Mr Barlow's Remarks on the Magnetic Needle. 14S 



shew that the hypothesis suggested by Lieutenant Foster, viz. 

 " of the magnetic pole having a daily motion about its mean 

 point in an orbit of about 2^' or 3' in radius," would serve to 

 explain all the general phenomena of the observed daily changes 

 in direction and in intensity of the magnetic needle in different 

 parts of the globe. I also stated that there were other changes, 

 or rather other sources of change, which served to modify the 

 observed results, and would require farther illustrations. These 

 illustrations will form the subject of the following pages, parti- 

 cularly with reference to Captain Sabine's experiments. First, 

 then, let us observe, that it is shewn in the former paper, that 

 if the phenomena alluded to may be generally represented by a 

 daily rotation of the magnetic pole about its mean point, it must 

 be by supposing the pole to be always inflected towards the 

 sun. It is on this supposition the former explanation as to time, 

 &c. has been founded, and the first question beyond this is, 

 ^' Does this inflection arise from the solar influence increasing or 

 diminishing the magnetism of that part of the globe on which it 

 is the greatest ? A very little consideration will suffice to shew, 

 that, if that hemisphere of the earth on which the sun shines at 

 any time has its magnetic power more strongly developed in 

 consequence of the increased heat, the resultant of all the forces 

 will approximate towards that part of the sphere, and will cause 

 an apparent approach of the pole towards that side. Now, we 

 have seen that this apparent approach of the magnetic pole has 

 been actually observed during the time the sun is advancing to- 

 wards the meridian of any place ; and hence we conclude that 

 this approach is caused by an increased magnetic action in 

 those parts of the earth immediately exposed to the solar in- 

 fluence. 



That this would be the case in the magnetism of an iron-ball 

 thus partially heated, is unquestionable, — Phil. Trans. 1821^ 

 Part I. ; and as, in all other known cases, the laws of iron,, 

 magnetised by induction, and those of terrestrial magnetism, sa 

 closely resemble each other, it may serve to condense our re- 

 marks, if we first confine them to the case of an iron-ball under 

 different temperatures resembling the actual state of the earth. 



With this view, it may be observed, that, according to the 

 hypothesis of iron receiving its magnetism by induction, it is 



