XVI 



Introduction to the Makerstoun Observations, 1844, 



Observatory was washed and cleaned. The vapour produced during the washing, it 

 is conceived, may have affected the suspension thread as found Nov. 11"^. 



Nov. 1^23"— 0". Torsion removed, + 8^°. Dec. 25'' 23''— 0^ Torsion removed, 

 - (iV- Dec. 2G'' O"" — 4''. Short scale magnet used in the declinometer, and deflecting 

 bar vibrated : at 22'', observations were made for the value of the torsion coefficient 

 of the suspension thread. Upon removing the short scale magnet after Dec. 26'' 4'", 

 the fil)res of the suspension thread became loose, when it is probable that the torsion 

 found at 23'' was introduced. 



Dec. 2G'' 23'" — 0\ Torsion removed, + 55°. Dec. 29'' 23''. Torsion removed, 0°. 



Dec. 30'' O*" — 3''. Short scale magnet used in the declinometer box, and deflect- 

 ing bar vibrated, after which the torsion was removed as follows : — 



Dee. 30* 3"— 4". Torsion removed,- 14°. 



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 18 seconds before the minute of observation, at the minute, and 

 18 seconds after the minute : the scale readings at these periods being a, b, and c, 



<7 + 2 6 + c 

 the mean position is deduced by the formula j . This method was employed 



till Oct. 15, 1844, when the copper ring for checking the vibrations having been re- 

 moved, the three scale readings afterwards were always made at the extremities of 

 the ai'cs of vibration ; the first reading being made at that extremity of the vibra^ 

 tion which occurred between 27 seconds and 9 seconds before the minute of obser- 

 vation. Even after the copper ring was removed, the arc of vibration was small, 

 seldom above 3', and 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 : — 



The middle wire of the theodolite telescope is brought to coincide with the ver- 

 tical wire of the fixed reading telescope (7.); the three verniers of the horizontal 

 circle are then read ; the theodolite telescope is turned (on the vertical axis of the 

 circle) until its middle wire coincides with the vertical line on the north meridian 



* See Introduction, 1843, p. xxi. 



