III.] , RAIN AND DEW. 53 



upright stand. In another form of the instrument, devised 

 by M. Regnault, the moisture is precipitated upon the sur- 

 face of a small vessel of polished silver. It will be seen, 

 however, that both instruments are but refined modifications 

 of our familiar experiment with the tumbler of cold water. 



After sunset, on a clear night, the grass and other objects 

 on the surface of the earth give off the heat which they 

 have absorbed during the day, while the sun has been 

 shining upon them, and their temperature is thus gradually 

 lowered. The air in contact with these objects is also 

 cooled ; and, as it gets cool, it grows less able to retain its 

 moisture, until at length the dew-point is reached, when 

 drops of liquid are deposited on the blades of grass. Some 

 bodies throw out, or radiate^ their heat much more freely 

 than others, and dew is therefore precipitated copiously 

 upon such good radiators. Thus it may sometimes be seen 

 in a garden that every blade of grass is bedecked with 

 sparkling dew-drops, while the neighbouring gravel path 

 remains almost dry. The grass has parted with its heat, 

 and consequently become chilled, more readily than the 

 gravel has cooled ; and the dew is therefore distilled more 

 abundantly upon the grass than upon the gravel. 



Whatever prevents the radiation or giving-off of heat 

 from terrestrial bodies tends to hinder the formation of dew. 

 A cloud, for example, acts in this way, since it reflects, or 

 throws back upon the earth, the heat which would otherwise 

 be projected into space. Dew is therefore most copious on 

 a cloudless night. A calm atmosphere also promotes the 

 formation of dew, for it is obvious that agitation by currents 

 of air must be unfavourable to local refrigeration ; while it 

 promotes the evaporation of any dew that may have been 

 deposited. 



It was not until the beginning of the present century that 



