Major-General Sabine on Colonial Magnetic Observatories. 299 
its usual or normal position to an amount much exceeding what 
might reasonably be attributed to irregularities in the ordinary pe- 
riodical fluctuations. The observations which had been made on 
the disturbances anterior to the institution of the Colonial Obser- 
vatories had been chiefly confined to the declination. A few of the 
German Observatories had recently begun to note the disturbances 
of the horizontal force; but as yet no conclusions whatsoever had 
been obtained as to their laws: in the words of the Committee’s 
Report, the disturbances “apparently observe no law.” By the 
instructions cited above, the field of research was enlarged, being 
made to comprehend the disturbance-phenomena of the three ele- 
ments; and the importance of their examination was urged, not 
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 
