Declinometee. xix 



is the quantity by which the deviations of the magnet from the plane of detorsion 

 should be multiplied, to obtain the decrements due to torsion. If n be the observed 



(TT. 

 1 + — J n — the true deviation. Following are the observations for the 



XT 



value of — : no use has been made of them for this correction. 

 F 



Jan. 13'^ 2^ 1843. Arc"^ w =\ _qao ; arc~^ u= \ g, , , ; mean value of p, =0-00154. 

 Aug. 7*^ 2^ Arc~^ w—\ _q()o; arc~^ u~ \ n,,^r. ; mean value of -^^ =0'00147. 



16. The second and most important error due to the torsion force is that pro- 

 duced by the varying plane of detorsion. Unless when the period and extent of 

 change is known, it can only be corrected practically. This is done occasionally in 

 the following manner : — The magnet being removed, a brass bar of nearly the same 

 dimensions and weight being suspended, and the box being completely closed, the 

 extremities of the arc of vibration are observed through the glazed lid. The marble 

 slab beneath having radii drawn for every 5° on each side of the magnetic meridian, 

 the position of rest being estimated, its deviation from the magnetic meridian is 

 known, and the arms of the torsion circle are turned an equal amount in the oppo- 

 site direction. Much care and time were bestowed on these observations, the ex- 

 tremities of at least two vibrations being observed, so that the torsion might be as 

 completely eliminated as possible. Some annoyance was experienced by the break- 

 ing of the suspension thread, which was formed of 20 fibres of untwisted silk. The 

 necessity of removing the magnet for the purpose of eliminating the torsion, was, it 

 is believed, occasionally a source of it, owing to the difficulty of holding the thread 

 with a force exactly equal to the weight of the magnet till the suspension of the 

 brass bar, and the consequent liability to strain, or by loosening, to alter the dispo- 

 sition of the fibres ; but much graver errors would have been introduced by leaving 

 the niagnet wholly untouched. 



17. The principal facts relating to the suspension thread. 



Jan. 2*^ 21''. 1843. The plane of detorsion was found + 26° from the magnetic 

 meridian (reckoning deviations to the east of the magnetic north positive). This 

 change, since December 20. 1842, it is probable, was produced in taking out the 

 magnet and inserting the brass bar, as it was found that a fibre' of the suspension 

 thread was then broken ; another fibre was probably broken on again inserting the 

 magnet, as on 



Jan. 6. 1843, the error of the plane of detorsion was found + 30° ; the broken 

 fibres were withdrawn on Jan. 13, and the torsion eliminated. 



May 26* S'^. Two fibres of the suspension thread were found broken ; the tor- 

 sion was eliminated. 



June 16* 2^*. Three fibres of the suspension thread were found broken ; all the 

 broken fibres were removed from the thread;, and the torsion was eliminated. 



