626 APPENDIX. 



needle three inches in length, resting upon hollow 

 curved agates. 



For the purpose of placing the instrument into the 

 magnetic meridian, there was an apparatus, consisting of 

 a cross piece, with a point and ball in the form of the 

 axis of the needle ; and on the point was placed a small 

 horizontal needle; and the instrument moved bodily 

 round (the index for the horizontal circle being placed 

 at zero), until the small needle was parallel 10 the 

 divided or vertical circle. The instrument was then 

 levelled in the usual manner ; but in case any accident 

 should have happened to the level, this operation could 

 be effected by the cross piece, before described, for 

 placing the instrument into the meridian; for, as it 

 acted upon the principle of the pendulum, the point 

 at the bottom of the ball would show, by the division on 

 the circle at 90°, the perpendicularity of the instrument, 

 or the correct horizontal motion. 



The dip was found at Fort Reliance in the usual 

 manner, with needle No. 1., by taking the means of 

 several readings, with the face of the needle to the face 

 of the instrument, and with the face of the needle re- 

 versed, both with the face of the instrument east and 

 with its face west; similar observations being made 

 with the poles of the needle inverted : but in making 

 observations for the dip with the needle No. 2., its 

 poles were in no instance inverted. 



If, then, we consider that the dip obtained with the 

 needle No. 1. is the correct dip at Fort Reliance, it is 

 evident that the dip deduced from the observations 

 there with the needle No. 2. will require a small cor- 

 rection, in consequence of its centre of gravity not coin- 

 ciding accurately with its axis ; and the result obtained 

 with this needle in all other cases will likewise require 



