OF NATURAL PHILOSOPHY. 157 



geminated through the celestial regions; and as 

 there are other good reasons for believing this to 

 be a vera causa, it has therefore been ascribed to 

 such a resistance. 



(160.) This 9th observation is of such import- 

 ance in science, that we shall exemplify it by an- 

 other instance or two. M. Arago, having suspended 

 a magnetic needle by a silk thread, and set it 

 in vibration, observed, that it came much sooner 

 to a state of rest when suspended over a plate of 

 copper, than when no such plate was beneath 

 it. Now, in both cases there were two verce 

 causes why it should come at length to rest, viz. the 

 resistance of the air, which opposes, and at length 

 destroys, all motions performed in it ; and the want 

 of perfect mobility in the silk thread. But the 

 effect of these causes being exactly known by the 

 observation made in the absence of the copper, and 

 being thus allowed for and subducted, a residual 

 phenomenon appeared, in the fact that a retarding 

 influence was exerted by the copper itself; and this 

 fact, once ascertained, speedily led to the knowledge 

 of an entirely new and unexpected class of relations. 

 To add one more instance. If it be true (as M. 

 Fourrier considers it demonstrated to be) that the 

 celestial regions have a temperature independent 

 of the sun, not greatly inferior to that at which 

 quicksilver congeals, and much superior to some de- 

 grees of cold which have been artificially produced, 

 two causes suggest themselves: one is that assigned 

 by the author above mentioned; the radiation of the 

 stars ; another may be proposed in the ether or elastic 

 medium mentioned in the last section, which the 



