SURVEYS OF NATURE. 



PART THE FOURTH. 



W 



E come now to a large ctivifion of creatures, entirely dif- 

 tindl from what have hitherto engaged our attention. It 

 is true, we have feen quadrupeds flying, or rather leaping (as the 

 fquirrel), and labouring a progreffive motion (as the bat), but far 

 from being able to endure, and continue this exercife, they foon 

 become enfeebled : ftill lefs have they power to attain great ele- 

 vation, or to float on the atmolphere as if at red. Their flight 

 "was % deviation from the general nature of quadrupeds, wherein 

 they approached to fomewhat of a bird; as the incapacity of fome 

 birds for fli<iht, deviates aifo from the peneral nature of their 

 kind, and fomewhat approaches a quadruped: nevertnelefs, the 

 great dillindion between thefe kinds of creatures is, the capacity 

 of one to riie in air, while the other is confined to reiidence on 

 the earth. 



To this quality of rifing contribute, (i) Their external fhape ; 

 (2) Their clothing of feathers ; (3) The great fl:rength of their 

 pecloral mulcles ; (4) Their capacity of inhaHng conhderable 

 .quantities of air: mofl: probably conned:ed with a principle where- 

 by they accommodate themlelves to the different degrees of rarity 

 in the air, as they afcend to confiderable heights. 



Part IV. No. 24. A The 



