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



each other. The remaining results appear between parentheses 

 in the table. To these belong, for example, No. 5, the Wild- 

 spitze, evidently too little blue. For the graphic representation, 

 and for the calculations which follow, the arithmetic mean of 

 the observations from No. 11 to 30 is taken, the following ex- 

 cepted:— Nos. 14, 17, 30, all of which are too low. No. 18 

 deserves a little notice here. Considering its lateral position, 

 the result given is too high; but this scarcely justifies the con- 

 clusion, that on this day a point in the zenith would have been 

 much darker than on the other days, had the clouds permitted 

 us to observe it. We are inclined to believe that the depth of 

 this " blue patch " between clouds was an optical illusion, created 

 by the contrast with the bright surrounding clouds. This ex- 

 planation suggested itself to us immediately after the experiment, 

 and induced us in the determinations of colour to choose the 

 freest situations possible. With the exception of No. 36, the 

 arithmetic mean of the observations made at Heiligenblut is 

 taken. Nos. 43 and 46 (Bludenz) are not taken into account, 

 because after the eclipse the sky was somewhat obscured by 

 clouds. In like manner the Lake of Como is omitted, on account 

 of its position being more southern than those of the other sta- 

 tions. The remaining observations are united to a curve in fig. 4. 

 Somewhat more regularly formed, after the manner of a broken 

 line, for every 1000 feet of ascent the following values are given: — 

 3,000 Par. feet. 40 per cent, cobalt. Diff. 



1 

 3 

 3 

 3 

 8 

 9 

 8 

 8 

 7 

 5 

 These numbers exhibit — 

 1. A very slow ascent at the lower end of the curve. 

 3. A quick ascent between 6000 and 10,000 feet. 

 3. A new but inconsiderable diminution of the increase, from 

 10,000 feet upwards. 



The first is due to the same cause as that of the light colour 

 in the vicinity of the horizon, that is, to a mixture of watery 

 vapour which collects in the valleys on account of the evapo- 

 ration from the bottom and sides*. The sudden acceleration 



* A similar brightening of the atmosphere by watery vapour is also exhi- 

 bited on the sea-shore ; the sky towards the land is always darker than 

 towards the sea. — Humboldt's Voyages, vol. ii. p. 123. 



