MAGNETIC NEEDLE IN LONDON, IN AUGUST 1828. 49 
ance as an ordinary needle, with its performance when used in the manner 
recommended by the Professor. 
For the purpose of distinction, one of the ends of the needle had a mark 
upon it which the other end had not. In consequence of the axis of the needle 
not being perfectly coincident with the centre of gravity, the dip shown by it 
was too great when the marked end was a North Pole ; and on the other hand, 
when the unmarked end was a North Pole, the dip shown was too small. After 
the two first experiments with the needle, this discrepancy was in great mea- 
sure removed, by taking off a very small quantity from the marked end of the 
needle by means of a grind-stone. The following are the results obtained 
with this needle used in the manner described : 
August 11th & 12th. From Noon to 4 P.M. Therm. 66°. 
Time of Vib n . 
Exp. /Marked end a S. Pole 53 s .6 24 Readings 66°23'.3\ M MT , rQ o 9fi ia 
I. LMarked end a N. Pole 48.4 24 Readings 72 30 .3 J 
Exp. /Marked end a S. Pole 52.8 24 Readings 66 23.1\ M ro r 
II. LMarked end a N. Pole 49.6 24 Readings 72 51.9J 1 L / ° 
Exp. 
III. 
Mean ... 69 46 .1 
/Marked end a S. Pole 46 .8 24 Readings 7 0 38 .81 M n 
LMarked end a N. Pole 52.8 24 Readings 69 49 .2 J 1 ean ' 
The same needle was then fitted with a small screw, inserted vertically in a 
line with the axis, and perpendicular to the needle ; and brass beads of dif- 
ferent sizes were successively used in the following experiments, in the manner 
and for the purposes recommended by Professor Meyer. Three distinct ob- 
servations were made with the weight undermost when the face of the instru- 
ment was towards the east, and the same number with the face towards the 
west, both ends of the needle being read off on all occasions : the same process 
was then gone through with the weight uppermost ; and finally the poles were 
changed, and the whole proceeding repeated. The dip is then deduced from 
the observations by the formula given for that purpose by Professor Meyer. 
Exp. 
IV. 
{ 
August 13th & 15th. Noon to 3 P.M. Therm. 67°. 
Time of Vib n . 
Marked end a S. Pole 53 s .6 
Marked end a N. Pole 54 .0 
/W. below ; 61°55'.5 
LW. above ; 80 30 .7 
fW. below ; 58 38.6 
LW. above ; 80 25.7. 
>Dip 69° 48'.3 
MDCCCXXIX. 
H 
