ON THE SUN'S HEAT. 



A Friday evening Lecture delivered before the Royal In- 

 stitution of Great Britain on January 21, 1887 : see also 

 Good Words for March and April 1887.] 



FROM human history we know that for several 

 thousand years the sun has been giving heat and 

 light to the earth as at present, possibly with 

 some considerable fluctuations, and possibly with 

 some not very small progressive variation. The 

 records of agriculture, and the natural history of 

 plants and animals within the time of human 

 history, abound with evidence that there has been 

 no exceedingly great change in the intensity of the 

 sun's heat and light within the last three thousand 

 years ; but for all that, there may have been 

 variations of quite as much as 5 or 10 per cent., as 

 we may judge by considering that the intensity of 

 the solar radiation to the earth is 6\ per cent. 



B B 



