Horizontal Intensity of the Earth's Magnetism. 121 



millims*. The latter number must be increased to 459*02 mil- 

 lims. (that is, by 0*45 millim.),in consequence of the local action 

 exercised by the bifilar galvanometer. 



From this is calculated the angle of deflection of the bifilar, 



and of the tangent- compass, 



6 = i arc tan |||^| = 3° 33'-20. 

 T ' 3681-6 



If now T denotes the horizontal intensity of the terrestrial 

 magnetism, i the intensity of the current, we get for the bifilar 

 galvanometer the equation 



D<£ = £TFcos<£t, 

 from which 



zT=6'7881. 



For the tangent-compass we have 



r 

 from which 



i 



^l + l5)cos^=T(l + ^ r )sm0, 



T = 1-986.9. 



From this is calculated 



z = 3-6717, T = 1-8488. 



Interposing six mercury units, the deflections were 218*68 

 and 232*79 millims. respectively, whence 



•3>' = l°34'-80, 



<£'=l°48'-65, 

 and 



i-^ 1-8704, T = 1-8499. 



T is here expressed in Gauss's absolute measure, and i in Weber's 

 absolute magnetic units. 



The concordance of the two values for T leaves nothing to be 

 desired. Their mean value is 1-8493. Calculating from the 

 secular formula the value of T for Gottingen, Oct. 23, 1868, we 

 have 1*8452. There is thus found, from the magnetic observa- 



* These numbers are the means of the deflections represented by 

 greater and smaller numbers. 



t The directive force is the elasticity of the suspending-wires — which is 

 corroborated by the fact that the time of oscillation seems independent of 

 the amplitude. Hence the statical moment of rotation is proportional to 

 the angle of deflection. 



