236 



Correspondence — Mr. W. H, Penning. 



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APPARENT A]S^D TEUE DIP. 



Sir, — Although the principle involved is always the same, the 

 methods given below for finding the true from the apparent dip are, 

 I believe, new ; and in the hope that they may be found by others 

 as serviceable as by myself, I send them for insertion in the Geo- 

 logical Magazine. 



To find direction of dip. — When the amount and direction of ^ two 

 lines of apparent dip are known, the direction of the true dip'may 

 be found by one of the following rules, A or B. 



Bide A. — When both the observed dips incline from, or towards^ 

 the angle enclosed by their lines (Fig. 1), the true dip is at right 

 angles to a line a h laid down by the following method. 



Fig. 1. 



Set off from the angle on each of the two lines of apparent dip, a 

 number of units corresponding to the number of degrees of dip 

 observed along the other line, and connect the two points by a line 

 a h. 



This line, a b, coincides with the strike, and is consequently at 

 light angles to the true dip's direction. 



Bide B. — When one observed dip inclines from, and the other 

 towards the angle enclosed by their lines (Fig. 2), the true dip 

 follows a line c d laid down by the following method. 



' Fig. 2. 



Set off from the angle on each of the two lines of apparent dip^ 

 a number of units corresponding to the number of degrees of dip 

 observed along that line, and connect the two 2:)oints by a line c d. 



This line coincides with the true dip's direction. 



