284 



RADIATION OF HEAT IMPEDED BY A 



[Ch. XIII. 



remarked that if the obliquity of the ecliptic could ever be 

 diminished to the extent of four degrees below its present 

 inclination, such a deviation would be of geological interest, 



lit to be disseminated 

 over a broader zone inside of the arctic and antarctic circles. 

 This greater spread of the solar rays, implying a shortening 

 of the polar night, would help in some slight degree to account 

 for a vegetation such as now characterises lower latitudes 



&» 



Miocene 



wider range towards the pole. If an adequate supply of light 

 w r as afforded, the warmth required by such a flora would 

 rarely have been wanting in past times, for, according to 

 principles before laid down, a more genial climate would 

 usually prevail in high latitudes, that is to say whenever 

 the earth's geography was in a normal state.* 



Radiation of heat impeded by a covering of snow. — Humboldt, 

 in his treatise on isothermal lines published before 1820,f ob- 

 served that as the winter in the southern hemisphere is now 



must 



hemisph 



observation would have still greater bearing on the problem 



now under consideration if, as would happen in extreme 



summer am 



nearly forty days instead of eight, 

 remembered that a covering of snow 



must, however, be 





* In Mr. Meech's valuable paper, be- length of path traversed by the oblique 

 fore cited (p. 273), he treats of the rays, is given by Sir J. Leslie and Mr. 

 effects of altered obliquity; but he states Traill in the article Climate, Encycl. 

 (pp. 21 and 43) that his results as to Britann. From this it appears, that 

 the intensity of solar radiation apply the total annual quantity of heat re- 

 only to the outside of the earth's atmo- ceived at the equator, latitude 45°, and 



sphere. If his readers fail to bear this 

 in mind, they will be in danger of greatly 



the poles will be as the numbers 115, 

 51, and 14 respectively. Even these 

 overrating the increased heat in polar figures represent the comparative quan- 



regions caused by different phases of 



• • 



tity of heat at the higher latitudes as 

 being more than the truth; for they 

 are computed on the supposition of con- 



precession, excentricity, and obliquity of 

 the ecliptic; for a large deduction will 

 have to be made for the greater amount stant sunshine. As cloud prevails to a 



greater extent in the high latitudes than 



of atmosphere through which the calo- 

 rific rays must pass in very high lati- 

 tudes. 



The investigation of the true calorific 

 effect of the sun's rays for every f>° 

 of altitude, allowing for the increased 



at the equator, the disproportion will 



be increased. 



f Memoires D'Arcueil, torn. in. p. 

 462, translated in Edin. Phil. Journ., 

 July 1820. 



