1 64 The Principles of Part IV. 



meafure, the influence of the air, and make 

 the ground lefs fertile. 



It is remarked, that not only a change of 

 fpecies, but alfo a change of grain, is necef- 

 lary : for the fame grain fown in the fame 

 ground is obferved to degenerate. This de- 

 pends on another caufe. It feldom happens, 

 I believe, that the vegetable food is of a due 

 mixture and confiftency. As foils are ge- 

 nerally either too wet or too dry, too light 

 or too ftiff; fo muft the vegetable food be 

 either too thin and watry, or too thick 

 and gluy. The vegetable muft at laft be 

 hurt from a conftant fuccemon of one fort 

 of food, and can only be recovered by a 

 foil poflefTed of oppolite qualities 



SECT. 



