Epidemics. 293 



ratios of motion ; but the difference of ratio was sufficient to 

 account for the ever constant change or variation of the 

 magnetic needle towards the Northern and Southern Poles. 



In more modern times we understand that in both hemi- 

 spheres, north and south, the poles have, in relation to the 

 earth's axis, an eastern and western centre of magnetic attrac- 

 tion, or there are two northern magnetic poles and two 

 southern. 



The time in which each completes a revolution of 360 is 

 variable. Dr. Barlow calculates the north-western pole to 

 complete its revolution once in 850 years, and Haustein cal- 

 culates the weaker pole as 860, and the stronger about 

 double that time, or 1,740 in the northern hemisphere, and in 

 the southern 1,307, and the stronger in 4,609 years. The 

 observations and calculations, though accurate enough over 

 a limited period of time, as those of the southern- hemi- 

 sphere, have been very recently made. But it is impossible, 

 from the shortness of time over which these observations 

 have run, to arrive at anything like a just estimate of 

 the time required to complete any given revolution in the 

 southern hemisphere, or eastern side of the northern hemi- 

 sphere. Again, the declination or secular variation of the 

 compass moves from east to west, and back again. In 1580 

 it was slightly to the east, in 1660 it was due north, and 

 1818 to 1823 it attained to its extreme westerly direction, 

 and it has since been moving eastward. 



To complete one oscillation, or complete extension of 

 variation from extreme west to extreme east, it is calculated 

 that a period of 320 years is required, and, of course, to 

 regain the extreme west again would be another 320 years ; 

 which periods, added together, give the epidemic period here 

 maintained as based upon historical data extending over 

 1,900 years, or from the time of Pompey's conquests in the 

 east on to the present time, and not at all upon any mag- 



