368 Prof. NKCKER on the Determination of the 



M. DUFOUR of Geneva, well known by his writings as a mili- 

 tary engineer, has, at my request, pointed out to me the simple 

 graphical process which I am now to explain, and which may be 

 employed in all cases whatever, where the position of two lines 

 in the same plane is given. 



The data in this case are the same as in the preceding, viz. 

 the angles which two seams of the strata taken in two diiferent 

 sections of the rock, make witli their horizontal projection, and 

 the angle which each of these projections make with the mag- 

 netic or with the true meridian. 



To determine with the aid of these data the true dip and di- 

 rection of the plane, it is necessary, first, to bring the angles into 

 the same horizontal plane, then through the intersection of two 

 vertical planes passing through the horizontal projections of the 

 seams, to draw a vertical plane. This plane will cut the plane 

 containing the two seams, and the horizontal plane, at an angle, 

 which is the angle required. 



Fig. 1. 



In Fig. 1 . the vertical plane a b G is placed parallel to the 

 horizontal projection A B of one of the seams. The point A is 



