BiFiLAE OR Horizontal Force Magnetometer. xxxiii 



300 sc. div. being added to all the readings for 1845, 340 has been added to all 

 those for 1846, and the same zero, 500, is applicable to both. 



44. The mean time of one vibration of the bifilar magnet, is between 26s and 27^ : 

 the natural arc of vibration is generally very small, and when considerable, the time 

 of vibration was found less than from large artificial vibrations.* 25^ or 26^ has been 

 used in the observations for 1845 and 1846. 



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

 scale coinciding with the vertical wire of the fixed 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, 5, and c, the mean is deduced from the 



formula . • The mean thus obtained is corrected to the temperature of 



26° Fahr., a constant quantity 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 cor- 

 rected means n, given in this volume, are obtained from the formulae 



1845. « = N + 8000 + (^-26°) 1-90 



1846. w = N + 340-0 + (^-26°) 1-975 



1*90 and 1-975 being the temperature coefficients in scale divisions in 1845 and 1846 

 respectively. 



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

 sults, are obtained by the formulae 



1845. /= (n- 500) 0000140 



1846. f=(n- 500) 0-000135 



0-000140 and 0-000135 being the values of k for 1845 and 1846 respectively. 



No correction has been applied for the eflfect 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 f 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. 

 The needle is placed at right angles to the plane of the magnetic meridian, it is ac- 

 curately adjusted to horizontality by a screw e which balances the needle, another 

 screw d working vertically, regulates its sensibility. The apparatus, and a ther- 

 mometer h which gives the temperature of the needle, is covered by a rectangular 

 box k having glazed openings on both sides opposite the spider crosses ; those on one 

 side allowing light to be thrown on the crosses from two small mirrors, (one of which 



* Introduction, 1841-2, pp. xxviii,, xxix. 

 MAG. AND MET. OBS. 1845 AND 1846. i 



