xvi Inteoduction to the Makeestoun Obseevations, 1845 and 1846. 



zoutal intensity. Dec. 29-* 191\ 1845. Torsion removed, - 11". Dec. 30*^ 7" 40"°, 

 1845. The deflectinc; bar was vibrated in the declinometer box after which the tor- 

 sion was removed, + 7^°. Dec. 31^ O**. Torsion removed, 0°. The suspension 

 thread found slightly stretched ; it was wound up about ^ inch, and torsion removed, 

 + 12°. The readings of the declinometer before and after the removal of the tor- 

 sion were compared with the readings of the unifilar in the intensity house ; allow- 

 ance being made for the torsion removed, both comparisons gave the same differ- 

 ence of readings. 



Feb. 15* 231^ 1846. Torsion removed, - 9°. Feb. 16<i O"^— 2^ magnet with 

 short scale in declinometer box ; 3^ — 5"^ deflecting bar vibrated in declinometer box. 



Feb. 16* 5i^ 1846. Torsion removed, + 5°. 



April 13*^ 22i^ 1846. Torsion removed, + 3°. At 23^ the magnet with short 

 scale was placed in the declinometer box. April 14*^ 7^^, the deflecting bar vibrated 

 in the declinometer box ; at 8^^, torsion removed, + 8^°. 



April 14*^ 20^ 1846. Torsion removed, + 3^°. Comparisons of the unifilar and 

 declinometer readings were made before and after removing the torsion, the difi'er- 

 ences of the readings agreed when allowance was made for the torsion removed : 

 these comparisons shew that no torsion is introduced during the manipulations con- 

 nected with the exchange of magnets, and the suspension of the brass bar. 



The declination magnet remained untouched from April 14* 20*', 1846, till 

 May 12* 5^, 1847, when the torsion removed was — M^". 



13. The times of vibration of the declination magnets are as follow : — 



s. 



The time of one vibration of the declination long scale magnet=17'8* 

 short scale =17-0 



14. The observations of the declinometer were made in the following manner : — 

 The points of the scale which coincided with the vertical wire of the reading tele- 

 scope were noted at the extremity of the magnet's vibration occurring between 

 27 seconds and 9 seconds before the minute of observation, and at the extremities 

 of the two following vibrations : the scale readings at these periods being a, 6, and 



c, the mean position is deduced by the formula j . The arc of vibration is 



seldom above 3', and is generally less than 2' ; during disturbances, however, the 

 arc is often considerable, and frequently in these cases only two observations are 

 made, the mean position being obtained at once from the mean of the two readings 

 at the extremities of the vibration which occur between 18^ and 0^ before, and between 

 0^ and 18^ after, the minute of observation. 



15. All the observations of declination in this volume are absolute. They are 

 rendered so as follows : — 



* See Introduction, 1843, p. xxi. 



