VARIABLE WEATHER. 379 



poa (poa annua), a delicate and agreeable food to 

 the horse and the sheep, by pressing down the 

 stoles or runners, and levelling the wornf castes, 

 the plant thus throwing out a succession of foliage ; 

 for this poa, by reason of its abundant fibrous roots, 

 endures drought better than many of its congeners^ 

 by which means a pathway becomes frequently a 

 verdant line in a brown and arid pasture. In a 

 wet season, water appears to nourish plants, or to 

 supply their requirements principally : in a dry 

 one, nutriment must be obtained from the soil by 

 means of the fibre of the root, and hence particles 

 are imbibed chemically different ; a dry or a drained 

 soil, producing short and scanty herbage, will fre- 

 quently improve the condition of cattle more than 

 an adjoining meadow having a profusion of food, 

 though probably no chemical analysis could indi- 

 cate the difference. These periodical winds again, 

 violent and distressing as they often prove, are yet 

 unquestionably essential in the economy of nature : 

 our two seasons, in which these commotions of the 

 air most usually become manifest, are about the 

 equinoxes of autumn and spring, periods which in. 

 many respects have a similarity with each other. 

 In the autumn of our year, the foliage of trees and 

 plants, &c. putrefies and decays ; marshes and dull 

 waters, clogged by their own products^ stagnate, 

 and discharge large portions of hydrogen, carbonic 

 gas, &c. injurious and even fatal to animal exist- 

 ence : in summer all these baneful exhalations are 

 neutralized and rendered wholesome by the vast 



