70 SPECIAL RESULTS IN THE URANOLOGICAL 



escaped the observation of La Condamine and Bouguer in 

 the Peruvian plains, and of Garcin ( 134 ) in Arabia, India, 

 and on the coasts of the Persian Gulf, near Bender Abassi. 



As the aspect of the starry heavens at the season of per- 

 petually clear and perfectly cloudless tropical nights had for 

 me a peculiar charm, I took the pains of always noting in 

 my journals the altitude above the horizon at which the 

 scintillation ceased with different hygrometrical readings. 

 Cumana, and the rainless part of the Peruvian sea-coast, 

 before the season of Garua or fog, were particularly suited 

 for such observations. According to the mean results of 

 my observations, the larger fixed stars would appear for the 

 most part to scintillate only when below 10 or 12 from 

 the horizon. At greater altitudes they shed a mild and 

 planetary light. The difference is best recognised by fol- 

 lowing the same star in its gradual ascent or descent, mea- 

 suring at the same time its angle of altitude, or calculating 

 it, if the latitude of the place and the time be known. 

 Sometimes, in equally clear and equally calm nights, the 

 region of scintillation extends to 20 and even 25 of alti* 

 tude ; yet there could hardly ever be traced any connection 

 between these differences, and the state of the thermometer 

 and hygrometer as observed in the lowest, and only accessi- 

 ble, atmospheric stratum. I have seen in successive nights, 

 after considerable scintillation of stars between 60 and 70 

 high, with Saussure's hair hygrometer at 85, the scintilla- 

 tion cease entirely at 15 above the horizon, although the 

 humidity of the air had so much increased that the hygrometer 

 had advanced to 93. It is not the quantity of aqueous 

 vapour which the atmosphere holds in solution, but the 

 unequal distribution of vapour in the superimposed strata, 



