67 



limits of the Roman empire *, almost at those of 

 the known world ; and in space, as in the con- 

 ceptions of the human understanding", fantastic 

 images are created where positive ideas end. , 

 The inhabitants of Atures and Maypures, 

 whatever the missionaries may have asserted in 

 their works, are not more struck with deafness 

 by the noise of the great cataracts, than the Ca~ 

 tadupes of the Nile. When this noise is heard 

 in the plain that surrounds the mission, at the 

 distance of more than a league, you seem to be 

 near a coast skirted by reefs and breakers. The 

 noise is three times as loud by night as by day, 

 and gives an inexpressible charm to these soli- 

 tary scenes. What can be the cause of this in- 

 creased intensity of sound in a desert, where 

 nothing seems to interrupt the silence of nature ? 

 The velocity of the propagation of sound, far 

 from augmenting, decreases with the lowering 

 of the temperature. The intensity diminishes 

 in air agitated by a wind, which is contrary to 

 the direction of the sound ; it diminishes also by 

 dilatation of the air, and is weaker in the higher 

 than in the lower regions of the atmosphere, 

 where the number of particles of air in motion 



between Fazuclo, arid Alata ? (Bruce's Travels, vol. v, p. 

 105, 316.) 



* Claustra imperii ro/nani, says Tacitus. In the name of 

 the Island Philce we recognize the Coptic word phe-lakk, the 

 extremity (the end of -Egypt). 



f2 



