124 



ABSOLUTE OBSERVATIONS AXD ADJUSTMENTS. 



The following deflections of the bifilar magnet with the unifilar magnet (N 5) were observed 

 for the determination of the scale value of the instrument. 



Table 7. 



The scale value appears therefore to have been practically constant throughout the whole 

 period of the observations, and = • 000251 X. 



Declinometer. 



The declinometer, one on Lamont's principle, having a cylindrical magnet 2 '5 in (6 cm.) 

 in length was adjusted by suspending the magnet by a bundle of unspun silk fibi'es 

 and (after the instrument had been levelled and the torsion removed from the suspension 

 thread) bringing the reflection of the central division of the scale into coincidence with the 

 cross wire of the telescope. 



Each division of the scale was = 60" • 6, and since of the coefficient of torsion ^^ varied 



from • 00266 to • 0044, the value of one scale division ranged between 60" • 76 and 60" • 87. 



In the reduction of these observations the scale divisions have been taken as minutes ; the 

 recorded deviations are therefore too small by about 1'3 per cent. 



Once finally adjusted, this instrument, like the bifilar, was left untouched until dismounted 

 on the morning of the 1st September 1883. 



Balance Magnetometer. 



The instrument for observing the variations of vertical intensity was a Lloyd's balance 

 magnetometer with 12-inch (30 cm.) magnet. It was adjusted by levelling the base slab and 

 bringing the magnet into the plane of the magnetic meridian. 



It was soon found that the magnet was largely afi'ected by changes of declination, and 

 required continual re-adjustment to bring it back into the meridian. 



The slow oscillation of this long magnet was a frequent source of error in reading off' its scale. 



The scale value was determined from the times of vibration of the magnet observed both in 

 the vertical and horizontal planes, which were 16 (/') and 10 (t) seconds respectively. The value 



of the ratio -^ was therefore 2*56, and the resulting value of one division of the scale 

 "0000093 Y. 



