HAIL AND DEW. 641 



ascertain for himself. We have another frozen fall of water from the clouds 

 viz., hail, which may possibly be the development of sleet. 



HAIL is formed by the falling rain being frozen in its descent, or when 

 different currents meet in the atmosphere. A hail-storm is accompanied with 

 a rushing sound, as if the hail-stones were striking against each other. They 

 are very destructive, and actual hail showers occur in summer more frequently 

 than in winter, and a peculiarity noticeable with regard to hail is its infre- 

 quent occurrence during the night. 



Records of destructive hail storms are plentiful. The hail assumes a 

 great size, weighing sometimes as much as two ounces, and measuring several 

 inches round. Thunder and lightning are very frequent accompaniments of 

 hail showers. 



DEW is moisture of the atmosphere deposited on a cool surface 

 another form of condensation, in fact. Cold water in a tumbler will 

 produce a " dew " upon the outside of the glass when carried into a warm 

 atmosphere. Such is the dew upon the grass. It is produced by the air 

 depositing moisture as it becomes colder after a warm day when much 

 vapour was absorbed. Warm air can hold more water than cold air, and, 

 the saturation point being reached, the excess falls as dew at the dew (or 

 saturation) point. We have previously remarked that one use of clouds was 

 to prevent rapid radiation of heat which they keep below. Under these 

 circumstances viz., when a night is cloudy we shall find much less dew 

 upon the grass than when a night has been quite clear, because the heat has 

 left the atmosphere for the higher regions, and has then been kept down 

 by the clouds ; but on a clear night the air has become cooled rapidly 

 by radiation, and having arrived at saturation point, condensation takes place. 



Dew does not fall, it is deposited ; and may be more or less according 

 to circumstances, for shelter impedes the radiation, and some objects radiate 

 less heat than others. .Hence some objects will be covered with dew and 

 others scarcely wetted. 



When the temperature of the air is very low, down to freezing point, 

 the particles of moisture become frozen, and appear as hoar-frost upon the 

 ground. Thus dew and hoar-frost are the same thing under different 

 atmospheric conditions, as are water and ice and vapour. 



We have now come round again almost to whence we started. We 

 have seen the land and water, and the parts that water, in its various forms, 

 plays upon the land, and its effects in the air as rain, etc. We have noticed 

 the winds and air currents as well as the ocean and its currents. We know 

 what becomes of rain and how it is produced, and how the sea works upon 

 the shore, and how clouds benefit us. There are besides some less common 

 phenomena which we \\ill now proceed to examine. 



