6o8 MAGNETIC FIELD. 



The oscillations of the bifilar in the meridian may again be 

 combined with those which would be obtained in another azimuth. 

 If the bifilar is deflected through an angle 0, near 90, the director 

 couple A', determined by the oscillations, is (1147) 



S ' 



sin 6 sin (w - 0) ' 



We have then 



and consequently 



A, sin <o-0 sin0 



. 



A sin w sin w 



When the angle 6 is exactly 90, it follows that 



i - cos w A, 



<H = -J 



cos w A 



and the ratio of the director couples A : and A' is given by the 

 ratio of the squares of the corresponding numbers of oscillations 

 N x and N'. 



1180. It is generally assumed that the coefficients / and <f> are 

 equal, and the true influence of the magnetic moment under the 

 influence of the earth is determined. 



Lament* arranged the magnet on the frame of a magnetic 

 theodolite in a vertical position, so that one of its ends is near 

 the horizontal plane which passes through the needle of the de- 

 clinometer, and this bar is always in a plane perpendicular to the 

 needle at the moment of observation. 



The pole S of the magnet being alternately above and below, 

 the resultant magnetic moment acquires the values M(i+/Z) and 

 M(i-/'Z); the corresponding deflections a and a satisfy equation 



sn a sn a 



If the induced magnetic moment remains sensibly the same 

 when it is parallel or contrary to the rigid magnetism, then, ob- 



* LAMONT. Handbuch des Erdmagnetismus, p. 152. 1849. 



