MATHEMATICS AND NATURAL PHILOSOPHY. 457 



period embraced ; which, as already observed, presupposes a knowledge of the 

 <easual deviations." 



It is clear from these extracts that in the discussion of the observations, the first 

 point, in the order of time, ought necessarily to be an investigation into " the 

 laws, extent, and mutual relations of the 'transient and," (as they were called at 

 the time the Report was written,) {'irregular changes," as a preliminary step w 

 the elimination of their influence on the observations, from which a correct know- 

 ledge and analysis of the progressive and periodical changes were to be obtained. 

 It will be proper to show therefore, in the first place, what the Observatories have 

 accomplished in regard to the so-called casual or transitory variations. 



Casual Variations. — All that was known regarding these phenomena at the 

 period when the Report of the Committee of Physics was Written, was, that there 

 occurred occasionally, and, as it was supposed, irregularly, disturbances in the 

 horizontal direction of the needle, which were known to prevail, with an accord 

 which it was impossible to ascribe to accident, simultaneously over considerable 

 spaces of the earth's surface, and were believed to be in some unknown manner 

 -connected, either as cause or effect, with the appearances of the aurora borealis. 

 The chief feature by which the presence of a disturbance of this class could be 

 recognised at any instant of observation.-^or by which its existence might be sub- 

 sequently inferred independently of concert or comparison with other Observa- 

 tories,— -appeared to be, the deflection of the needle from its usual or normal' posi- 

 tion to an amount much exceeding what might reasonably be attributed to irre- 

 gularities in the ordinary periodical fluctuations. The observations which had 

 been made on the disturbances anterior* to the institution of the Colonial Observa- 

 tories bad been chiefly confined to the declination. A few of the German Obser^ 

 vatories 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 elements; and the impor- 

 tance 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 know- 

 ledge 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 observations, 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 ob- 

 servations 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 

 pa the temperate zone of the southern hemisphere, and a third in the tropics — the' 

 laws or conditions regulating or determining the occurrence of the magnetic dis- 

 turbances. The method by which this separation has been effected has been ex- 

 plained on several recent occasions, and will be found. fully described in the Phil- 

 VL. II.-rF* 



