MAGNETIC OBSERVATIONS. 357 



1. THE MAGNETIC ROCK STRIKES EAST OR WEST OF NORTH AND DIPS 



VERTICALLY. 



Let. US first take the case of a rock striking east of nortli. At the 

 stations within range of the local influence on the east side of such a rock 

 belt the horizontal needle is pulled west of the meridian, reaches a west- 

 ward maximum, then points north, then on the west side of the belt, east 

 of the meridian, and reaches an eastward maximum. It is observed, how- 

 ever, that the westward deflections on the east side of the belt are generally 

 not so great as the corresponding eastward deflections on the west side of 

 the belt. The reason for this is easily seen. At each station east of the 

 belt the local pull acts along the normal to the belt drawn tlirough the 

 station. This normal makes with the local magnetic meridian an acute 

 angle. The needle will come to rest within this acute angle along the line 

 of the resultant of the horizontal components of the two forces, the earth's 

 and the local force, which determine its position. However strong the 

 local pull may be, the horizontal needle can not be deflected past the 

 normal. 



At the corresponding stations on the west side of the disturbing belt 

 the local pull also acts along the normal from the station to the belt, and 

 has the same numerical value. But in this case the normal makes an 

 obtuse angle with the magnetic meridian. For two points equallj^ distant 

 from the magnetic belt, one on the east and the other on the west, the 

 resultant for the western point will, therefore, make a larger angle with the 

 meridian than that for the eastern. 



On the other hand, when the rock strikes west of nortli, it is observed 

 that the horizontal deflections are greater on the east side than on the west, 

 and the explanation is entirely similar to that given above. 



The dip-needle observations at the same stations show general phe- 

 nomena quite like those in the case in which the strike of the rock coincided 

 with the meridian. They gradually increase to a maximum near the sta- 

 tion, where the horizontal needle stands at zero between the converging 

 arrows, and gradually decrease from this maximum on the other side. It 

 is noted, however, that the readings are not equal at corresponding stations 

 on opposite sides of the maximum. When the strike is east of north, the 

 western station shows a higher dip than the eastern; when the strike is 



