to the Aritarctic Renions, ~ 237 



^ts 



moveable weight. The test of the adjustment is similar to that of 

 the corresponding adjustment of the ordinary balance. The move- 

 able wire of one of the microscopes being brought to bisect the 

 cross, if the adjustment is complete, it will bisect the cross at the 

 other extremity upon reversal ; if not, the position of the needle will 

 indicate in what manner the weight is to be moved. 



A horizontal line being thus obtained, the fixed wires of the micro- 

 scopes are to be adjusted to it, by moving the capstan-headed screws 

 with which they are connected. 



4. The last adjustment is that of the magnetic needle itself. This 

 adjustment is twofold: 1. of the needle to the horizontal position; 

 and 2. of the centre of gravity of the needle to the magnetic axis. 

 To effect this double adjustment the needle is furnished with two 

 moving weights, one on each arm. These weights (it has been 

 already stated) are screws moving in fixed nuts, one in a direction 

 parallel to the magnetic axis of the needle, and the other in a di- 

 rection at right angles to it. By the movement of the former the 

 needle is brought to the horizontal position ; and by that of the 

 latter, the centre of gravity is made to coincide with the magnetic 

 axis. The latter part of the adjustment is tested by inverting the 

 needle on its supports ; the inclination of the needle should not be 

 altered by this inversion when the adjustment is complete. 



Observations. — In observing the variations of the vertical force 

 with this instrument, it is only necessary to bring the moveable wire 

 of each micrometer to bisect the opposite cross of the needle ; unless 

 in seasons of disturbance, the needle will be found at each instant 

 to have assumed its position of equilibrium. The interval between 

 the fixed and moveable wires, expressed in angular measure, is the 

 deviation of the needle from the horizontal position ; and the changes 

 of the vertical force are thence obtained by multiplying by a constant 

 coefficient. 



If n denote the number of minutes, and parts of a minute, in the 

 observed angle of deviation, the changes of the force are expressed 

 (as in the case of the other component) by the formula 



-^ = kn; 



in which the constant coefficient is 



k = cos a cotan sin 1'. 

 The quantity F in the preceding formula is the product of the ver- 

 tical component of the earth's magnetic force multiplied by the mo- 

 ment of free magnetism of the needle ; or 



F = mY. 

 Accordingly the results thus deduced require a correction for the 

 effects of temperature upon the quantity m. This correction is 

 similar to that applied to the horizontal intensity ; and the cor- 

 rected expression of the changes of the vertical component is ac- 

 cordingly 



; ^^kn + q{t-Z2); 



