144 Mr Barlow's Remarhs on the Dtp and Intensity 



supposed, both in my investigation and in that of M. Poisson, 

 that each particle of ^the magnetic fluid has tlie same intensity 

 of action at the same distance ; and, on this supposition, all the 

 conditions of its action are deduced. But if, instead of this 

 uniform action, we suppose the equatorial parts of a sphere of 

 iron to act with greater intensity, as would actually be the case 

 if the temperature of the iron were increased in those parts, 

 let us inquire in what manner, and in what degree, this would 

 influence the laws deduced from assuming an uniform intensity 

 at equal distances. And if it result from this inquiry, that an 

 increased intensity, and a diminution of the natural dip about 

 the equatorial regions, would be the necessary consequence of 

 such a supposition, and an increase of the natural dip, with a 

 corresponding decrease of intensity in those parts towards the 

 poles of the iron shell ; and if, moreover, referring to actual ob- 

 servations and experiments on the terrestrial globe, it should be ' 

 found that corresponding phenomena have been observed in the 

 frigid and torrid zones, we shall, I think, have strong reasons 

 for assuming that the phenomena in both cases are due to the 

 same cause, viz. an unequal temperature and a corresponding 

 inequality of magnetic intensity ; but still not such as can be 

 explained by assuming a pole of intensity distinct from that of 

 direction. 



Here, then, at once will be seen the principle on which I pro- 

 pose to explain, first, the apparent anomalies which Captain Sa- 

 bine detected in his experimental results between the intensity 

 and dip in the frigid, temperate, and torrid zones ; and, second- 

 ly, some of the modifications in the daily changes of magnetic 

 intensity, &c. which seem to be dependent on causes not em- 

 braced by the general hypothesis of the daily rotation of the 

 magnetic pole of the earth about its mean point. 



It is, I believe, to Dr Young we are indebted for the first 

 analytical formulae for expressing the intensity of terrestrial mag- 

 netism, as depending on the dip of the needle, viz. 



Intensity of dipping needle I — A s.1 -. ^ . » 



•^ rr & Y 4 — 3 sm^ 5 



Intensity of horizontal needle I = A ^/ 7: s-. 



^ V 3 -t- sec^ ^ 



Where ^ is the dip, and I the magnetic latitude of the place of 



