106 TERRESTRIAL MAGNETISM. 



had been the subject of very careful examination by Chris- 

 tie^ 117 ) Arago, Hansteen, Gauss, and Kupffer. But as, 

 notwithstanding the great improvements in the construction 

 of dipping-needles, their time of vibration in the vertical 

 plane could not be ascertained with the same accuracy as 

 that of needles vibrating in the horizontal plane, a know- 

 ledge of the horary variations of the total force could not 

 be obtained from the latter, without a more exact knowledge 

 of the horary variations of the inclination than could be 

 gained by the instrumental means in use before the existence 

 of the magnetic observatories. The establishment of 

 magnetic stations in the northern and southern hemi- 

 spheres has since afforded the great advantage of supply- 

 ing results at once the most numerous and the best 

 assured, with magnetometers measuring the horary varia- 

 tions of the horizontal and vertical components of the 

 force, and thus enabling their theoretical equivalents, 

 the horary variations of the inclination and of the total force, 

 to be learnt. It will be sufficient here to consider two 

 of these stations, ( ll8 ) both extra-tropical, and situated 

 at nearly equal distances on either side of the equator ; viz. 

 Toronto in Canada, in 43 32' N. lat., and Hobarton in 

 Van Diemen Island in 42 53' S. lat.; their difference of 

 longitude being- about 15 hours. In the simultaneous 

 system of hourly observations, the observations of the 

 winter months of the one station are made during the 

 summer of the other, and the greater part of the night 

 observations at the one correspond in like manner to the 

 day observations at the other. The declination is at 

 Toronto 1 33' W., and at Hobarton 9 57' E. ; the incli- 

 nation and force at the two stations are very similar, the 



