USE OF THE BIFILAR MAGNETOMETER. 



271 



height of suspension, which is of such great importance for the 

 objects of this instrument, renders it very desirable that access 

 to the points of suspension should be rarely or never required. 



acts on the declination-bar C, {or if A C B = 90°, AD=\AB. This latter 

 case is the actual one, because C D is perpendicular to A B (the magnetic axis 

 of the declination-bar must be situated in the magnetic meridian, and the mag- 

 netic axis A B, of the intensity-bar must be placed perpendicularly to it) ; then 

 A C being the half diameter, ^Z) is the sine of A C £>, A B the secant of J5^ C, 

 or the cosecant of ^ CD; consequently, 



AD: AB= sin AC D: cosec ^ CD = Vi : V'S = 1 : 3. 



The direction of the force with which the intensity-bar acts on the declination- 

 bar is therefore that of the magnetic meridian C D : it may consequently have 

 some influence on the time of vibration of the declination-bar, the directive 

 force of which is somewhat changed by it ; but it will exert no influence on its 

 direction, so long as this direction coincides with the assumed mean meridian 

 C D : the deviations from it will, however, be somewhat diminished or increased 

 by this force, according as it acts conjointly with, or in opposition to, the ter- 

 restrial magnetic force ; but even this is provided for if we alter the value in 

 arc of the divisions of the scale, in which the deviations from the mean meri- 



dian are expressed proportionately to the force of direction, i. e. hy^^— ^ . -= — 



T^ VI liT m 



where, according to the above proposition, -j-^, . V2 = -jyrs • cot A C D 



= -j-j: . . ^3 1 designates the magnitude of the directive force produced by the 



intensity -bar, and Tm designates the directive force of the earth. 



The latter assertion, in so far as it relates to the changes of the intensity, is 

 proved by letting fall a perpendicular on C A at C, which intersects the pro- 

 longation of the line C D at E. It then results from the similarity of the tri- 

 angles A CD, ABC, E AD, EC A, that E D = i E C, because A D was 

 equal to ^ A B ; 



North 

 C 



consequently, if CD is bisected in F, CFz=i CE. Now, as all that has been 

 Baid of A, AB, C,A B C,AD, and C D, is true also of C, C E,A,CAE,C F, 



and A F, it results that A F is the direction, and ^ . i^, is the magnitude 



CF AC^ ^ 



of the force with which the declination-bar acts on the intensity-bar. If now 



