BiPILAR OE HOEIZONTAL FOKCE MAGNETOMETER. 



XXXI 



Table 4. — Observations of Deflection for the Determination of the Coefficient of 



the Bifilar Magnetometer. 



37. 5th, In May and August 1847, a theodolite magnetometer by Mr Jones of 

 London was converted into a unifilar horizontal force magnetometer, the suspended 

 bar having been deflected through an angle {u — S) as in Dr Lamont's method, the 

 variations of horizontal force were deduced from the scale readings, reduced to angu- 

 lar measure by the formula 



aX 



= — cot (m — a) (au — All) + g (t — 'o) 



where u is the angle which the deflected magnet makes with the astronomical meri- 

 dian (negative when to the east) and 8 is the westerly declination, a u being the arc 

 value of the change of reading, and a 8 the arc value of the simultaneous change of 

 declination obtained from the declinometer : q being the temperature coefficient of 

 the deflecting bar ( = 0-00021), io the standard temperature of the deflecting bar, and 

 t the temperature of observation. 



In May 1847, only three comparisons of the two instruments were obtained 



