INTRODUCTION. XV 
to the west and east respectively: before that date they were in the reverse posi- 
tions ; consequently, 
Joint effect of Bifilar and Balance Magnets on the Declination Magnet 
before July 30. 1841 = — 0°30 Se. div. 
after {ot Sige xeoeaho0'30, Se. din: 
14. Effect of the Copper Ring. 
No observations were made to determine the error due to the copper ring till 
1843. Previously, however, it had been placed over the Balance Magnet which 
indicates immediately the presence of the smallest quantity of iron ; but the needle 
remained motionless. 
In 1843, series of observations were made by reading the position of the De- 
clination Magnet with the copper ring, in its place and away ; the results were con- 
tradictory, being, for the effect of the ring, from —1.82 to + 1.89 Se. div., and lead 
to the belief that the effect must be small, if anything. The differences, it was pre- 
sumed, were due to the generation of currents of air by lifting and shutting the box, 
to the changes of Declination occurring at the time, and to the greatly increased are 
of vibration when the ring was removed. 
In July 1843, after an extra Declinometer had been obtained, the results were 
equally contradictory, the differences were attributed to aerial currents as before,— 
the result of the best observations being nearly zero: the ring was therefore allowed 
to remain. 
In October 1844, a careful series of observations was made with all the guards 
of double boxes, &c., when it was found from the consistent results, that the effect 
of the copper ring when in its usual position was equivalent to about — 1-0 Se. div. 
It is now believed that the differences of the partial results obtained in 1843 were 
to some extent due to slight changes in the position of the ring in the different ob- 
servations.* 
The observations for the effect of the copper ring are reserved for the Intro- 
duction to the Volume for 1843. No correction has been applied for its effect in 
1841 and 1842, as the position of the ring must have varied slightly during that 
period. It is believed that the effect must have been between about — 05. 
15. Error of Collimation of the plane glass in the south side of the Declino- 
meter-box. 
No observation was made to determine this error till 1843. The magnet rest- 
ing on blocks, the glass being in its usual position, reversed, and away, no difference 
could be detected in the scale readings. 
* It should be remarked that the method of taking away and replacing, recommended in the 
Report of the Royal Society, does not seem that best fitted for determining the ewistence of a disturbing 
cause. During the Observations of 1844, it was found, that, by slightly varying the position of the 
ring, the effect might be much magnified, or reduced to zero. 
