oT8 SCIENTIFIC RECORD FOR 1884. 



iiir, iucreasing and diminisbiug directly with it, so that the items 2 and 

 3 are probably exjilicable thereby. 



5. It l'ollo\?s that this an<^le (whose auuual meau value for the even- 

 ing twilight is in Athens 15'^.\), southern Spain 15<^.6, South Atlantic 

 Ocean 15o.6) must be greater in high latitudes than in low, and on the 

 ocean than in dry continental regions, but this conclusion needs further 

 confirmation by other observations. 



G. Since the twilight depression of the sun is thus periodically vari- 

 able it follows that the various solutions of the problem of the short- 

 est twilight thus far offered are no longer sufficient. {Z. 0. G. M., xix, 

 p. 57.) 



382. Dr. G. Hellmann, in a second communication on the twilight 

 phenomena, analyses numerous observations from all parts of the world 

 and formulates the following conclusions relative to the difference be- 

 tween this phenomenon in southern Spain and in Germany. 



1. The phases of the phenomenon are better developed in Spain than 

 in Germany, the chronological order is more determined, the strata of 

 different colors are easier to distinguish from each other, the limit of 

 the bright segment against the dark sky is verj^ sharp. 



2. In Germany the extension of the colors in time and space are 

 greater than' in Spain, except for the anti-twilight. 



3. In reference to the colors, green was almost regularly observed in 

 all its shades in Spain but rarely in Germany. 



4. The red colors of the twilight in Spain had in Germany very fre- 

 quently a decidedly flesh color and purple tint. 



5. Violent colors in Germany were more decided than in Spain. 



6. In Sjiain there was a greater contrast between the twilight phe- 

 nomenon of the summer and winter (the dry and rainy jjeriods) than in 

 Germany. 



The above conclusions, and in fact this whole essay, were completed 

 before the appearance of the remarkable skj' colors of lS83-'84, and in 

 reference to these Hellman states in a note that, although he has fol- 

 lowed it closely and collected numerous reports from others, yet he will 

 not at j)resent attempt any analysis or explanation. He however notes 

 the great ignorance of observers generally as to what constitutes the 

 ordinary jfud typical phenomena of the twilight, and at present his own 

 feeling is that the real peculiarity of the phenomena of 1883-84 con- 

 sisted in their universality, the length of duration, the intensity and 

 variety of colors. {Z. 0. G. M., xix, p. 102.) 



383. Professor Kiessliug, of Hamburg, has reproduced the experi- 

 ments of Coulier, Mascart, and Aitken on the effect of dust in the atmos- 

 phere. He says that if we understand the word dust to refer to the 

 total of all foreign substances in the atmosphere, iucludiug coarse and 

 fine i)articles of dust, microscopic or organic forms, the products of pu- 

 trefa(;tion and oxidation, even when they are entirely gaseous, a,s, for 

 iustauce, the expirations of human beings and animals, there will be 



