THE BEDSHIEL KAlMS 29? 



Some time after the hills and valleys of Britain had been 

 shaped by rain and rivers, and other agents, into nearly 

 their present form, the whole of North-western Europe 

 appears to have undergone several important changes of 

 level. Slowly, and at a rate perhaps not exceeding a few 

 inches in a century, and generally less even than that, the 

 land was carried above the sea ; and just as imperceptibly 

 was it again depressed below that level. Very important 

 changes in climatal conditions resulted from these oscillations 

 of level, to understand which we need first to consider 

 what are the chief causes which render our climate at the 

 present day as mild as it is. 



Of first importance to remember in this connection is 

 the fact that, in the absence of any sources of heat, the 

 temperature of the air in contact with the earth would be 

 that of the Absolute Zero of Temperature, which is generally 

 considered to be 490 degrees Fahrenheit below what is 

 commonly understood to be zero. It may conduce to a 

 clearer understanding of the subject before us if we endeavour 

 to realise and bear in mind this important fact. The chief 

 factor concerned in keeping the temperature some five or 

 six hundred degrees above this frightfully low point is, of 

 course, the heat of the Sun. The internal heat of the Earth 

 is not without a share in the work, and it is even possible 

 that we may be warmed just a little by heat emitted from other 

 celestial bodies than the Sun. It is not difficult to compute 

 what percentage of the total heat that reaches the Earth 

 from our chief luminary should, theoretically, be received by 

 any given zone in any given time. The climates marked 

 out in accordance with the results of these computations are 

 usually termed "Astronomical Climates." They do not by 

 any means agree with the facts actually observed, except 

 over very limited areas of the Earth's sui-face. Why they 

 do not agree, is a matter for our present consideration. What 

 we actually find is evidence of there being numerous coun- 

 teracting causes at work, some of which tend to lower the 

 theoretical temperature in some parts, and others to raise 

 that temperature elsewhere. Some of these disturbing causes 

 may seem at first sight to be of but trivial importance ; 

 but, nevertheless, slight though their effects may seem, they 



