j6o ke Principles of Part IV. 



LET us, then, examine the methods by 

 which it is kept loofe. Thefe may be di- 

 vided into natural and artificial. 



S *E C T. II. 



Effetfs of the atmofpbere. 



THE alternate viciflitudes of the air, 

 are the chief means that nature 

 makes ufe of to attain this end. Heat and 

 cold, moifture and drought, contract and 

 dilate it by turns j and, by thefe alternate 

 motions, (hake the particles afunder. But 

 there are no means fo efficacious as froft 

 and thaw. Every one muft have obferved, 

 how loofe the foil is after a froft. Many 

 vegetables are at this time ejected out of 

 the earth altogether. 



FROST feems to act in different ways. 



ift, By changing into an elaftic ftate much 



of the fixed air, which muft fhake and 



open the ground, to gain, an exit. 2dly, 



By 



