268 ALEXANDER VON HUMBOLDT. 



of the magnetic needle, which, by means of Borda's new 

 instrument, may be found to within 20 minutes. I enclose you 

 the results I obtained with it during the voyage. . . . 



' The chronometer I have with me is one of Louis Berthoud's, 

 No. 27 ; it has been frequently made use of by travellers, and 

 its excellence is well known to Borda ; it has preserved with 

 me its usual uniform rate. 



' I must confess it needs a remarkable stock of patience to 

 make astronomical observations with sufficient accuracy in 

 this extreme heat, and to do one's work con amore ! You will 

 however notice that this heat, however oppressive it may be, 

 does not seem to diminish my activity. . . . 



' I wish I could convey to you an idea of the exquisite beauty 

 of the nights here, or give you any notion of the transparency 

 of the air ; by the light of Venus I have frequently been able 

 with a lens to read the vernier of my small sextant ! Venus 

 really plays the part of a moon here. She presents a large and 

 luminous surface two degrees in diameter, and displays the 

 most beautiful colours of the rainbow, even when the air is 

 quite clear and the sky is perfectly blue. I am persuaded that 

 the splendour of the heavens is nowhere so remarkable as in 

 these latitudes, for south of the equator the magnificent 

 northern constellations soon cease to be visible. Not but what 

 the constellations of the southern hemisphere have their own 

 peculiar charm. Sagittarius, the Southern Crown, the South- 

 ern Cross, the Southern Triangle, the Altar, are all conspi- 

 cuous for the brilliancy of their stars, while Centaurus almost 

 rivals in splendour our own Orion, which I have here seen 

 at such an altitude as to wring from me an expression of deep 

 emotion and throw me into a glow of excitement. 



' Another very remarkable and surprising phenomenon which 

 I noticed even the second day of my arrival here is the ebb 

 and flow in the atmosphere. 



' Eemember me affectionately to our friend Blumenbach. 

 Oh how often I think of him as I see around me these marvellous 

 treasures of nature ! Tell him that the geological formation 

 here is of a most interesting character. Mountains of mica- 

 schist, basalt, gypsum, and gneiss. Sulphur and petroleum, 

 rising with considerable force from very small openings, throw 



