Declinometee. 
xvii 
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 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. 
Taele 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 
= u. 
Declinometer 
minus u. 
Means of two 
on, and of two 
away. 
Effect of 
Copper Ring, 
On 
Sc. Div. 
Sc. Div. 
Sc. Div. 
Sc. Div. 
Sc. Div. 
275-52 
9-51 
266-01 
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 
-171 
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 
318 
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- 
vations = — 1*7 long scale divisions. The position of the ring in this case, how- 
MAG. AND MET. OBS. 1843. e 
