628 APPENDIX. 



The variation was determined by means of a Kater's 

 compass made by Jones ; and, when used, great care was 

 taken to remove it from the proximity of any iron or 

 other metallic substance which might be supposed to 

 derange it. 



Owing, I consider, to the great diminution of the 

 directive force acting on the horizontal needle, the 

 variation could not be determined with any degree of 

 certainty after we arrived at the mouth of the Thlew- 

 ee-choh ; but whether the differences in the variation 

 which I obtained at different times of the day were due 

 to sluggishness in the needle, or to an actual change in 

 the direction of the force acting upon the needle, to the 

 amount observed, I will not venture to say, though there 

 cannot be much doubt that the latter cause had some 

 influence. 



The Diurnal Variation. 



The diurnal changes in the direction of the needle 

 were determined with an instrument constructed by 

 Jones expressly for this expedition. 



The instrument consisted of a rectangular brass box, 

 ten inches long, and two and a quarter wide ; with pieces 

 of plate glass at each end, and on the top ; and was per- 

 fectly air-tight. It had two levels, and stood on three 

 foot-screws, by means of which it was levelled. The 

 needle was 8| inches long; and could vibrate in an 

 arc of ten degrees on each side of the magnetic me- 

 ridian. It could be used either vibrating on a centre, 

 or by suspension, or both ; as a pillar, with the ne- 

 cessary apparatus for preventing torsion, screwed on 

 the top of the instrument. There was a small telescope, 

 quite independent of the instrument, for reading off the 



