RADIATING HEAT. 35 



" whose action differed considerably from gravitation or the 

 ordinary attracting force of the sun ; since those portions of 

 the comet which constitute the tail are acted upon by a re- 

 pulsive force proceeding from the body of the sun."* The 

 splendid comet of 1744, which was described by Heinsius, 

 led my deceased friend to similar conjectures. 



The actions of radiating heat in the regions of space are 

 regarded as less problematical than electro-magnetic phenom- 

 ena. According to Fourier and Poisson, the temperature of 

 the regions of space is the result of radiation of heat from the 

 sun and all astral bodies, minus the quantity lost by absorp- 

 tion in traversing the regions of space filled with ether. t 

 Frequent mention is made in antiquity by the Greek and 

 Roraan| writers of this stellar heat; not only because, from 

 a universally prevalent assumption, the stars appertained to 

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

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

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

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

 tions of the above-mentioned eminent French mathemati- 

 cians, Fourier and Poisson, have been the means of direct- 

 ing attention to the average determination of the tempera- 

 ture of the regions of space ; and the more strongly since the 

 importance of such determinations on account of the radia- 

 tion of heat from the earth's surface toward 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 hith- 

 erto 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 



* Bessel. op. cit., s. 186-192, 229. 



t Fourier, Thforie Analytiqne de la Chaleur, 1822, p. ix. (Annales 

 de Chimie el de Physique, torn, iii., 1816, p. 350; torn, iv., 1817, p. 128; 

 torn, vi., 1817, p. 259; torn, xiii., 1820, p. 418.) Poisson, in his Thiorie 

 Mathtmatiqve de la Chaleur ( 196, p. 436, 200, p. 447, and 228, p. 

 521), attempts to give the numerical estimates of the stellar heat {cha- 

 leur stellaire) lost by absorption in the ether of the regions of space. 



+ On the heating power of the stars, see Aristot., De Meteor., 1, 3, 

 p. 340, lin. 28 ; and on the elevation of the atmospheric strata at which 

 neat is at the minimum, consult Seneca, in Nat. Quasi., ii., 10: 'Su- 

 periora enini aeris calorem vicinorum siderum sentiunt." 



$ Plut., De plac. Pkilos., ii., 13. m 



