104 cosmos. 



latitude of the place. In some serene and calm nights, the 

 region of scintillation extended to an elevation of 20 or even 

 25 ; but a connection could scarcely ever be traced between 

 the differences of altitude or intensity of the scintillation 

 and the hygrometric and thermometric conditions, observ- 

 able in the lower and only accessible region of the atmosphere. 

 I have observed, during successive nights, after considerable 

 scintillation of stars, having an altitude of 60 or 70, when 

 Saussure's hair-hygrometer stood at 85, that the scintillation 

 entirely ceased when the stars were 15 above the horizon, 

 although the moisture of the atmosphere was so considerably 

 increased that the hygrometer had risen to 93. The intricate 

 compensatory phenomena of interference of the rays of light 

 are modified, not by the quantity of aqueous vapour con- 

 tained in solution in the atmosphere, but by the unequal 

 distribution of vapours in the superimposed strata, and by 

 the upper currents of cold and warm air, which are not 

 perceptible in the lower regions of the atmosphere. The 

 scintillation of stars at a great altitude was also strikingly 

 increased during the thin yellowish red mist, which tinges 

 the heavens shortly before an earthquake. These obser- 

 vations only refer to the serenely bright and rainless seasons 

 of the year, within the tropics, from 10 to 12 north and 

 south of the equator. The phenomena of light exhibited 

 at the commencement of the rainy season, during the sun's 

 zenith-passage, depend on very general, yet powerful, and 

 almost tempestuous causes. The sudden decrease of the north- 

 east trade-wind, and the interruption of the passage of regular 

 upper currents from the equator to the poles, and of lower 

 currents from the poles to the equator, generate clouds, and 

 thus daily give rise, at definite recurring periods, to storms of 

 wind and torrents of rain. 1 have observed during several 

 successive years that in regions where the scintillation of the 

 fixed stars is of rare occurrence, the approach of the rainy 



