348 ANNUAL OF SCIENTIFIC DISCOVERT. 



TRANSPARENCY OF THE AIR. 



The inhabitants of valleys know that one of the surest signs of rain 

 is the clearness of outline and blue color of the distant mountains; 

 it indicates great dampness in the air. But how does it happen 

 that this dampness aids the transmission of light, while it hinders 

 that of radiating heat, as Tyndall has shown ? According to Col. 

 Jackson, the watery vapor dissolves the impurities which are 

 mingled with the air, and thus renders it very transparent. En- 

 tering into these ideas, and recalling the large proportion of at- 

 mospheric dust of all kinds which later works have made known 

 to exist, De la Rive has concluded that not only does the atmos- 

 pheric dust become trans^Darent on absorbing .the watery vapor, 

 but that the water absorbed renders the dust heavier and makes it 

 fall to the ground. He also admits that if the presence of watery 

 vapor renders the air transparent when it contains dust or organic 

 particles, its presence is no longer necessary for this end in the 

 absence of dust. This explains why, in winter, mountains appear 

 so clearly at a distance, even when the air is very dry ; why the 

 air is so clear over plains of snow, and why it is always the sam"^ 

 on the peak of Teneriffe in consequence of the east wind. In the 

 warm season, and in the months when organic life has the great- 

 est activity, the air is most charged with this kind of dry vapor, 

 which in calm weather diminishes in so wondcrfal a manner the 

 visibility of distant objects. Of the various kinds of dust, only 

 those which are soluble in water, as common salt, according to 

 Bunsen, are always in the air, even in regions remote from the 

 coast; these are the kinds of dust which contribute to the fertili- 

 zation of the soil ; the chemist Barral has even found phosphates. 



According to Marshal Vaillant, atmospheric refraction has very 

 much to do with these phenomena. In the hot seasons the air is 

 warmed by contact with the soil, expands and changes its density 

 and refracting power, — effects which do not happen when the air is 

 cold. These considerations have induced De la Rive to include 

 the transparency of the atmosphere in the list of meteorological 

 elements, to be regularly observed in order to establish the precise 

 relations which exist between this particular element and all 

 others, such as the pressure, temperature, moisture, hour of the 

 day, and epoch of the year; a kind of observation of interest not 

 only to science, properly so-called, but also perhaps to medicine. 

 With the add of Mr. Thury, De la Rive has had constructed a 

 photometer designed to measure the variations in the transpar- 

 ency of the air at different seasons. — Amer. Jnurn. of Science^ 

 Jan., 1868. 



SUMMARY OF FACTS IN ASTRONOMY AND METEOROLOGY. 



Color of the Sky. — As seen from a balloon by M. Flammarion, 

 the sky above the height of 3,000 metres appears dark and impen- 

 etrable ; it is deep grayish-blue near the zenith, azure blue in the 



