362 THE CEYSTAL FALLS IRON-BEARING DISTRICT. 



the effects produced upon the magnetic needles are correspondingl}^ com- 



phcated. 



For the purposes of iUustration it is sufficient to consider a few extreme 



H' V 



cases, and to represent the values of — - and — for these graphically. Let 



' -L GO GO 



it first be assumed that the two parallel belts are vertical, that the distance 

 between them is 8, and that azz3, and h = 2. This represents the conditions 

 when the distance of separation is large compared with h. The ordinates to 



XT' 



the curve of PI. XL VIII, fig. 1, give the values of — which correspond to 



the different stations of observation. Those parts of the curve which are 

 above the horizontal axis of coordinates represent the portions of the trav- 

 erse in which H' is directed toward the west; the parts below, those in 

 which it is directed toward the east. It is seen that besides the middle point 

 there are two other points of no horizontal deflection, which do not exactly 

 correspond with the points vertically over the middle of the magnetic forma- 

 tions, but are somewhat nearer the adjoining edges; and also four points of 

 maximum deflection, one on each side of each rock. The maximum points 

 inside of the two formations have smaller deflections than those outside, as 

 shown by the relative lengths of the ordinates to the curve, and also 

 between the inside maximum points the horizontal components are directed 

 away from the middle point. 



V 

 The curve of — , represented by the dotted line, has two maximum 



values, which fall nearly over the two rocks. 



If next we assume that the distance between the rocks is 8, and that 

 a— 3, and //-:8, we obtain the curve of PI. XLVIII, fig. 2, which represents 



TT' 



the value of — when h is large compared with the distance of separation. 



This case shows but one point of no horizontal deflection between the two 

 rocks and but two points of maximum deflection, one on the outside of 



each. 



V 

 The curve of — , represented by the dotted line, shows the interesting 



feature of three maximum points, one at the center and one over each 

 rock. If h were relatively a little greater, these would evidently coincide 

 at the center. 



