INCLINATION. 597 



by the method of separate touch, and the same operation is made 

 on the other face. We have thus a first magnetisation, which 

 will serve for a series of observations. 



The needle is placed in the compass and the vertical circle is 

 placed in the first azimuth. We shall call that the face of the 

 instrument at which the observer looks in making the readings. 

 If the mark of the needle is above for this first magnetisation 

 the various observations may be described as follows : 



FIRST MAGNETISATION. MARK ABOVE. 

 First Azimuth. 



MARK IN FRONT. MARK BEHIND. 



Face to the S.E. Face to the N.W. Face to the S.E. Face to the N.W 

 (0 (*) (8) (7) 



Second Azimuth. 

 Face to the N.E. Face to the S.W. Face to the N. E. Face to the S.W. 



(3) (4) (6) (5) 



By making the observations in the above order, the needle need 

 only be touched once. 



It is then magnetised in the opposite direction by the same 

 number of passes : the mark will be found below, and the read- 

 ings are made in the same order. 



We may add that for each position we should not be satisfied 

 with a single reading above and below. It is necessary to recom- 

 mence several times, by raising the needle with its stirrup and 

 letting it rest on the agates, and then taking the mean of the 

 readings. 



We may determine the magnetic declination with a less" error 

 than a minute ; but, whatever be the perfection of the instruments, 

 it is difficult to get the dip to within a minute. 



1168. The inclination may be determined from the equation 

 Z = H tan I, if we know the ratio of the horizontal and vertical 

 components of the terrestrial field. 



We might determine, for instance, the ratio of the directive 

 couples MH and MZ of a magnet movable successively about a 

 vertical axis and a horizontal axis perpendicular to the meridian 

 (1191). 



