DECLINOMETER. XVil 
The adopted effect 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 are 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 aérial 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. 
Unifilar reading, 
E minus 130 Se. Div., fs Means of two 
Copper Ring | Declinometer reduced to Declinometer on, and of two Effect of 
on or away. Reading. Declinometer MINUS Ue away. Copper Ring, 
Se. Diy. ¢. Div. Se. Div. 
275-52 
274-63 . 265-70 
271-16 . 267-35 
271-63 . 265-74 
267-45 : 267-50 
271-56 “02 265-96 
269-02 : 267-50 
270-62 : 265-60 
269-72 . 267-27 
273-38 . 265-23 
271-67 
| The mean of the partial results for the position of the ring during these obser- 
vations = — 1-7 long scale divisions. The position of the ring in this case, how- 
MAG. AND MET. oBs. 1843. e 
