116 feature's 



the cause of it has been understood. We even 

 yet speak of the dew " falling " like rain. In 

 former times some scientists supposed that it 

 was a fine rain that fell from the higher 

 regions of the atmosphere. Others supposed 

 it to be an emanation from the earth, while 

 still others supposed it was an exudation from 

 the stars. 



"By his knowledge the depths are broken up and the 

 clouds drop down dew " (Prov. iii. 20). 



The first experiments carried on in a scien- 

 tific way were by Dr. Wells, a physician of 

 London, between the years 1811 and 1814. 



Everyone has noticed in warm weather the 

 familiar phenomenon of water condensed into 

 drops on the outside of a pitcher or tumbler 

 containing cold water. This condensation is 

 dew. It always forms when the conditions 

 are right, summer and winter. In cold 

 weather we call it frost. It has been stated 

 in a former chapter on evaporation that the 

 capacity of the air for holding moisture in a 

 transparent form depends upon its tempera- 

 ture. If the temperature is at the freezing 

 point it will contain the 160th part of the at- 

 mosphere's own weight as aqueous vapor. If 

 it is 60 degrees Fahrenheit the air will retain 

 six grains of transparent moisture to the 

 square foot of air, while at 80 degrees it will 

 contain nearly eleven grains. When the air 



