126 PHYSICAL PHENOMENA OF UPPER REGIONS OF ATMOSPHERE. 



The most important facts observed from the balloon were entirely- 

 unexpected. I will briefly state them : 



1. Clouds formed of ice crystals occur very frequently; they consti- 

 tute the cirrus clouds that float at very great heights. 



2. The direction of the wind changes at different heights. 



3. The temperature does not always decrease regularly with the 

 increase in altitude, cold strata and warm strata often alternating with 

 each other. 



The second method of studying the atmosphere is by the establish- 

 ment of mountain observatories, upon isolated peaks when possible. 

 At these stations the unexpected inversion of the temperature at vari- 

 ous altitudes is daily verified. 



The ice clouds are too high to be directly reached by mountain 

 observatories. 



A view of the principal mountain observatories of France will prob- 

 ably interest you. 



Photographs of the following observatories were then thrown on 

 the screen : 



Pic du Midi (altitude 2,800 meters), in the Pyrenees. 



Mont Ventaux (altitude 1,900 meters), in Provence. 



Puy-de-Dome (altitude 1,900 meters), in Auvergne. 



Eiffel Tower (altitude 330 meters), at Paris. 



This last observatory, thanks to the lightness of its construction of 

 open work, may almost be considered as a captive balloon fixed perma- 

 nently at 300 meters above the ground. 



Halos. — Since ice clouds are situated at altitudes (0,000 to 10,000 

 meters) greater than that of the highest mountain observatories, we 

 are condemned to the use of the balloon alone for all observations upon 

 them. Fortunately the presence of ice crystals is revealed by an opti- 

 cal phenomenon that can be observed even at ordinary levels — the halo. 

 This is a brilliant circle having a radius of about 22 degrees that sur- 

 rounds the sun or moon. It has a reddish tint within and is slightly 

 bluish at its outer border. It is explained, as are many appearances of 

 a similar kind, by the refraction of the light of the sun or the moon 

 in passing through icy needles. In fact the ice crystals are hexagonal 

 prisms, the faces of which are inclined to each other, two by two, at an 

 angle of 60 degrees. These, scattered through the air and facing in 

 every direction, refract the light, but the refracted rays can not pass 

 beyond the angle of 22 degrees imposed upon them by the minimum of 

 deviation discovered by Sir Isaac Newton. The limit of the refracted 

 rays is then a cone of 22 degrees around the line that passes from the 

 eye to the luminous body. 



Experiment imitating a halo. — By forming crystals in a transparent 

 medium made by mixing appropriate liquids, there is exactly repro- 

 duced the mingling of the warm, moist strata of the atmosphere with 

 the cold ones which produces ice crystals. 



To do this place in a glass jar a saturated aqueous solution of potash 

 alum, and send through the jar a luminous beam projecting the image 

 of a circular opening like that of the sun upon the dark sky. Then 



