OF NATURAL HISTORY. I7t 



fuiRcient to illiiftrate thofe invifible motions of the air by 

 which Ibunds are conveyed from one place to another, and 

 to give an idea of echoes, or refledl:ed undulations of that 

 fluid. 



The celerity with which founds, or undulations of airj 

 move, has been exactly computed. All founds, whether 

 3cute or grave, ftrong or weak, move at the rate of 1142 

 feet in a fecond of time. Hence, whenever the lightning of 

 thunder, or the fire of artillery, are feen, their actual dif- 

 tances from the obferver may be eafily afcertained by the 

 vibrations of a pendulum. This velocity, it is true, may be 

 a little augmented or diminiflied by favourable or by con- 

 trary winds, and by heat or cold. But the difference, even 

 in high winds, is fo trifling, that, for any ufeful purpofe, it 

 fcarcely merits attention. 



Infants hear bluntly, becaufe the bones of their ears are 

 foft and cartilaginous ; and, of courfe, the tremulations excited 

 in them by the motions of the air are comparatively weak. 

 Young children, accordingly, are extremely fond of noife. 

 It roufes their attention, and conveys to them the agreeable 

 fenfation of found , but feeble founds are not perceivedj 

 which give infants, like deaf perfons, the appearance of inat- 

 tention, or rather of flupidity. 



The force or intenfity of found is augmented by reflecllon 

 from furrounding bodies. It is from this caufe that the hu- 

 man voice, or any other noife, is always weaker, and lefs 

 diftinftly heard, in the open air than in a houfe. 



The modifications of found are not lefs various than thofe 

 of taftes or odours. The ear is capable of diftinguifking 

 Ibme hundred tones in found, and probably as many degrees 

 of ftrength in the fame tones. By combining thefe, many 

 thoufand fimple founds, which differ either in tone or in 

 ftrength, are perceived and diftinguifhed by the ear. A 

 violin, a fiut^, a French-horn, may each of them give the 



