636 



SCIENTIFIC RECREATIONS. 



varied phenomena of perihelia, etc., are due also to the snowy or icy flakes 

 of the cirri and cirri-cumuli, caused by the refraction of light from the frozen 

 particles. These cirri clouds are indicative of changeable weather as "Mares' 

 tail " skies, and long wisps of cloud, foretelling storm. 



The cirro-cumulus is the true " mackerel " sky, and is formed by the 

 cirri falling a little and breaking off into small pieces of cumulus, which is a 

 summer (day) cloud generally, and appears in the beautifully massive and 

 rounded forms so familiar. The stratus is, as its name implies, a cloudy layer 

 formed like strata of rock. It is generally observable at night and in the 

 winter. It often appears suddenly in the sky consequent upon diminished 

 pressure or a rapid fall of temperature. It is low-lying cloud sometimes, 

 and at night forms fogs. 



The cirro-stratus is perceived in long parallel lines, and indicates rain ; 

 when made-up rows of little curved clouds it is a certain prophet of storm, 



m 



Fig. 718. Cirrus cloud. 



and when viewed as haze is also indicative of rain or snow. " Mock-suns " 

 and halos are often observed in the cirro-stratus. 



The nimbus is the rain-cloud, or condition of a cloud in which rain falls 

 from it. It is upon this rain-cloud we can perceive the rainbow, and on no 

 other cloud, but otherwise only in the sky. 



We have now seen the varieties of cloud and their common origin with 

 fogs and mists, which differ from them only in the elevation at which they 

 come into existence, according to the condition of the atmosphere. 



The uses of clouds are many and varied. Their first and most apparent 

 use seems to be the collection and distribution of rain upon the earth. But 

 besides this, they shelter us from the too great heat of the sun, and check 

 the evaporation at night. Supposing we had no clouds we should have no 

 rain. If we had no rain the earth would dry up, and the globe would appear 

 as the side of the moon appears a waterless desert. The invisible vapour 

 in the atmosphere will produce cloud, but the moon can have no atmosphere 

 in that sense. Vapour will also absorb heat, and intercept the sun's heat 

 rays, acting much as clouds do in preventing radiation and great changes of 

 temperature.* 



* See " Molecular Physics " (Tyndall). 



