25i Mr. Dobbie on the Magnetic Puvoer 



earth itself, if it contains any substance susceptible of it, which 

 experience proves it does contain : and it is no less certain 

 that the polar region contains the means of effecting that se- 

 paration of the rays of light, in the refractive power of innu- 

 merable angular masses of ice. 



The same reasoning applied to the aurora horealis is equally 

 conclusive. It has lately been found that a very small mirror 

 reflecting the rays of the sun, is seen at a much greater di- 

 stance than could have been previously expected. I have 

 often observed this powerful effect of reflection in ploughed limd: 

 for instance, a dazzling reflection is frequently perceived at a 

 considerable distance ; in following it out, the cause is dis- 

 covered to be a small bit of glass or an insignificant fragment 

 of glazed earthenware, &c. If such an effect is produced by so 

 small a reflecting surface, with the disadvantage of the full glare 

 of da\', what must be the effect when the sun's rays fall upon 

 a world bespangled witli reflectors as the polar regions are ? 

 with the advantage of the darkened hemisphere, like a great 

 camera ohscura opened to receive the images of the sun (as 

 certainly the aurora- are), though distorted and reflected at se- 

 cond hand by the soft bosom of the yielding atmosphere : if 

 this second reflector were more perfect, these pha^nomena 

 would probably be the most splentlent the eye could witness 

 in tlic natural world. 



Thus, by ascribing to known facts their legitimate and 

 known effects, and those effects agreeing in every particular 

 ■with the phasnomena in question, it falls little or nothing short 

 of demonstration tliat such are the true causes. This theory 

 of the earth's magnetism, and that of the aurora bor calls in the 

 essay referred to, mutually confirm one another. The illu- 

 mined arch described in the latter lias appeared more fre^ 

 quently than I was aware of at that time, and each appearance 

 of it has happened so near either of the equinoxes, as scarcely 

 to leave a doubt that the then relative position of the earth and 

 sun is essential to its production ; which establishes the prinr 

 ciple on which that phjenomenon is explainetl. 



The simple mean here suggested, by which the phasnor 

 mena of the pohir aurora and magnetism are at once effected, 

 is striking and sublime : all the operations of nature are so 

 when their true causes are discovered, and afford but small 

 encouragement to the inventors of complex theories : indeed, 

 the jierpetual failure of all such baseless fabrics may well put 

 out of countenance that propensity. True knowledge is only 

 attained by the observation and right application of facts. 

 Nature is extremely frugal in the means employed for the at- 

 tainment of any particular end. A remarkable instance of this 

 is to be found in the principle of attraction, which, being a 

 power inherent in matter, effects all motion, from that of the 



smallest 



