INVESTIGATION 



ELEVEN (OR TEN) YEAR PERIOD, AND OF THE DISTURBANCES OF THE HORIZONTAL 

 COMPONENT OF THE MAGNETIC FORCE. 



VOLUME XI of the Smithsonian Contributions to Knowledge contained a discus- 

 sion, in three parts, of the observations for magnetic declination. The first part 

 referring to the eleven (or ten) year period in the amplitude of the solar diurnal 

 variation, and of the disturbances of the magnetic declination ; the second, to the 

 annual inequality of the solar diurnal variation, and the third, to the influence of 

 the moon on the magnetic declination. The present discussion refers to the 

 changes of horizontal force, and will be carried on in the same order as the former, 

 so as to dispense with explanations in the mode of treatment, unless in those por- 

 tions involving the peculiarities of the horizontal force instrument and record. 

 Charles A. Schott, Esq., has rendered me the same assistance in this work, stated 

 in the introduction to Part I. 



The horizontal force instrument was one of Gauss's large bifilar magnetometers, 

 made by Meyerstein, of Gottingen, the weight of the magnetic bar being about 

 twenty-five pounds, and its length being thirty-six inches and five-eighths. The 

 suspension wires were slightly inclined, the smaller distances between them being 

 above the larger. The value of one division of the scale in parts of the horizontal 

 force was determined to be : 



in May, 1840, .... 0.000035 

 in June, 1841, .... 0.000038 



The mean, or 0.0000365 is the value used throughout the series. The sensibility 

 of the instrument was thus very considerable. The instrument having been pro- 

 perly adjusted with the bar at right angles to the mean magnetic meridian, the 

 torsion angle Z was found to be 71 43'. The relation 7c = a cotan. Z expresses 

 the value of one scale division li in parts of the horizonal force, a being the value 

 of a scale division in parts of the radius, or 0.00011 = 0'.38, and Z the angle of 

 torsion. Increase of readings on the scale corresponded to decrease of horizontal 

 force. 



The instruments were placed in position by the equations deduced by Professor 

 Lloyd, for the case of the declinometer in equilibrium with the horizontal and 

 vertical force magnetometers, the position of instablc equilibrium being taken 



