PHYSICAL PHENOMENA OF UPPER REGIONS OF ATMOSPHERE. 



127 



add to the contents of the jar a quarter of its total volume of rectified 

 spirits; the alum, insoluble in the alcoholic mixture, precipitates in 

 very minute crystals that float within the liquid. The image of the 

 sun first becomes dim as in a fog, but soon a brilliant and slightly 

 iridescent circle is seen, simulating very closely the appearance of a 

 halo. The experiment is brilliant and instructive. 



This phenomenon is well known to country people; it is a certain 

 sign of rain when it appears during a warm day, even Avhen no other 

 sign predicts a meteorological disturbance. 



Alteration and inversion of temperature.— In neighboring observatories 

 situated at widely different altitudes, like those of Puy-de-Dome and 

 Clermont, we often find that warm currents exist in the upper regions. 

 It is to successive inversions of this character that Mr. Amsler, of 

 Schaffouse, attributes the beautiful phenomenon known in Switzerland 

 as the "Alpengluhen," which consists in a renewal of the illumination of 

 the snowy summits of the Alps some moments after the setting of the 

 sun has darkened them. 



There was thrown on the screen a photograph of the summits of the 

 Bernese Oberland, the Jungfrau, the Monch, the Eiger; the view being 

 taken from St. Beatenberg, near the Lake of Thun. A picturesque 

 imitation of the phenomenon just cited was given by means of a colored 

 glass and suitable diaphragms. 



The explanation of Mr. Amsler is founded on the change of direction 

 of curvature that is given to the trajectory of the luminous rays accord- 

 ing as the air at the bottom of the valleys is warmer or colder than 

 that of the more elevated regions. 



Fig. 1. 



Before sunset, the earth's surface, heated by the solar rays, gives 

 the trajectory a curvature, S A M B, like that of a mirage; that is, 

 convex toward the earth; the sun, while setting at S', causes the 

 shadow of the summit, A, to be projected upon the summit, B, which 

 it would seem ought henceforward to remain in shadow, since the sun 

 continues to descend and its last ray is S' A M' B'. But if, during the 

 interval, the air of the valley becomes sufficiently cool, the trajectory 

 curves in the opposite direction, S ;/ A M" B", and the summit, B, is 

 illumined anew. 



Experiments shoiving the inversion of curves of luminous trajectories. — 

 By using some care we can place in a transparent jar, 20 cm. in diam- 

 eter, three strata of liquid, a lower one of chloride of zinc, heavy but 



