NIPHER — MAGNETIC OBSERVATIONS IN MISSOURI, 1S7S. 95 



One deflection series was thus reduced to the temperature of 

 the other, for which temperature the oscillation series was also 

 corrected. 



Inclination. 



Observations of inclination were all made with dip circle No. 

 9, made by Henry Barrow & Co. of London. It was provided 

 with two needles, marked 2 and 3, each 9^ inches long, and 

 with axles 0.021 of an inch in diameter. The agate planes on 

 which the needles rested were f of an inch apart. The circle 

 was divided to lo", and the readings were made by simple micro- 

 scopes, which enables the reading to be estimated to single min- 

 utes with tolerable accuracy. 



The horizontal circle of the instrument was four inches in 

 diameter, divided to half degrees, and provided with a vernier 

 reading to minutes, but with no tangent screw. 



The instrument was at first set in the magnectic meridian, by 

 taking the mean of the readings of the horizontal circle for all 

 possible reversals, when the angle of dip was 90°, which gives its 

 position when the plane of the ring is at right angles to the mag- 

 netic meridian. This was found to occupy too much time, and 

 it was found afterwards as satisfactory to set the instrument by 

 means of a good pocket compass before introducing the needle. 



At one or two stations, at first, two readings were made for 

 each position of the needle and circle, but thereafter three read- 

 ings were made. This was done by gently raising the axis in the 

 Ts and bringing it down upon the agate planes so as to cause 

 very small oscillations. The polarity of the needles was always 

 reversed just before a series, being magnetized by three strokes 

 on each side for the first observations at any station, and by four 

 for the subsequent reversal at that station. At Columbia, Lou- 

 isiana, Canton, and Keokuk, the needles were, however, stroked 

 only on one side, owing to a misunderstanding of the instruc- 

 tions. 



Following is appended a description of the stations where 

 observations were made, with an abstract of the results obtained. 

 They bear evidence to some local attraction which is known to 

 exist at several points, particularly in the vicinity of Wright City, 

 in Warren Co. 



