338 PHYSIOGRAPHY. [chap. 



illuminated hemisphere, on which the sun's rays were 

 constantly shining, would, of course, become intensely hot ; 

 while the darkened hemisphere would become intensely 

 cold, by the unchecked radiation of its heat into space. 

 Under such circumstances, the hottest part of the 

 world would be the middle of the sun-facing hemisphere, 

 because the solar rays would there fall square upon its 

 surface ; while the heat would diminish, in all directions 

 towards the circumference, because the rays would be 

 received in a more slanting direction on those parts which 

 were farther from the centre of the lit-up half. 



If the earth had no atmosphere, the contrast of climate 

 between the two halves of the globe would be most intense ; 

 for the half turned towards the sun would monopolise all the 

 heat sent to it, while the other half would constantly lose heat 

 by radiation into space. But, if the earth were enveloped in 

 an atmosphere, currents would be raised in this air, and these 

 currents would tend to moderate the climate. From the 

 highly-heated centre of the illuminated hemisphere, the 

 warmed air would rise and spread, on all sides, through the 

 higher regioi)^ of the atmosphere ; while the less-heated, and 

 therefore denser, air from the surrounding parts, would rush 

 in, through the lower regions of the atmosphere, to supply 

 its place. Hence, any one on the surface of such an earth 

 would find winds blowing from all points of the compass 

 directly towards the middle of the sun-facing hemisphere. 



If the earth now began to rotate, what would happen 

 would depend upon the direction of the imaginary line, or 

 axis, round which it turned. The axis coincides with the 

 earth's polar diameter, and the points on the surface which 

 were described in the last chapter as the earth's poles are 

 the extremities of this imaginary line. Suppose, first, that the 

 axis coincided with a prolonged radius of the sun, as in the 



