Bifilar or Horizontal Force Magnetometer. xxvii 



29. Giving the means of groups, column 8, the weights in column 9, we find 



Mean value of X for 1843 reduced to mean bifilar reading for 1843, =3*3752 



for 1844 1844, =33801 



These give the secular change from 1843 to 1844 in parts of X, =0*00145 

 The secular change from the bifilar magnetometer, p. 356, =0*00389 



From the observations, therefore, of the absolute value of X it would appear 

 that the secular change indicated by the bifilar magnetometer from 1843 to 1844 is 

 too great. 



Bifilar or Horizontal Force Magnetometer. 



30. This instrument was made by Grubb of Dublin, and is similar in its general 

 construction to the declinometer, having two boxes, gilt internally and externally as 



in the latter instrument. The magnet a «, whose dimensions are 15 inches, § inch, 

 and \ inch, is placed in a stirrup b, which carries below it a tube c, having a lens d 

 at the extremity next the reading telescope, and a glass scale at the other : the scale 

 has 280 divisions, and the graduation at the 300th division ; increasing readings of 

 the scale indicate increasing magnetic force ; the axle of a grooved wheel e fits into 

 the suspension eyes of the stirrup b ; the magnet, with these appendages, is borne 

 by a silver wire/, passing round the grooved wheel, and having its two extremities 

 pegged into a suspension roller g : the roller is supported by the torsion circle h, 

 which also bears, beneath the roller, a micrometer-headed screw i, right-handed where 

 it meets one extremity of the wire, left-handed where it meets the other. The screw 

 serves to render the distance of the extremities of the wire equal to the diameter of 

 the grooved wheel ; the screw and suspension roller turn with the verniers k. A 

 copper ring encircles the magnet, in order to check the vibrations. A thermometer 

 by Adie and Son is enclosed by a glass tube passing through both boxes, the stem 



