344 The Kev. Dr. Lloyd on the Determination of 



also used as a deflector, and C the deflected needle. The two latter needles 

 should not have their poles reversed, nor be touched with a magnet. 



The observations may be conveniently taken in the following order: — 



1. Needle A is to be placed on the agate planes, the other two needles 

 being removed, and a complete observation of inclination taken in the four 

 usual positions of the needle and limb, and with the poles of the needle direct 

 and inverted. 



2. Needle A is then to be removed, and the loaded needle, B, substituted ; 

 and its inchnation to the horizon, ?/, is to be observed in the four positions of 

 the needle and limb. The deviation of this needle from the position due to the 

 Earth's magnetic force alone is m = 6 - 1/. the angle r} being positive when mea- 

 sured at the same side of the horizontal line with 6, and negative in the contrary 

 case. 



3. Needle B is now to be removed from the agate planes to its supports 

 between tlie microscopes, and needle C substituted ; and the inclination of the 

 latter to the horizon is then to be observed in one position of the needle and 

 limb. The observation is to be repeated with the north end of needle B turned 

 in the opposite direction, by the revolution of the moveable arms which carry 

 the microscopes ; half the difference of the readings in the two positions is the 

 angle of deflection, u'. 



The total intensity is given by the formula 



i. 



B- • ^"^^ 



which is fitted, without any artifice, for logarithmic computation.* 



In strictness a correction is required for the effect of the change of tempera- 

 ture of needle B, in the two observations in which it is employed ; but as one 

 of these observations may be made to follow the other quickly, and as the 

 needle may be placed in both in nearly the same circumstances, the correction 

 may generally be disregarded. It will be necessary, however, that this needle, 



* If the weight be attached to the loaded needle at a fi.xed point, the formula becomes 



T> _ \U Wr . cos 7 

 \ sin u sin u' 



