MAGNETIC EFFECT OF SYMMETRICAL IRON. 153 



at the same level as the compass (making sin b = 0), or 

 below the compass (making cos 5 = 0), the expression 

 for VN vanishes. 



Second. The terms - HM + HP . cos 2 A shew that 

 there is a term of fluctuating value in the meridional 

 direction ; if however P vanishes, that is if all the iron 

 be below the compass, the fluctuation with change of 

 azimuth vanishes. In any case, the force towards the 

 north is affected on the whole by HM : and when on 



/2 



the whole cos b is < , / - , M is positive, and the ter- 

 restrial horizontal force is on the whole diminished. 



Third. The term HP. sin 2 A indicates a force 

 changing its sign in every quadrant, which produces the 

 quadrantal deviation described in Articles 61, 65, 66. 

 It has no existence if the iron is entirely below the 

 compass. It changes sign when cos 2<z changes its sign : 

 that is, when the iron is mainly at the sides of the 

 compass. 



The tangent of deviation actually produced will be 



deviating force , TT1 T7 - ,. 



Y^ . When V occurs as factor in the de- 



_Q 



y 



viating force, the quotient ~ is the same as tangent dip. 



When H occurs as factor, the deviation is independent 

 of terrestrial force or dip. 



The terms affecting the vertical force are of little 

 interest in general. They would be comparable with 

 observations only when the vertical force is accurately 

 examined (as for instance by dip-observations). 



