114 THE PHILOSOPHY 



the animal, without adding to its weight, which mufl: be diminifh- 

 ed ; becaufe the fpecific gravity of the external air is fuperior to that 

 of the interrral air, which is rendered more rare by the heat of the 

 animal's body. This opinion was corroborated, by confidering that 

 the feathers of birds, and particularly thofe of the wings, contain a 

 great quantity of air. With his ufual ingenuoufnefs, however, Mr 

 Hunter, in oppofition to his firft conjedure, informs us, that the 

 oftrich, which does not fly, was amply provided with air-cells dif- 

 peifed through its body; that the wood-cock, and foaie other flying 

 birds, were not fo liberally fupplied with thefe cells as the oflrich; 

 and that the bat had no fuch peculiarity of flrudure. With regard 

 to the oftrich, though it is not intended to fly, it runs with amazing 

 rapidity, and, confequently, requires fimilar refources of air. 



He next conjedured, from analogy, that the air-cells in birds 

 ought to be confidered as an appendage to the lungs; becaufe in the 

 fnake, viper, and feveral other amphibious animals, the lungs are 

 continued, in the form of two bags, through the whole abdomen, 

 the upper part of which can only perform the ofl'ice of refpiration 

 with any degree of effed; becaufe the lower part has comparatively 

 few air-veflels. ' The air,' fays Mr Hunter, * muft pafs through 

 ' this upper part before it gets to the lower in infpiration, and muft 

 ' alfo repafs in expiration ; fo that the refpiratory furface has more 



* air applied to it than what the lungs of themfelves could contain. 

 ' There is, in fa<ft, a great fimilarity between birds and that clafs of 

 ' animals called amphibious ; and, although a bird and a fnake are 

 ' not the fame in the conftrudion of the refpiratory organs, yet the 



* circumflance of the air paffing in both beyond the lungs, into the 



* cavity of the abdomen, naturally leads us to fuppofe, that a ftrue- 



* ture fo fimilar is defigned in each to anfwer a fimilar purpofe. This 

 ' analogy is ftill farther fupported by the lungs in both confifting of 

 ' large cells. Now, in amphibious animals, the ufe of fuch a con- 



' formatioa 



