d 8 s -65 32°0 



Dec. 9 d — 20 d 6 s -90 33°-7 



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 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 con- 

 stant. During the same period, the mean position of the needle had varied about 160 

 micrometer 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 temperature 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 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 dis- 

 turbance, April 17 d , 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 S '8, in the time of 

 vibration. f 



* See Transactions of the Royal Society, Edinburgh, vol. xvi., p. 69, Table I. 

 t Ibid., p. 72, Table IV. 



