Declinometer. xv 



This value has not been used for this correction ; it has been employed in the 

 observations of absolute horizontal intensity made in 1845 and 1846. 



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

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

 of change is known, this can only bo corrected by removing it ; this is done occa- 

 sionally in the following manner : — The magnet, with its stirrup, being removed, a 

 brass bar and stirrup of nearly the same weight and dimensions is suspended ; the 

 rectangular box being removed, tlie cylindrica] box being completely closed, and the 

 shutters removed from the glazed lid, the extremities of the arc of vibration are 

 observed through the latter ; the marble slab beneath having radii drawn for every 

 5" on each side of the magnetic meridian, the positions of rest at the extremities 

 of the arcs are estimated to ^° ; the deviation of the mean position from the magnetic 

 meridian is known, and the arms of the torsion circle are turned through an equal 

 angle in the opposite direction ; the position of rest is then in the magnetic meri- 

 dian. Much time and care were bestowed upon these observations, as the error 

 due to this cause is by far the most serious that occurs in connection with the de- 

 clinometer. 



12. The following are all the observations for the elimination of torsion made 

 in 1845 and 1846, together with all the occasions on which the magnet was touched 

 or removed from its box. When the mean position of rest for the north end of the 

 brass bar was found to the east of the magnetic north, the torsion existing is con- 

 sidered positive, and the effect of+ 10° of torsion= — 0''84 (from the previous obser- 

 vation for the value of o.) In the first case below, the north end of the brass bar 

 was found to rest 7^° west of the magnetic north ; and the brass bar was thereafter 

 made to coincide with the magnetic meridian. 



Feb. 5*^ S^^ 1845. Torsion removed, — 7^°. The torsion could not have been 

 more than 3° at first, but the north end of the brass bar moved slowly westward as 

 if afi'ected by currents, or as if the suspension thread were affected by moisture. 



March 23d 231^^ 1845. Torsion removed, - 181°. 



April 2* 2^\ 1845. Torsion removed, - 3^°. The line of detorsion of the 

 supension thread varied during the observation ; at first it was about N. 3° E. and 

 S. 3° W. ; ultimately it was N. 5° E. and S. 5° W. The torsion circle was turned 3^°. 



May 8<i 31^ 1845. Torsion removed, - i°. June 18"* 2i^ 1845. Torsion re- 

 moved, + 1^°, 



Sept. 21*^ 22^'', 1845. Torsion removed, + 19°. This amount of torsion may 

 possibly be due to the dampness arising from washing the floor of the Observatory. 

 September 20"^, the brass bar was suspended, with some difficulty in the manipula- 

 tion, but it is not conceived that much torsion could have been thus introduced. 



Dec. 29^ 01^ 1845. Torsion removed, 0°. The magnet with the short scale 

 was placed in the declinometer box after this examination of torsion, the long scale 

 magnet being removed to the intensity house for an observation of absolute hori- 



