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' 



Exp. /Marked end a S.Pole 53^6 

 I. LMarked end a N. Pole 



24 Readings 66° 23 



Exp. fMarked end a S. Pole 



II. LMarked end a N. Pole 



Exp. /Marked end a S. Pole 



III. l.Marked end a N. Pole 



48.4 



32.8 

 49.6 



46.8 

 52.8 



24 Readings 



24 Readings 

 24 Readings 



24 Readings 

 24 Readings 



72 3o':3}Mean69°26'.8 



72 51 Ig}^^^"^^ ^^'^ 



70 38.8' 

 69 49.2. 



I J' Mean 70 14.0 

 Mean. . . 69 46.1 



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. 



August 13th & 15th. Noon to 3 P.M. Therm. 67°. 



Time of Vib°. 



53^6 



p r Marked end a S 

 IV 



Pole 

 I Marked end a N. Pole 54 .0 



fW. below; 61°55'.5 

 LW. above ; 80 30 .7 

 rW. below; 58 38.6 

 LW. above ; 80 25 .7 



} 



Dip69°48'.3 



MDCCCXXIX. 



H 



