166 SENSATION OF AIR. 



We are not authorized to say that it moves without 

 any friction ; but its friction is only the friction of 

 particles ; and, with moderate velocities, the resist- 

 ance of air rubbing on air is very small. 



The atmospheric air is at once the most delicate 

 and the most powerful of all springs. It actually 

 yields to the touch of a sunbeam, and yet it can 

 cleave rocks, and shake the surfaces of countries to 

 pieces in earthquakes. It is more nice in the de- 

 tection of pressure than any instrument that we can 

 contrive, and no thermometer can measure heat 

 with nearly the precision of an air one. The air 

 is, indeed, not only fine beyond all sensation, but it 

 is the immediate object of all the senses. It is the 

 air which the eye sees, the ear hears, the nose 

 scents, and the finger touches. We know nothing 

 of what sight might be in a vacuum, or space where 

 there were no air, because the eye would be de- 

 stroyed if it were, in such a place, even though the 

 apparatus were so contrived as that the operation 

 of breathing could still be carried on. Once remove 

 the pressure of the atmosphere, and the fluids of the 

 eye would burst the vessels and coats, and there 

 would be an end of its curious structure, as well as 

 its power of seeing. 



Smell and taste are not in the air, but still the fra- 

 grance and the sapidity are " melted or dissolved in 

 air," before we can perceive them ; and in those 

 internal parts of the body which we may suppose 

 that the atmospheric air does not reach, we have 

 no perception of any thing like either smell or taste. 

 Then as to hearing, it is the air that we hear. Air 

 is the instrument, and the only instrument of sound ; 

 and if it were taken away, all nature would be as 

 dumb as a little bell is when it is tolled or struck 

 within an exhausted receiver. Indeed, it not only 

 requires air, but it requires some body or substance 

 of air to produce a sound that can be heard ; for we 

 are not able, by even the best air-pump, to exhaust 



