42 COSMOS. 



a universally prevalent assumption, the stars appertained tc 

 the region of the fiery ether, but because they were supposed 

 to be themselves of a fiery nature 27 the fixed stars and the 

 sun being, according to the doctrine of Aristarchus of Samos, 

 of one and the same nature. In recent times, the observa- 

 tions of the above-mentioned eminent French mathematicians, 

 Fourier and Poisson, have been the means of directing attention 

 to the average determination of the temperature of th e regions 

 of space ; and the more strongly since the importance of 

 such determinations on account of the radiation of heat from 

 the earth's surface towards the vault of heaven, has at length 

 been appreciated in their relation to all thermal conditions, 

 and to the very habitability of our planet. According to 

 Fourier's Analytic Theory of Heat, the temperature of celestial 

 space (des espaces planetaires ou celestes) is rather below the 

 mean temperature of the poles, or even perhaps below the 

 lowest degree of cold hitherto observed in the polar regions. 

 Fourier estimates it at from 58 to 76 (from 40 

 to 48 Reaum.). The icy pole (pole glacial), or the point 

 of the greatest cold, no more corresponds with the terrestrial 

 pole than does the thermal equator, which connects together 

 the hottest points of all meridians with the geographical 

 equator. Arago concludes, from the gradual decrease of mean 

 temperatures, that the degree of cold at the northern ter- 

 restrial pole is 1 3, if the maximum cold observed by Captain 

 Back at Fort Reliance (62 46' lat.) in January, 1834, were 

 actuaUy 70 ( 56'6 Cent., or - 45'3 Reaum.). 28 The 



1 , 3, p. 340, lin. 28 ; and on the elevation of the atmospheric 

 strata at which heat is at the minimum, consult Seneca in Nat. 

 Qucest.<ii. 10 : " Superiora enim aeris caloremvicinorumsiderum 

 scntiunt." 



27 Plut. de plac. Philos., ii. 13. 



28 Arago, Sur la temperature du Pole et des espaces celestes 

 in the Annuaire du Bureau des Long, pour 1825, p. 189, el 



