364 Mr Barlow on the Diurnal Variation 



After such close accordances as those we have shewn, be- 

 tween theory and observation, in so many, and in such distant 

 and peculiar situations on the globe, no one can, I think, hesi- 

 tate in pronouncing, that that theory, in its general character, 

 must be correct, although there may be variations due to dif- 

 ferent causes, which modify the results, as the wind and other 

 circumstances influence the tides, without, however, in any way 

 throwing a doubt upon the theory, which makes the moon 

 the great primary agent, productive of those daily changes. 

 These influencing or disturbing causes may furnish the sub- 

 ject of another communication. This I shall conclude by exa- 

 mining what is the present state of our knowledge, and Avhat 

 are our future hopes of arriving at a more perfect knowledge 

 of the laws, which govern the phenomena of the magnetic needle 

 in different parts of the earth. With reference to the former, 

 I think we may venture to say, \st^ We know, that, from what- 

 ever source the earth derives its magnetism, it is by some in- 

 ductive principle, the phenomena which it exhibit being wholly 

 inconsistent with a state of determinate magnetic polarization,-— 

 " Essay on Magnetic Attraction,'" p. 208. ; 9,dly, I have shewn, 

 by an incontestible experiment, that such a power may be indu- 

 ced in a globe of any matter, and all the phenomena of terres- 

 trial magnetism exhibited by the agency of electricity or galva- 

 nism, independently of any magnetic body whatever; Qdly^ 

 Professor Leibech has shewn, that such an electric motion may 

 be induced in a body composed of different metals, by merely 

 destroying the equilibrium of caloric within them ; and, ^tlily^ 

 It is shewn, by the highly valuable experiments of Captain 

 Parry and Lieutenant Foster, that a magnetic disturbance is 

 actually produced on the needle, and on the general magnetism 

 of the earth, through the medium of the solar influence. 



We have thus at least a glimpse of the probable cause of that 

 hitherto mysterious, but invaluable property, of a magnetized 

 needle, which disposes it to take up a determinate position ; and, 

 as we owe this conclusion, in a great measure, to the happy 

 thought of Lieutenant Foster, which led him to make a series 

 of simultaneous observations on the intensity of the horizontal 

 and dipping needles, we do sincerely hope he may be enabled 



