MAGNETIC OBSERVATIONS. 345 



again the westward pointing- in its tuna gTadually decreases, nntil finally 

 the needle reaches its normal eastward declination, after passing- throug-h a 

 second zero. The dip-needle readings at the same stations generally 

 increase slowly at first, and then rapidly, and soon reach a maximum at the 

 first zero point between the converging arrows ; beyond this to the east they 

 decrease correspondingly, so that the dip cui've is symmetrical east and west 

 of the maximum These statements will be made clear by a reference to 

 fig. 15, which represents an actual traverse in T. 45 N., R. 31 W. 



2. DEFLECTIONS OF THE HORIZONTAL NEEDLE. 



It is evident that in crossing a rock belt which stretches away indefi- 

 nitely in both directions, only a limited part of it will afi"ect the readings on 



v.^ 



Fig. 15.— Masnetic cross section in T. 45 N., R. SI W. 



a given cross section. Since the pull of the poles of a magnet on a com- 

 pass needle diminishes with the square of the distance of separation, it 

 follows that the limits to the material that would noticeabty disturb com- 

 paratively insensitive instruments would soon be reached. If we consider 

 for the moment only the horizontal components, and call the distance a 

 (fig. 16) at which the needle would respond to the attraction of material 

 possessing the magnetic force of that with which we are dealing, then at 

 any station, P, the material inside a chcle di'awn with P as a center and 

 radius a (shaded in the figure) would exert force on P, the material outside 

 would not. If the circle drawn from a station, P', does not reacli to the 

 magnetic belt, the needles at P' will not be disturbed.-' 



For reasons of symmetry, it is seen that the attraction of the magnetic 



' The actual distances at which disturbances of the needles can be detected are exceedingly 

 variable, since they depend (as will be shown hereafter) not only upon the lithological character of 

 the magnetic formation, but also upon its strike, dip, thiclcness, extent, and nearness to the surface. 

 One formation in which all the conditions are exceptionally favorable distinctly deflects the dial- 

 compass needle at a distance of 3+ miles. 



