374 Prof. NECKER on the Determination of the 



which, though it may be thought at first rather complicated, will 

 soon be attained by a little practice, and found very simple. 



Let the given lines or seams be, 



1st, A line dipping 45 S. b. E., or rising 45 N. b. W., which 



is the same thing. 

 d, A line dipping 56 E, b, N., or rising the same number 



of degrees W. b. S. 



We have, first, to observe, that, as has been said before, the 

 hinge or diameter of the brass semicircle correspond to the di- 

 rection of the stratum, in which direction all the seams appear 

 horizontal. Secondly, that the line marked on the semicircle 

 perpendicularly to the hinge or diameter, is the horizontal pro- 

 jection of the true line of dip or inclination of the stratum, and 

 that this true line of dip is, of all the lines which may be drawn 

 in the plane, that which makes the greatest angle with its hori- 

 zontal projection. The horizontal projection of this line perpen- 

 dicular to the hinge I shall name the Index Line. 



Now, we must begin by supposing, that, of the two lines 

 given, the one that dips with the greater angle may be the true 

 line of dip, and accordingly we will direct the index line to the 

 W. b. S. of the brass circle ; then placing the protractor vertically 

 on the index line, we will move upwards the half ring till its 

 sharpened edge comes in contact with the 5Gth degree of the 

 vertical arc. Looking then to the direction of the other given 

 .line, or to the N. b. W. of the compass, and moving the protrac- 

 tor around the centre till it conies to be vertical upon this line, 

 we will look whether the edge of the ring touches its 45th 

 degree, for, in that ase, the position of the ring would cor- 

 respond exactly to that of the required plane. But, in the pre- 

 sent instance, it is not necessary even to move the protractor to 

 see that the N. b. W- Jine is that which corresponds to the dia- 



