Balance or Vertical Force Magnetometer. xli 



Aug. 24. 1843. The temperature experiments were repeated ; tlie brass rings 

 containing the spider crosses were removed, and the needle was immersed in water 

 of different temperatures ; at the end of the observations the needle was found con- 

 siderably tarnished ; the stains were partially removed by rubbing with crocus. 

 After the temperature observations the needle was vibrated horizontally, and then 

 adjusted. When the needle was placed on the agate planes it was altogether out of 

 balance, the S. end falling till caught by the Y ; the horizontal screw was then 

 turned in till the reading of the micrometer was about 200 minus; both microme- 

 ters read the same, and the N. end was much better in focus than before. 



Sept. 1. 1843. The balance needle was again removed in order to determine 

 the temperature coefficient, the previous observations not being satisfactory. 



Sept. 2. Temperature observations continued, the needle being placed in water 

 as before. The stains were partially removed by rubbing with crocus, 



Sept. 4. After vibrating the balance needle horizontally, and determining the 

 value of the divisions of the micrometer head, observations were made to verify the 

 horizontality of the fixed wires in the micrometers, when it was found necessary to 

 move the fixed wire of the right micrometer down 8 micrometer divisions. 



The micrometer heads were exchanged for convenience of reading, as the read- 

 ings are almost always negative, or the north pole of the needle is generally above 

 the horizontal. 



57. The deviation of the line joining the spider crosses from the magnetic axis 

 of the needle was determined, by repeated reversals, to be + 8 micrometer divisions ; 

 merely lifting the needle by the Ys was found to give readings differing sometimes 

 20, 30, and even 40 divisions. The horizontal screw was moved till a negative 

 reading of about — 130 was obtained ; and as it had been previously found that 

 lifting by the Ys altered the reading, the needle was not left till consistent read- 

 ings were obtained after the different lifts. The following were the readings at each 

 lift after the last movement of the horizontal screw : — 



- 120, - 126, - 121, - 120, - 127, - 138, - 134, - 131, - 134, - 136, - 131. 



September 26"^ 4'' — 6^, 1843. The balance magnetometer box was removed in 

 order to be covered, externally and internally, with gilt paper. 



November 13, 1843. The balance needle was again removed for the purpose of 

 determining its temperature coefficient; the brass rings and spider crosses were 

 removed, and the needle immersed in water as before. The needle was adjusted 

 the same day. The value of the micrometer division was also determined, and the 

 deviation of the line joining the spider crosses from the magnetic axis was found 

 — 36 divisions. The needle was left reading about — 60 divisions. 



58. Mode of observation and reduction. 



The moveable wire of the right micrometer is made to bisect the spider cross 5" 

 before the minute of observation, and that of the left micrometer 5* after the minute 



MAG. AND met. OBS. 1843. I 



