Declinometer. 



xvii 



The adopted eflfect of both magnets, and, therefore, the correction of the scale 

 reading at the magnetic axis for the zero, = + 0-16 long scale divisions, or + 0-14 

 short scale divisions. 



12. Correction for the effect of the copper ring or damper. 



In 1843 many series of observations were made in order to determine this cor- 

 rection ; the reading of the declination magnet was observed with the copper ring 

 in its place, and when removed from the box. The results were contradictory, 

 being, for the effect of the copper ring, from — 1-82 to + 1-89 long scale divisions, 

 and led to the belief that the effect was small, if anything : the differences in the 

 results, it was presumed, were due to the currents of air generated in lifting and 

 shutting the box, to the changes of declination occurring at the time, and to the 

 greatly increasing arc of vibration when the ring was removed. In July 1843, 

 after an extra declinometer was obtained (§ 4), the results were equally contradic- 

 tory, the differences being attributed to aerial currents, as before. The result of 

 the best observations being nearly zero, the ring was therefore allowed to remain. 



Oct: 7. 1844. Careful series of observations were made, every precaution was 

 taken, with the aid of the double boxes, to prevent internal currents of air, and the 

 position of the ring on the marble slab was marked, so that it occupied exactly the 

 same place after each removal. It is conceived that it was owing to a failure in the 

 latter precaution that many of the irregularities of the previous results were due. 

 In the following series, each of the declinometer and unifilar readings is a mean of 

 two or three series of comparative observations, the series being made at intervals 

 of about ten minutes. 



Table 5. — Observations to determine the effect of the Copper Ring or Damper on 

 the Long Scale Readings of the Declination Magnet. 



Copper Ring 

 on or away. 



Declinometer 

 Reading. 



Unifilar reading, 



minus 130 Sc. Div., 



reduced to 



Declinometer 



= M. 



Declinometer 



minus u. 



Means of two 



on, and of two 



away. 



Effect of 

 Copper Ring. 



On 



Sc. Div. 



275-52 



Sc. DiT. 



9-51 



Sc. Div. 



266-01 



Sc. Div. 



Sc. Div. 



Away 



274-63 



7-38 



267-25 



265-70 



-1-55 



On 



271-16 



5-76 



265-40 



267-35 



-1-95 



Away 



271-63 



4-18 



267-45 



265-74 



-1-71 



On 



267-45 



1-37 



266-08 



267-50 



-1-42 



Away 



271-56 



4-02 



267-54 



265-96 



-1-58 



On 



269-02 



3-18 



265-84 



267-50 



-1-66 



Away 



270-62 



3-16 



267-46 



265-60 



-1-86 



On 



269-72 



4-36 



265-36 



267-27 



-1-91 



Away 



273-38 



6-30 



267-08 



265-23 



-1-85 



On 



271-67 



6-57 



265-10 







The mean of the partial results for the position of the ring during these obser- 

 ns = — 1-7 long scale divisions. The position of the ring in this case, how- 



MAG. AND MET. OBS. 1843. e 



