PHYSICAL KNOWLEDGE. 25 



deceptive form. Clear views of nature, even if merely his- 

 torical, are sufficient preservatives against these dogmatizing 

 fancies. The history of the atmosphere, and of the annual 

 variations of its temperature, extends already sufficiently far 

 back to shew that these consist in repeated small oscillations 

 around the mean temperature of a station, thereby dispelling 

 the exaggerated fear of a general and progressive deteriora- 

 tion of the climates of Europe. Encke's comet, which is 

 one of the three interior comets, completing its course 

 in 1200 days, must, from its position and the form of its 

 path, be as harmless to the inhabitants of our globe as 

 Halley's great comet of 1759 and 1835, which has a perioi. 

 of seventy-six years. The path of another comet of short 

 period, Biela's, which completes its course in six years, 

 does, indeed, intersect the earth's path, but it can only 

 approach us when its perihelion coincides with our winter 

 solstice. 



The quantity of heat received by a planetary body (the 

 unequal distribution of which determines the great meteoro- 

 logical processes of our atmosphere) depends conjointly on 

 the light-evolving power of the sun (i. e. the nature of its 

 surface, or the state of its gaseous covering), and on the re- 

 lative positions of the sun and planet. There are, indeed, 

 periodical variations, which the form of the earth's orbit and 

 the obliquity of the ecliptic undergo in obedience to flie 

 universal law of gravitation ; but these changes are so slow, 

 and restricted within such small limits, that their thermic 

 effects would hardly be appreciable by our present ther- 

 mometric instruments in many thousands of years . Supposed 

 cosmical causes of diminished temperature or moisture, or 

 of epidemic diseases, of which the idea has been enter- 



