506 EDITOR'S NOTES. 



establishing decisively, by observations at a sufficient 

 number of well-selected localities, whether this line of 

 least force, or some other line which according to va- 

 rious hypotheses might be deemed to have a preference, 

 be really the zero line of the mean diurnal variation; viz. 

 the line in which, apart from the semi-annual effect pro- 

 duced according as the sun has north or south declination, 

 the diurnal variation disappears, and in which the semi- 

 annual effect also disappears about the times of the 

 equinoxes, but only then. Those who incline to ascribe 

 all the minor magnetic variations, and prominently 

 amongst them the diurnal variation, to thermic agency, 

 or to thermo-electric currents generated on the surface 

 of the globe or in the higher regions of the atmosphere, 

 might, according to distinctions in their particular views, 

 be disposed to prefer either the terrestrial equator, or 

 some variable line, such as "the parallel of latitude 

 which has the sun in its zenith," as the most probable 

 line from which effects should commence, which are 

 supposed to proceed from the more heated to the 

 less heated portions of the globe. Amongst those 

 who, on the other hand, might deem it more pro- 

 bable that ^effects which are in themselves magnetic, 

 might have a reference in their distinguishing pheno- 

 mena to characteristic lines of the earth's magnet- 

 ism, there would doubtless be many who would be 

 disposed to give a preference to the line of no dip, 

 so frequently termed the magnetic equator. In such 

 important questions we naturally desire indisputable 

 evidence; but this we scarcely as yet possess, though 

 the facts already known appear to point most distinctly 

 to the line of least force as meriting a decided preference. 



