58 TERRESTRIAL ADAPTATIONS. 



and of organic nature, with the view of fixing 

 the reader's attention upon the correspondence 

 of the two. 



1 . The succession and alternation, at any given 

 place, of heat and cold, rain and sunshine, wind 

 and calm, and other atmospheric changes, appear 

 at first sight to be extremely irregular, and not 

 subject to any law. It is, however, easy to see, 

 with a little attention, that there is a certain 

 degree of constancy in the average weather and 

 seasons of each place, though the particular facts 

 of which these generalities are made up seem to 

 be out of the reach of fixed laws. And when we 

 apply any numerical measure to these particular 

 occurrences, and take the average of the numbers 

 thus observed, we generally find a remarkably 

 close correspondence in the numbers belonging to 

 the whole, or to analogous portions of successive 

 years. This will be found to apply to the mea- 

 sures given by the thermometer, the barometer, 

 the hygrometer, the raingage, and similar instru- 

 ments. Thus it is found that very hot summers, 

 or very cold winters, raise or depress the mean 

 annual temperature very little above or below 

 the general standard. 



The heat may be expressed by degrees of the 

 thermometer ; the temperature of the day is esti- 

 mated by this measure taken at a certain period 

 of the day, which period has been found by ex- 

 perience to correspond with the daily average ; 



