52 SPECIAL RESULTS IN THE URANOLOGICAL 



saltic precipice, before we, who were looking for him with 

 telescopes, discovered him. In a short time my companion 

 (the ill-fated son of the Marques, Carlos Montufar, who 

 afterwards fell a victim in the civil war) and myself were 

 also able to distinguish the white moving figure with the 

 naked eye. Bonpland was wrapped in a white cotton 

 mantle, the poncho of the country. Allowing from 3 to 5 

 feet for the breadth of the shoulders, as the mantle some- 

 times clung close, and sometimes seemed to fly loosely in 

 the wind, and taking the known distance, we have from 

 7" to 12" as the angle under which the moving, object was 

 distinctly seen. "White objects on a black ground are seen, 

 according to Hueck's repeated experiments, at a greater dis- 

 tance than black objects on a white ground. The weather 

 was clear, and the ray passed through the stratum of thin 

 air, proper to an elevation of 14412 Trench (15360 Eng.) 

 feet above the level of the sea, to our station at Chillo, itself 

 8046 (8575 Eng.) feet high. The distance from the eye 

 to the object was 85596 (91225 Eng.) feet, or 14'8 geo- 

 graphical miles. The heights of the barometer and ther- 

 mometer at the two stations were very different: at the 

 upper, probably, 194 lines (17*23 English inches), and 8 

 Cent. (46.4 Eahr.) ; and at the lower, by exact observation, 

 250-2 lines (22'22 English inches), and 1S'7 Cent. (65'66 

 Eahr.) Gauss's heliotropic light, which has become so 

 important an auxiliary in our German trigonometrical 

 measurements, reflected from the Brocken to the Hohen- 

 hagen, was seen there at a distance of 213000 French feet 

 (227008 Eng.), more than 36 geographical miles; and 

 often at points at which the angle subtended by a three-inch 

 mirror only amounted to. 0'H3. 



The visibility of distant objects is modified by the absorp- 



