358 PLEASANT WA YS IN SCIENCE. 



to the formation of dew. Hence arose the singular theory, 

 that as the sun pours down heat upon the earth, so the stars 

 (and also the moon and planets) pour down cold. 



Nothing is more common we may note in passing 

 than this method of philosophizing, especially in all that 

 concerns weather-changes; and perhaps it would be im- 

 possible to find a more signal instance of the mistakes into 

 which men are likely to fall when they adopt this false 

 method of reasoning ; for, so far is it from being true that 

 the stars shed cold upon the earth, that the exact reverse 

 is the case. It has been established by astronomers and 

 physicists that an important portion of the earth's heat-supply 

 is derived from the stars. 



Following on these fanciful speculations came Aristotle's 

 theory of dew celebrated as one of the most remarkable 

 instances of the approximation which may sometimes be 

 made to the truth by clever reasoning on insufficient ob- 

 servations. For we must not fall into the mistake of suppos- 

 ing, as many have done, that Aristotle framed hypotheses 

 without making observations; indeed, there has seldom 

 lived a philosopher who has made more observations than 

 he did. His mistake was that he extended his observations 

 too widely, not making enough on each subject He 

 imagined that, by a string of syllogisms, he could make 

 a few supply the place of many observations. 



Aristotle added two important facts to our knowledge 

 respecting dew namely, first, that dew is only formed in 

 serene weather ; and secondly, that it is not formed on the 

 summits of mountains. Modern observations show the 

 more correct statement of the case to be that dew is 

 seldom formed either in windy weather or on the tops of 

 mountains. Now, Aristotle reasoned in a subtle and able 

 manner on these two observations. He saw that dew must 

 be the result of processes which are interfered with when 

 the air is agitated, and which do not extend high above 

 the earth's surface ; he conjectured, therefore, that dew is 

 simply caused by the discharge of vapour from the air. 



