399 



alone as a means of eliminating their influence on the periodic and 

 progressive changes, but also on the independent ground, that " the 

 theory of the transitory changes might prove itself one of the most 

 interesting and important points to which the attention of magnetic 

 inquirers can be turned, as they are no doubt intimately connected 

 with the general causes of terrestrial magnetism, and will probably 

 lead us to a much more perfect knowledge of those causes than we 

 now possess." 



The feature which has been referred to as furnishing the principal 

 if not the only certain characteristic of a disturbance of this class, 

 viz. the magnitude of the departure from the usual or normal state 

 at the instant of observation, has, in the discussion of the observa- 

 tions, been made available for the investigation of their laws : it has 

 afforded the means of recognizing and separating from the entire 

 mass of hourly observations, taken during several years, a sufficient 

 body of observations to furnish the necessary data for investigating 

 at three points of the earth's surface one in the temperate zone of 

 the northern hemisphere, a second in the temperate zone of the 

 southern hemisphere, and a third in the tropics the laws or con- 

 ditions regulating or determining the occurrence of the magnetic 

 disturbances. The method by which this separation has been 

 effected has been explained on several recent occasions, and will be 

 found fully described in the Phil. Trans, for 1856, Art. XV. By 

 processes of this description, the disturbances of principal mag- 

 nitude in each of the three elements, the Declination, Inclination 

 and Total Force, have been separated from the other observations, 

 at the three observatories of Toronto, Hobarton and St. Helena, 

 and submitted to an analysis of which the full particulars will be 

 found in the preliminary portions of the volumes which record the 

 observations. By the adoption of a uniform magnitude as constitu- 

 ting a disturbance throughout the whole period comprised by the 

 analysis, the amount of disturbance in the several years, months, and 

 hours is rendered intercomparable. The result of this investigation 

 (which could not be otherwise than a very laborious operation, 

 since the observations at a single one of these stations, Toronto, con- 

 siderably exceeded 100,000 in number, each of which had to be 

 passed through several distinct processes,) has made known to us 

 that the phenomena of this class, which may in future with propriety 



