xvi INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 AND 1846. 
zontal intensity. Dec. 291 193", 1845. Torsion removed, — 11°. Dec.-30% 7° 40™; 
1845. The deflecting bar was vibrated in the declinometer box after which the tor- 
sion was removed, + 74°. Dec. 314 0". 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. 154 233", 1846. Torsion removed, — 9°. Feb. 16a 0*—2", magnet with 
short scale in declinometer box; 3%—5" deflecting bar vibrated in declinometer box. 
Feb. 164 53", 1846. Torsion removed, + 5°. 
April 134 221", 1846. Torsion removed, + 3°. At 23" the magnet with short 
scale was placed in the declinometer box. April 14473”, the deflecting bar vibrated 
in the declinometer box ; at 84", torsion removed, + 84°. 
April 144 20%, 1846. Torsion removed, + 34°. Comparisons of the unifilar and 
declinometer readings were made before and after removing the torsion, the differ- 
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 144 20", 1846, till 
May 124 5°, 1847, when the torsion removed was — 14}’. 
13. The times of vibration of the declination magnets are as follow :— 
Ss. 
The time of one vibration of the declination long scale magnet=17:8* 
Docasnisaemetadsusudeserennaceecneestcectentaretie tise 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, b, and 
a+2b+e 
c, the mean position is deduced by the formula AW The are of vibration is 
seldom above 3’, and is generally less than 2’; during disturbances, however, the 
are 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. 
