xxxiv Introduction to the Makekstoun Observations, 1844. 



45. The observations of the bifilar were made as follows : The point of the 

 scale coinciding with th(' vertical wire of the tixed telescope is estimated to the 

 tenth of a scale division at 25^ before the minute of observation, at the minute, and 

 25= after it ; the three readings being a, b, and c, the mean is deduced from the 



formula ,— -• The mean thus obtained is corrected to the temperature of 



26" Fahr., a constant quantity of 300'0 has been added to all the corrected means. 

 If N be the observed mean, and t be the observed temperature of the bifilar bar, 

 the corrected means n, given pages 1 to 170, are obtained from the formula 



« = N + 300-0 + (t-26°) 1'90 



1-yO being the temperature coefficient in scale divisions. 



The means / in parts of the whole horizontal force given in the abstracts of re- 

 sults, pages 355 to 373, are obtained by the formula 



/= (n- 500) 0000140 

 0-000140 being the value of k for 1844. 



No correction has been applied for the effect of the balance magnet, which is 

 constant. 



Balance or Vertical Force Magnetometer. 



46. The balance magnetometer was made by Robinson of London ; it is com- 

 posed of a magnetic needle cfc, 12 inches long, about | inch broad, and about i 



inch thick, with knife-edged axle/, which rests upon agate planes ; brass rings c- c are 

 attached to the extremities of the needle, each ring carrying a cross of spider threads. 



