92 Dr. H. ScMaghttweit-s Observations in the Alps 



creasing to 282 0, 18; then, without interrupting the series, allows 

 the difference to sink again to 148 0, 97. The bismuth and anti- 

 mony equivalent for this is 12 o, 30; whereas for a difference of 

 152 0, 29 between the iron and platinum, before the difference of 

 temperature had reached the above amount (282°'18), the antimony 

 anci bismuth equivalent is only ll°-69. This fluctuation in the 

 7th series causes the curve derived from plotting to present 

 somewhat of the appearance of a railway section over undulating 

 ground, whereas in all the other cases it presents a gradual and 

 almost uniform ascent. The ' sudden leaps ' noticed by M. 

 Regnauit, whose cause he considered it impossible to ascertain, 

 appear to be thus capable of satisfactory explanation. 



XIV. Observations in the Alps on the Optical Phenomena of the 

 Atmosphere. By Dr. Hermann Schlagintweit. 

 [Concluded from p. 16.] 

 Colour of the atmosphere. Different kinds of Cy ammeters. Alte- 

 ration of the intensity of the blue with the height. Determinations with 

 the tricoloured Cyanometer. Cloud colours. 



Colour of the Atmosphere. 



THE blue colour of the sky, as well as the transparency of the 

 atmosphere, deepens as we ascend. De Luc* has already no- 

 ticed this. Saussure and Humboldt have published a long series 

 of experiments on the subject. The instrument used by both was 

 the cyanometer of Saussure. It consists of a number of strips 

 of paper, washed over with different shades of prussian blue. 

 The differences of shade are so regulated, that two strips, which 

 at a certain distance coidd not be distinguished from each other, 

 constituted divisions upon the scale. As normal distance, Saus- 

 sure assumed that at which the black circle of a diaphanometer 

 If" in diameter disappeared. Black was added by little ancl 

 little until perfect black was obtained. At zero the scale was 

 perfectly white, at the extreme end perfectly black. Within 

 these two limits the scale was divided into 53 degrees. With 

 this the colour of the sky was compared, and the nearest degree 

 was set down as the expression thereof t- 



* Modifications de V Atmosphere, vol. iv. § 117> P- 930. 

 t As an example of Saussure's degrees, we may mention that the mean 

 position of his cyanometer amounts — 



For Germany, to . . . 15 — 1/ 



For the torrid zone . . 20—24 



On Mont Blanc ... 39 



Humboldt found {Tableau Physique, p. 103) — 



In the tropics 23 



On the peak of Teneriffe . . 41 



On the Andes at 3000 toises \ 43 

 which has also been observed by Gay-Lussac. 



