THE EARTH IN METEORIC SHADOW. 155 



weather of our northern February. And the same with 

 the cold days of April and May. 



We seem obliged, then, to look for some cosmical 

 cause of the anomalous falls in the earth's temperature ; 

 and as it is difficult, if not impossible, to conceive any 

 cause which could directly affect the sun's temperature 

 on special days of the terrestrial year, we seem naturally 

 led to infer that somewhere between the earth and the 

 sun there lies either constantly or usually something of 

 the nature of a cloud which intercepts a portion of the 

 sun's light and heat. If we consider for a moment 

 what any special part of the earth's year means, we 

 seem forced to this conclusion. The 9th of February, 

 for instance, is the time when, in her course around 

 the sun, the earth arrives at a particular part of her 

 path ; her coming there can in no way affect the sun's 

 light and heat, which therefore cannot be diminished 

 at that date (systematically) any more than at any 

 other time. Yet the heat received by the earth when 

 she gets there is less ; hence it seems to follow 

 inevitably that the earth there passes through a region 

 where less heat (and therefore less light) is received 

 because of some shadow-throwing matter. 



Now, when Erman enunciated his theory of the 

 interposition of meteoric streams, it was natural that 

 such meteor systems as those which produce the 

 August and November showers should be regarded as the 

 shadow-throwing matter. Those were the only impor- 

 tant meteor systems recognised, and there was no known 

 reason for supposing that many such systems exist. 



