PORTION OF THE COSMOS. VELOCITY OF LIGHT. 7 1 



and the upper currents of cold and warm air, not perceptible 

 iii the lower regions, which modify the intricate compensa- 

 tory movement of the interferences of the luminous rays. 

 The presence of a very thin orange-coloured mist, which 

 tinged the sky a short time before earthquake shocks, in- 

 creased the scintillation of the stars at high altitudes in a 

 striking manner. All these remarks apply to the perfectly 

 clear, cloudless, and rainless season of the tropical zone 10 

 or 12 degrees north and south of the equator. The changes 

 which take place in the phenomena of the light of the stars 

 at the commencement of the rainy season, during the passage 

 of the sun through the zenith, depend on very general and 

 almost tempestuous meteorological causes. The sudden 

 slackening of the north-east trade wind, and the inter- 

 ruption of the regular upper currents from the equator to 

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

 equator, produce the formation of clouds, and the daily 

 occurrence, at certain hours, of thunder and heavy rain. 

 I have remarked in several successive years, in places where 

 the scintillation of stars is at other times a rare occurrence, 

 that the approach of the rainy season is announced many 

 days beforehand by the tremulous light of stars high in the 

 heavens. Sheet lightning, and single flashes in the distant 

 horizon, without visible cloud, or in narrow perpendicularly 

 rising columns of cloud, are accompanying signs. In seve- 

 ral of my works I have tried to describe these changes in 

 the physiognomy of the heavens, which are the characteristic 

 precursors of the rainy season. 



On the subject of the velocity of light, and the probability 

 that it requires a certain time for its propagation, we find 



