REPLY TO CRITICS. 63 



determine with accuracy the intensity of the sun's 

 heat from the temperature of the ground ; for the 

 surface of the ground in equatorial regions is kept at 

 a much lower temperature by the air blowing over it 

 than is due to the intensity of the sun's heat. It thus 

 becomes a very intricate problem to determine how 

 much the surface of the ground is kept below the 

 maximum temperature by the heat absorbed by the 

 moving air. 



