292 



Sun was almost at the zenith ; the earthy wher- 

 ever it appeared sterile and destitute of vegeta- 

 tion, was at the temperature of 48° or 50° *, 

 Not a breath of air was felt at the height at 

 which we were on our mules ; yet, in the midst 

 of this apparent calm, whirls of dust incessantly 

 arose, driven on by those small currents of air^ 

 that glide only over the surface of the ground, 

 and are occasioned by the difference of tem- 

 perature, which the naked sand and the spots 

 covered with herbs acquire. These sand winds 

 augment the suffocating heat of the air. Every 

 grain of quartz, hotter than the surrounding air^ 

 radiates heat in every direction ; and it is dif- 

 ficult to observe the temperature of the atmos- 

 phere, without these particles of sand striking 

 against the bulb of the thermometer. All 

 around us, the plains seemed to ascend toward 

 the sky, and that vast and profound solitude 

 appeared to our eyes like an ocean covered with 

 seaweeds. According to the unequal mass of 

 * vapours diffused through the atmosphere, and 

 the variable decrement in the temperature of the 

 different strata of air, the horizon in some parts 

 was clear and distinct ; in other parts it ap- 

 peared undulating, sinuous, and as if striped. 

 The earth there was confounded with the sky. 



* Reaumur's thermometer, buried in the sand, rose to 

 38-4° and 40 9 , , 



