REPLY TO CRITICS. 43 



effectually cut off* the sun's rays and prevent them 

 from reaching the earth, then I do not wonder that he 

 should feel embarrassed in arguing with me. But if 

 he supposes my meaning to be, as it of course is, that 

 those two opposite conditions, existing at totalfy dif- 

 ferent times or in totally different places at the same 

 time, should lead to similar results, namely the cooling 

 of the air and consequent conservation of snow, then 

 there is no ground whatever for any embarrassment 

 about the matter. 



" We might therefore show," he states, " that if the 

 snow, air, fog, or whatever throws back the rays of 

 the sun into space is so excellent a reflector of heat, it 

 is a correspondingly poor radiator; and the same fog 

 which will not be dissipated by the summer heat will 

 not be affected by the winter's cold, and will therefore 

 serve as a screen to prevent the radiation of heat from 

 the earth during the winter." 



There are few points in connection with terrestrial 

 physics which appear to be so much misunderstood as 

 that of the influence of fogs on climate. One chief 

 cause of these misapprehensions is the somewhat com- 

 plex nature of the subject arising from the fact that 

 aqueous vapour acts so very differently under different 

 conditions. When the vapour exists in the air as an 

 invisible gas, we have often an intensely clear and 

 transparent sky, allowing the sun's rays to pass to the 

 ground with little or no interruption; and if the 

 surface of the ground be covered with snow, a large 

 portion of the incident rays are reflected back into 

 space without heating either the snow or the air. 

 The general effect of this loss of heat is, of course, to 

 lower the general temperature. But when this vapour 

 condenses into thick fogs it acts in a totally different 

 manner. The transparency to a great extent dis- 



