APPENDIX. 397 



For the present discussion it is best to accept Mr. Croll's 

 estimate, and to compare the amount of heat which he supposes 

 to be transferred from one hemisphere to the other with the 

 total amount which is received annually from the sun on each 

 hemisphere. For this purpose I have taken the known areas 

 of the torrid, temperate, and frigid zones respectively, and, 

 following Mr. Croll, I have adopted Mr. Meech's estimate of 

 the average amount of heat, per unit of surface, received from 

 the sun in each zone, irrespective of absorption by the atmo- 

 sphere. To estimate the proportion of heat which actually 

 reaches the surface, I have adopted Pouillet's measure of the 

 proportion of solar radiation cut off at vertical incidence, which 

 is 24 per cent. I assume 28 per cent, to be the average loss in 

 the torrid zone, 50 per cent, in the temperate zone, and 75 per 

 cent, in the frigid zone.* The resulting figures, showing the 

 proportional amount of heat annually received on the surface of 

 each zone, and on the entire hemisphere, are as follows : — 



Torrid zone ... ... ... ... 3370 



Temperate zone ... ... ... 2304 



Frigid zone ... ... ... ... 112 



Whole hemisphere ... ... ... 57^6 



Calculating, on the same basis, the amount received on a 

 zone one mile wide at the equator, allowing a loss of 25 per 

 cent, from atmospheric absorption, and multiplying the result by 

 104, I obtain the number 233'i, or rather more than one twenty- 

 fifth part of the entire heat annually received from the sun by 

 each hemisphere. 



To trace the results of such a transfer of heat from one 

 hemisphere to the other, I shall adopt a mode of reasoning, 

 sanctioned by the great authority of Sir John Herschel, to 

 which Mr. Croll frequently resorts. It is by solar heat that the 

 surface of the earth is raised above the temperature of space, 

 which is assumed to be 239 degrees below the zero of Fahrenheit's 

 scale. Adopting Ferrel's estimate, I take the mean temperature 

 of the northern hemisphere at 59"5° Fahr., or 298^ degrees above 

 the temperature of space. To maintain this temperature, it 



* Viewed in the light of Mr. Langley's recent researches on solar 

 radiation, all these numerical determinations are probably far from the 

 truth ; but the errors do not much affect the present argument. 



