588 



NA TURE 



[April 23. 1896 



PHYSICAL PHENOMENA OF THE HIGH 

 REGIONS OF THE ATMOSPHERE.^ 



THE first and decisive cause of nearly all physical phenomena 

 occurring in the terrestrial atmosphere is the solar heat. 

 The atmosphere may therefore be considered an immense heat 

 machine, of which the sun is the focus ; the boiler is represented 

 by the soil or the clouds heated by its rays, and the condenser 

 by the radiation towards the interplanetary space. 



The means by which physicists and meteorologists study the 

 various regions of the atmosphere are very limited ; they are 



. B" 



obliged to content themselves, more often than not, with very in- 

 direct observations, and to proceed by induction. In fact, the 

 most interesting phenomena occur in the high regions — that 

 is to say, at almost inaccessible heights. The object of this 

 lecture is to show by some experiments that meteorological 

 physicists are beginning to approach very closely the real ex- 

 planation of natural phenomena. Vou will see, in fact, that in 

 certain cases, not only an exact image of these phenomena is 

 obtained, but often a veritable synthesis of them may be pro- 

 duced by the employment of processes entirely analogous to those 

 which really operate in nature. 



I will begin by enumerating the means in use amongst 

 meteorologists for studying the different regions of the 

 atmosphere. 



The most direct method is the use of the aerostat ; the aero- 

 stat or balloon makes it possible to take instruments of measure- 

 ment to the very heart of the atmospheric regions one wishes to 

 study. Unfortunately the method is difficult, expensive, and 

 -also dangerous ; it is therefore only used in exceptional cases. 

 The balloon ascents which have resulted best are those of Gay- 

 Lussic (1804), of Glaisher (1862), and recently of Dr. Berson, 

 of Strassfurt (1894), who ascended more than 9000 metres. 



The most important facts observed in the balloon were very 

 unexpected ; here is the r^^«/;//of them : 



(i) There exist very frequently clouds formed of crystals of 

 ■ ice ; they constitute the cirrus, which 

 float at very great heights. 



(2) The direction of the wind changes 

 at different heights. 



(3) The temperature does not always 

 diminish regularly with the altitude; 

 very often cold layers and hot layers are 

 met with alternately. 



The second direct method for study- 

 ing the atmosphere is the constructing 

 of mountain observatories, as much as 

 possible on isolated peaks. In these 

 observatories the reality is daily verified 

 of these unforeseen inversions of wind 

 and temperature at different altitudes. 



As for the clouds of ice, they are too 

 high to be attained directly by the 

 mountain observatories. 



It will, perhaps, be interesting for you to know the principal 

 mountain observatories constructed in France. 



[Projection of the photographs of the following observatories : 

 Pic du Midi (altitude 2800 metres) in the Pyrenees. 

 Mont Ventoux ,, 1900 ,, in Provence. 



Puyde-Dome . ,, 1900 ,, in Auvergne. 

 The Eiffel Tower „ 330 ,, in Paris. 



1 Discourse delivered by Prof. Cornu, at the Royal Institution. (Translated 

 .by Winifred Lockyer.) 



This last observatory, owing to the lightness of its construc- 

 tion, entirely in open work, may almost be considered a captive 

 balloon, permanent and fixed, 300 metres above the ground.] 



Halos. — We have said that mountain observatories do not 

 attain the region of the clouds of ice {6000 to 10,000 metres in 

 altitude) ; it would, therefore, be only possible to observe them in 

 a balloon. Fortunately these crystals of ice reveal themselves 

 by an optical phenomenon, the halo, which is even seen from the 

 low levels. It is a brilliant circle, with radius of about 22'', 

 which encircles the sun or moon ; it has a reddish tint inside, 

 and slightly bluish tint towards the exterior. It is explained, 

 as well as many appearances of the same 

 kind, by the refraction of the light of the 

 body through ice crystals ; in fact, the crystals 

 of ice are hexagonal prisms, of which the 

 faces are in pairs inclined at 60°. These 

 crystals, disseminated in the air, and pointing 

 in all directions, refract the light, but the 

 refracted rays cannot exceed the slant of 22" 

 which the ininiintim deviation discovered Vjy 

 Sir Isaac Newton imposes on them ; the limit 

 of the refracted rays is, therefore, a cone of 

 22° round the line which joins the eye with 

 the sun or moon. 



[Experiment imitating the Halo. — Crystals 

 are produced in a transparent medium, con- 

 sisting of a mixture of appropriate liquids ; 

 in this way the mixture of hot and damp 

 I layers of the atmosphere is reproduced precisely, with the cold 

 i layers, which form the crystals of ice. 



I For this purpose a saturated aqueous solution of potash alum 

 is placed in a glass cell, and through this cell a stream of light is 

 ; made to pass, projecting the image of a circular opening repre- 

 senting the sun on a dark sky. Then a quarter of the total 

 j volume of rectified alcohol is added ; the alum, insoluble in 

 ; alcoholised water, is precipitated in very small crystals which 

 I float about in the liquid. The image of the sun is at first in- 

 i distinct, as in a mist, but soon a brilliant circle, with delicate 

 i rainbow tints, appears, and represents exactly the appearance of 

 the halo. The experiment is brilliant and instructive.] 



This phenomenon is well known by country people ; it is a 

 sure sign of rain when it appears on a hot day, even if no other 

 indications predict a metereological disturbance. 



Alternation and Inversion of Temperatures. — In neighbouring 



observatories situated at very different altitudes, such as that ot 



Puy-de-D6me and Clermont, the existence of hot currents are 



I often noted in the high regions. It is to successive inversions 



j of the same nature that Mr. Amsler, of Schaffhousen, attributes 



j that beautiful phenomenon known in Switzerland as " Alpen- 



I gllihen," and which consists of a second illumination of the 



snowy caps of the Alps some minutes after they had become 



dark by the setting of the sun. 



I [Projection of a photograph of the summits of the Bernese 



Fig. 2. — (i) Glycerine J, water 3 ; (2) glycerine §, water J; (3) anhydrous chloride of zinc, J, 



Oberland, the Jungfrau, the Monch, the Eiger ; the view having 

 been taken from St. Eeatenberg, close to the lake of Thun. 

 Picturesque imitation of the phenomenon by a coloured glass 

 and proper diaphragms.] 



Mr. Amsler's explanation is founded on the change of the 

 direction of the curvature of the trajectory of the luminous ravs 

 depending on whether the air at the bottom of the valleys is 

 warmer or colder than that of the higher regions. 



Before the setting of the sun the ground warmed by the solar 

 heat imprints on the trajectory a curve analogous to that of the 



NO. 1382, VOL. -53] 



