110 Trans. Acad. Sci. of St. Louis. 



scope and scale. The scale was divided into centimeters 

 and tenths. The scale value was 1 mm=3.37 minutes of 

 deflection of the needle. 



The structure upon which the needle and telescope 

 were mounted, was a frame constructed of 2x4 inch tim- 

 ber bolted together with brass bolts, and the legs or 

 corner posts of the frame extended two feet down into 

 clay and gravel soil. The structure was braced longi- 

 tudinally and transversely, the braces being held in place 

 by large brass screws. The structure thus formed was 

 8 ft. in length in a north and south direction with respect 

 to the magnetic meridian, and 4 ft. in width. 



The needle was deflected 90° from the magnetic merid- 

 ian, by means of two bar magnets 2 ft. in length 

 whose axes made an angle of 45° with the meridian. The 

 resultant field was thus the same as the horizontal com- 

 ponent of the earth's field. This resultant field was 

 then partly compensated by two bar magnets 4 ft. in 

 length, on either side of the needle, and at the ends of 

 the table, about 4 ft. distant from the needle. The final 

 resultant field in which the needle was thus placed was 

 about 0.05 that of the horizontal component of the earth's 

 field. This was determined by the oscillation method, 

 before the damping liquid was introduced. The time of 

 vibration was corrected for the torsional effect of the 

 suspension fiber. 



With this arrangement, the needle is very sensitive to 

 changes in the horizontal component of the earth's field, 

 and it has the advantage of permitting these changes 

 to be observed at any instant. Some preliminary obser- 

 vations were made with the control magnets exposed 

 to the air. The tent was provided with a fly, which per- 

 mitted a free circulation of air between it and the roof. 

 It however became evident that the variation in posi- 

 tion of the needle was materially influenced by a variation 

 in the temperature of the magnets, although the maxi- 

 mum temperature of the day during July and August 

 did not. exceed 85° F. This temperature effect was 



