BALANCE OR VERTICAL FORCE MAGNETOMETER. XXXVI 
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. 274 1844, the times of vibration after the adjustment were 
Feb. 1°— 6%, 1844, Mean time of one vibration, 9°24 Temperature of needle, 33°-9 
Pepe Wein oS costes stecteunenee tiecanthe SOD sae oath cette emote 32°°0 
Dec: ee OR ch a co Sxitadesedalt gst deatbneenadpetatided GE DO ne Pees RR lk te 33°°7 
Deere LSAT, 1.2.8. oe. acrasnnesacseneceueseosee E(UBI) “eens veacooneratnoscoceuenenne 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 had varied about 300 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 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 174 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. 
Are of vibration at commencement, 32’. Time of one vibration, 85-05. 
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, 8*-12. 
* Transactions of the Royal Society, Edinburgh, vol. xvi., p. 72, Table IV. 
MAG. AND MET. oBs. 1845 AnD 1846. k 
