314 - Royal Society : — 



hours by the means above stated, we should be prepared to expect 

 that the ratios which are below unity do not represent the actual 

 variations of the disturbing influences at those hours quite so purely 

 as do the ratios which are above unity ; and that they are liable to 

 be affected, though in a very subordinate degree, by the abstraction 

 of the neutralized portion, when the aggregate values which they 

 represent are very small. 



Without, however, resting undue weight upon this suggestion, we 

 may safely say that the hours, when the ratios are below unity, are 

 hours of comparative tranquillity, and that their variations from hour 

 to hour are of afar less marked character than during the hours when 

 the ratios exceed unity. Thus viewed, the character of the disturb- 

 ance-diurnal variations may be conceived to have some analogy with 

 that of the phenomena of the regular solar-diurnal variation. We may 

 imagine the disturbance-variation (either the westerly or the easterly, 

 it is indifferent which is taken), — divided as it is into two portions, by 

 the ratios being in the one case above, and in the other below unity, — 

 to correspond in one of its divisions to the hours when the sun is 

 above the horizon, in the part of the hemisphere where the disturb- 

 ance may be imagined to originate, whilst the other division, or that 

 in which the ratios are below unity, and manifest hours of compara- 

 tive tranquillity, may be viewed as the hours of night at the same 

 locality. The solar hours at a station of observation which are 

 characterized . by disturbance ratios above unity, will in such case 

 correspond in absolute time with the hours of the day at the sup- 

 posed originating locality, modified (it may be) by a more or less 

 rapid transmission of the disturbance. It will be understood, that 

 in this hypothetical suggestion, the purpose in view is to aid the 

 imagination, if it may be so, in apprehending the ensemble of the 

 phenomena as far as they are yet known to us, rather than to ad- 

 vance a theoretical explanation, when we have not yet sufficient facts 

 before us by which it may be judged ; it may be remarked, however, 

 that the conception of a double locality of origination of the disturb- 

 ances (easterly and westerly) in the one hemisphere will present no 

 especial difficulty to those who are conversant with the general facts 

 of terrestrial magnetism. 



If our attention be limited to the consideration of the facts observed 

 at a single station, unaccompanied by a view of corresponding pheno- 

 mena elsewhere, we might be in danger of regarding some of the 

 features, particularly perhaps those which are not the most pro- 

 minent, as having an accidental rather than a systematic origin ; 

 and we might thus lose a portion of the instruction which they 

 may otherwise convey. On this account it has appeared desirable 

 to exhibit the phenomena as observed at a second station, in com- 

 parison with those at Kew ; and I have selected for this purpose the 

 results of a similar investigation to the present at Hobarton in 

 Tasmania ; not only because the facts have been remarkably well 

 determined there, but also because, though it is a very distant station, 

 differing widely in geographical latitude and longitude, and situated 

 indeed in a different hemisphere, there is a striking resemblance 



