398 



nicety, so as to determine with perfect precision the mean state of the 

 elements at the two extremes of the period embraced ; which, as 

 already observed, presupposes a knowledge of the casual deviations." 



It is clear from these extracts that in the discussion of the obser- 

 vations 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 to the elimi- 

 nation of their influence on the observations from which a correct 

 knowledge 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 phe- 

 nomena 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 un- 

 known manner connected, either as cause or effect, with the ap- 

 pearances of the aurora borealis. The chief feature by which the 

 presence of a disturbance of this class could be recognized at any 

 instant of observation, or by which its existence might be sub- 

 sequently inferred independently of concert or comparison with other 

 Observatories, appeared to be, the deflection of the needle from 

 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 



