SECTION III. 

 OBSERVATIONS AND DISCUSSION OF THE MAGNETIC INTENSITY. 



THE instrument used (a unifilar magnetometer) has already been described. 

 For the determination of the intensity, the long magnet A. 67 has exclusively 

 been used for oscillations and deflections. The effect of the torsion in the sus- 

 pension was found so small that it was neglected. The vibrations have been 

 observed in sets of two, one containing the readings of the chronometer when the 

 magnet was moving in the direction of the scale readings, and the other when the 

 magnet was moving in the opposite direction. 1 A mean time pocket chronometer 

 was generally used for noting the time, and its rate was too small to affect sensibly 

 the duration of a single vibration. In the deflections, the magnets were always 

 kept at right angles to one another; the distance of the middle of the deflecting 

 magnet, A. 67, from the suspended magnet, is given by a scale divided into feet and 

 decimals of a foot. 2 The observations were made by Mr. A. Sonntag. At Van 

 Rensselaer Harbor the observations extend over the time from January, 1854, to 

 May, 1855. Two other stations were occupied, one in June, 1855, at Hakluyt 

 Island, the other in July, on the coast between Parker Snow Point and Cape 

 York, at the return of the party. 



The necessary constants have been determined at Washington, D. C. 



Magnet A. 67 is nearly three inches in length, the two other magnets, I. 7 and 

 I. 10, are somewhat shorter. 



1 The vibrations given in the Narrative, vol. II., Appendix, No. XV., pp. 431 434, are, therefore, 

 double vibrations, and should have been noted as such. 



3 By some inadvertence, Appendix No. XV. of vol. II. of the Narrative contains the distances ex- 

 pressed in inches; it should have been given in feet and decimals, thus, 13 inches should be 1.3 feet, and 

 9 inches should read 0. 9 feet. 



