of Terrestrial Magnetism . 101 



in respect to the particular times of their occurrence when viewed 

 singly, they were in their mean effects strictly periodical pheno- 

 mena, exhibiting by the character of their periodical variation a 

 dependence on the sun as their primary source. To this im- 

 portant fact the disturbances of each of the three magnetic 

 elements — the declination, the dip, and the intensity of the 

 magnetic force — bore concurrent testimony, although the hours 

 of maximum and minimum of their respective diurnal progres- 

 sions were dissimilar, confirming the belief of distinct pheno- 

 menal laws in the disturbances of each of the elements. 



The knowledge of the existence of periodicity in respect to the 

 hours of the day was followed by the recognition of a periodical 

 variation of a more remarkable kind, but which did not at first 

 seem to connect itself with any other known phenomena, either 

 cosmical or terrestrial. The employment of a constant quantity 

 at each station, as a measure of disturbance at that station, had 

 been adopted w r ith the view of learning whether the aggregate 

 amount of disturbance in different years was subject to any note- 

 worthy variation. It soon appeared that this was the case. The 

 observations made hourly at the colonial observatories, situated 

 in very distant parts of the globe from each other, concurred in 

 showing a progressive annual increase in the amount of disturb- 

 ance from the year 1843 to the years 1848 and 1849, the two 

 last being the years when the period for which the Government 

 had sanctioned the hourly observations terminated. The in- 

 crease was far too great to be ascribed to an accidental cause, 

 the ratio of disturbance in 1848 and 1849 being more than 

 double that in 1843 and 1844. The progressive increase was 

 shown in each of the three elements, each having a distinct in- 

 strumental apparatus. The consistency of the evidence from so 

 many sources seemed decisive as to the fact of a progressive 

 increase during the years which have been named, wearing the 

 appearance of being a portion of a periodical variation of which 

 the cycle had yet to be discovered. Regret might well be felt 

 that at this conjuncture the course of hourly observation, by 

 which we had been thus far guided, had to be broken off, by the 

 termination of the period for which the colonial observations had 

 been sanctioned. Fortunately another indication was available 

 which included a rather longer interval. It appeared that the 

 increase of disturbances from 1843 to 1849 had been accom- 

 panied by a similar, though smaller, increase in the magnitude 

 of the range of the diurnal oscillation of the three magnetic 

 elements in those years, after due elimination had been made 

 of the greater part at least of the effects of the disturbed obser- 

 vations. A less onerous system of observation, which had been 

 commenced two years before and continued two years longer 



