COMMENTS AND ADJUSTMENTS. 



Instruments. — The observations for diurnal inequality as well as those for abso- 

 lute declination, were made with a Jones unifilar magnetometer (No. 3), kindly 

 loaned by Prof. A. D. Bache, Superintendent U. S. Coast Survey. The azimuth 

 circle reads to 20" and the centre division of the scale reads 280. The magnet 

 was suspended by means of a silk thread 9i inches in length. Several trials to 

 determine the effect of torsion gave such small quantities that it was not considered 

 necessary to take the same into account. The instrument was not originally 

 intended to give absolute declinations, but at the Winter Quarters the observer 

 succeeded in obtaining a few values for absolute declination by detaching the box, 

 containing the magnet, from the circle which bears the telescope. The same was 

 then moved in azimuth until a well defined object within the small range of its 

 vertical motion could be observed. The focus of the telescope was adjusted to the 

 distance. We find the instrument " perched on a pedestal of frozen gravel," the 

 contents of two barrels. This mounting was considered as stable as the rock 

 imderneath. On the 9th of June, 1854, Mr. Sonntag examined the instrument in 

 reference to local disturbance, and found no sensible deviation arising from such a 

 source. " The local deviation seems to have corrected itself; the iron in our com- 

 fortless little cell seems to have been so distributed that our results were not 

 affected by it." (Narrative, vol. I.) The adjustments were made according to Rid- 

 del's magnetical instructions. The mirror attached to the suspended magnet faces 

 the magnetic north. The following are the determinations for the angular value 

 of a scale division: — 



