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MAGNETIC OBSERVATIONS ON THE BOUNDARY LINE 



to obtain the corrections to be applied when no reversal of poles was made. These 

 corrections are -)-12' for needle No. 1, and — 15' for No. 2, in the position of the poles 

 called direct, and have been so applied in the following abstract of results. 



The difference of about 16' between the results by the two needles is too large to 

 admit of the mean being taken. An examination of the pivots ha\ing shown some 

 corrosion on a pivot of No. 2, the probability is that the results by No. 1 are to be 

 preferred, especially as the observations with that needle show a greater consistency 

 among themselves. The results by No. 1 are therefore adopted in the general table. 



Observations for horizontal intensitij were made with a unifilar magnetometer by 

 Jones. The deflections were measured on a nine-inch circle according to Lamont's 

 method, in which the two magnets remain at right angles. In the vibration experi- 

 ments, the time of -100 vibrations is usually observed, of which six values are obtained 

 in each set, in the usual way. Experiments to determine the moment of inertia of the 

 magnet and its attachments were made repeatedly by vibrating it loaded with a brass 

 ring, marked No. 11, of which the outside diameter is 2.899 inches, the inside diameter 

 2.462 inches, and the weight 719.90 grains ; hence its moment of inertia, K, = 9.04 

 expressed in feet and grains. The magnet used, marked X 7, is of the usual form, 

 being a hollow cylinder of 0.3 inch in diameter, and 3.66 inches in length, and carrying 

 a mirror. Its temperature coefficient was determined at Washington City in February, 

 1856, by vibrations at temperatm-e near 30° and 70°, and was ascertained 



q = 0.0003. 



The following is an abstract of the observations ; to which is to bfe added, that, in 

 the experiments without the inertia ring, the effect of 90° of torsion was 7'.5 ; in those 

 with the ring, 15'. 



