WRIGHT: A NEW DIP CHART 



441 



graphical method of projection* or by use of the standard pro- 

 jection-equation 



tan C = sin B .tan A (1) 



in which A is the true angular dip of the bed; B, the angle included 

 between its line of strike and the vertical section; C, the desired 

 angle of apparent dip as shown in the vertical section. 



A chart for the graphical solution of this equation was first 

 proposed by D. F. Hewett;- and more recently the same chart 

 (with a few additional curves and on a reduced scale) has been 

 published by H. Bancroft.^ On this chart the abscissae are 

 the azimuth angles B; the ordinates, the true angular dips, A; 

 and the curves, the angles C. For the C-curves below 80° the 



C <4 5° 



C > 4 5 



Fig. 1. Diagram to illustrate the relations underlying the construction of 

 the dip-chart (fig. 2). The sides are considered to be of unit length. The sine 

 values from to 1 (angles from 0° to 90°) are plotted directly as abscissae; tangent 

 values from to 1 (angles from 0° to 45°), as ordinates. The third variable is 

 represented by radial lines which pass through the origin and the tangent divi- 

 sions on the unit ordinate. 



interval is 5°; from 80° to 90° it is 2°. Under favorable con- 

 ditions {B > 20°, C < 60°) the angle C can be read off directly 

 with an error of about 0.5°; for dips greater than 60° the error 

 may exceed 1°. This degree of accuracy is sufficient for most 

 purposes. 



It is possible, however, to obtain more accurate results (cor- 

 rect to 0.1° under favorable conditions) by use of a diagram 

 (fig. 2) similar to that which was described sometime ago by the 

 writer in a paper on ''Graphical Methods in Microscopical Petro- 



1 Economic Geology, 9: 55. 1914. 



2 Economic Geology, 7: 190. 1912. 



' Bull. Am. Inst. Mg. Eng., p. 1769. July. 



1914. 



