NOTES. XV 



the determination of place in the heavens, an extension and success never 

 before attained. 



( 93 ) p. 44. Kosmos, Bd. ii. S. 209 (English edition, Vol. ii. p. 175). 



( 94 ) p. 45. The passage in which Strabo (Lib. iii. p. 138, Casaub.) seeks 

 to refute the views of Posidonius, runs, according to the manuscript, as 

 follows : " The image of the sun, both at sunrise and sunset, is enlarged 

 over the sea, because there the vapours ascend most abundantly from the 

 humid element ; for the eye, when it sees through vapours, as when it looks 

 through tubes, receives the images refracted and enlarged : and the same thing 

 happens when it sees the sun or moon set behind a thin dry cloud, in which 

 case they also appear of a red colour." This passage has, quite lately, been 

 supposed to be corrupt (Kramer, in Strabouis Geogr. 1844, Vol. i. p. 211) ; 

 and it has been proposed to read in lieu of 81 ouAwz/, St va.\tav (through 

 glass globes) (Schneider, Eclog. phys. Vol. ii. p. 273). The magnifying 

 power of hollow glass globes filled with water (Seneca, i. 6) was, indeed, 

 known to the ancients, as well as the effects of burning glasses, or " burning 

 crystals" (Aristoph. Nub. v. 765), and of Nero's emerald (Pliu. xxxvii. 5) ; 

 but certainly such globes could not serve for astronomical measuring instru- 

 ments (compare Kosmos, Bd. ii. S. 464, Note 44 ; English edition, Vol. ii, 

 Note 384). Altitudes of the Sun, taken through thin light clouds, or through 

 volcanic vapours, show no trace of the influence of refraction (Humboldt, 

 Recueil d'Observ. astr. Vol. i. p 123). Colonel Baeyer has been unable to 

 find any angular alteration of the heliotrope light when streaks of inist were 

 passing, or even with vapours purposely called forth, thus confirming 

 Arago's experiments. Peters, in Pulkova, in comparing groups of star- 

 altitudes measured when the sky was clear with others observed through 

 light clouds, finds no difference amounting to 0".017 (see his Recherches sur 

 la Parallaxe des Etoiles, 1848, p. 80 and 140143 ; Struve, Etudes stellaires 

 p. 98). On the employment of tubes in Arabian instruments, see Jourdain 

 sur 1'Observatoire de Meragah, p. 27 ; and A. Sedillot, Mem. sur les Instru- 

 ments astrpnomiques des Arabes, 1841, p. 198. Arabian astronomers have 

 also the merit of having first introduced large gnomons with small circular 

 openings. In the colossal sextants of Abu Mohammed al-Chokandi, the 

 limb, graduated to 5 minutes, received the image of the Sun itself. " A midi 

 les rayous du Soleil passaieut par une ouverture pratiquee dans la voute de 

 1'Observatoire qui couvrait 1'instrument, suivaieut le tuyau, et formaient sur la 

 concavite du sextant une image circulaire, dont le centre donnait, sur 1'arc 

 gradue, le complement de la hauteur du Soleil. Get instrument ne differe de 



