100 Dr. H. Schlagintweit's Observations in the Alps 



If the side portions of the heavens be examined, a decrease in 

 the deptli of bine is observable which does not seem to be irre- 

 gular. Alexander von Humboldt's observations on the Atlantic 

 Ocean (18° 53' N. L.) — at a place, therefore, where lateral dis- 

 turbances through local changes of temperature, so common in 

 mountains, was not to be feared — proved that the blue colour 

 varied nearly as the cosine of the zenith distance *. In commu- 

 nicating these observations, we retain the original division of the 

 cyanometer of Saussure into degrees. 



The same law may be recognised through many of our obser- 

 vations, but rarely with this exactitude. The zenith distance 

 must be always measured by a proper instrument, and not esti- 

 mated by the eye alone. In the latter case the peculiar shape 

 of the dome of the firmament might be the source of considerable 

 error. Thus by contemporaneous observations made on the 

 Johannishutte, we found — 



Series A. 



The similarity between these results and those in the foregoing 

 tabic will appear more evident when it is remembered, that one 

 degree of Saussure corresponds to 3 or 4 per cent, of cobalt in 

 our case. The unequal increase between every two of Saussure' s 

 degrees, when the latter are expressed in per-centage of cobalt 

 (compare p. 95 above), can have no disturbing influence here. 



* Humboldt's Voyaye, vol. ii. p. 122. Observed on June 30, J 799. 



