PASSAGE OF SUN'S RAYS THROUGH AIR. 117 



great part of their lighting, as well as of their 

 heating power. Experiment, so far a-s it has been 

 possible, has distinctly proved the fact, in reference 

 to this behaviour, that the sun's rays (equal sur- 

 faces being supposed to be offered to their vertical 

 influence) have a greater heating power on moun- 

 tains than in valleys. In the Alps, for instance, 

 they lost, in perfectly clear weather, during their 

 passage through an atmospheric depth of six thou- 

 sand feet, one-fifth of their heating power. The 

 experiments were made on Eaulhorn, simultane- 

 ously above and below. (Forbes.) 



The atmosphere in the different regions of the 

 earth is by no means, even on the brightest days, 

 equally free from mists, winch, as I have explained, 

 appropriate some of the heat. Thus the sun's 

 rays in England, at the level of the sea, manifest 

 only three-fifths of the heating power, winch they 

 possess at the Cape of Good Hope. In general, 

 the nearer you approach to the equator, travelling 

 from the poles, the clearer will you find the air, 

 and therefore the more pervious to the sun's rays. 

 As a rule it may be said, that, in the same latitudes, 

 we number more bright days, and enjoy a clearer 

 air, towards the interior of the continents than in 

 the neighbourhood of the sea-coast. 



The parts of the sun's rays which, after these 

 subtractions that they suffer in the atmosphere, 

 succeed in reaching the surface of the earth, exert 



