SCINTILLATION OP THE STARS. 79 



ghcrtiy before an earthquake. These ohservatlons only refei 

 to the serenely bright and rainless seasons of the year with- 

 in the tropics, from 10° to 12° north and south of the equa- 

 tor. The phenomena of light exhibited at the commence- 

 ment of the rainy season, during the sun's zenith-passage, 

 depend on very general, yet powerful, and almost tempestu- 

 ous causes. The sudden decrease of the northeast 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. I 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 season is an- 

 nounced many days beforehand by a flickering light of the 

 stars at great altitudes above the horizon. This phenome- 

 non is accompanied by sheet lightning, and single flashes on 

 the distant horizon, sometimes without any visible cloud, and 

 at others darting through narrow, vertically ascending col- 

 umns of clouds. In several of my writings I have endeav 

 ored to delineate these precursory characteristics and physi- 

 ognomical changes in the atmosphere.* 



The second book of Lord Bacon's Novum Orgamim gives 

 us the earliest views on the velocity of light and the prob 

 ability of its requiring a certain time for its transmission. 

 He speaks of the time required by a ray of light to traverse 

 the enormous distances of the universe, and proposes tho 



* See Voyage aux Rigions Equln., t. i., p. 511 and 512, and t. ii., p 

 202-208; also my Vie^cs of Nature,^. IG, 138. 



" En Arable, de meme qu'^ Bendei'-Abassi, port fameux du Golfe 

 Persique, I'air est parfaitement serein presque toute I'anuee. Le prin- 

 temps, I'ete, et I'automne se passent, sans qu'on y voie la moindre rosee. 

 Dans COS memes temps tout le monde couche dehors sur le haut des 

 maisons. Quand on est ainsi couche, il n'est pas possible d'exprimer le 

 plaisir qu'on prend k contempler la beaute du ciel, I'eclat des etoiles. 

 C'estune lamiere pui-e, ferme et eclatante, sans etincellement. Ce n'est 

 qu'au milieu de I'hiver que la scintillation, quoique tres foible, s'y fait 

 upercevoir." 



'' In Arabia," says Gai'cin, "as also at Bender- Abassi, a celebrated 

 port on the Persian Gulf, the air is perfectly serene throughout nearly 

 the whole of the year. Spring, summer, and autumn pass without ex 

 hibiting a trace of dew. Dm'ing these seasons all the inhabitants sleep 

 on the roofs of their houses. It is impossible to describe the pleasure 

 experienced in contemplating the beauty of the sky, and the brightness 

 of the stars, while thus lying in the open air. The light of the stars is 

 pure, steady, and brilliant ; and it is only in the middle of the winter 

 that a slight degi'ce of scintillation is observed." — Garcin, in Hift, dt 

 VAcad. des Sc, 1743 p. 30. 



