6g INTRODUCTION. 



in the horizontal direction (according to the length 

 of the needle). If the artist has so accurately 

 balanced the needle that the centre of gravity is 

 in every direction but very little distant from the 

 centre, which is found by the near coincidence of 

 the four observations, it is sufficient, especially 

 when the dip is small, to take the mean as the true 

 dip. But if there are considerable differences, it 

 is necessary to make a particular allowance for the 

 false position of the point of gravity. The mode 

 of proceeding is as follows : 



1. On days when the motion of the ship is 

 small, place the dipping needle on a separate 

 stand upon the deck, as far as possible from the 

 considerable masses of iron on board; (for ob- 

 servations on board ship, it is not a bad way to 

 suspend the instrument in the manner of a com- 

 pass, between moveable rings;) then turn the instru- 

 ment, according to the compass, into the direction 

 of the magnetic meridian ; put the needle in, and 

 observe the different degrees to which the extremes 

 of its vibrations fall. The mean of these vibrations, 

 if they are not altered during the observation by an 

 accidental motion of the vessel affecting them, 

 gives the first observation, in which we suppose the 

 division to turn to the east : we will call it A. 

 Now, turn the instrument round its vertical axis 

 180 degrees, so that the circle of division may 

 point to the west ; the needle will then turn, and 



