1bow Dew ie ^formed, u? 



is charged with this vapor to the point of satu- 

 ration (which point varies with the tempera- 

 ture) a slight depression of the temperature 

 is sufficient to condense this vapor into cloud 

 or drops of water. Between 1812 and 1814 

 Dr. Wells made a series of experiments with 

 flocks of cotton wool. He weighed out pieces 

 of equal weight and attached a number of 

 them to the upper side of a board and as many 

 more to the lower side, and exposed it to the 

 night air under varying conditions. One ex- 

 periment was made with a board four feet 

 from the earth, so that half of the bunches of 

 cotton faced the ground and the other half the 

 sky. He found upon weighing these after a 

 night's exposure under a clear sky that the 

 cotton wool on top of the board had gained 

 fourteen grains in weight from the moisture, 

 or dew, that had formed upon it, while the 

 same amount of cotton on the under side of 

 the board had only increased four grains. He 

 tried further experiments by making little 

 paper houses, or boxes, to cover a certain por- 

 tion of grass or vegetation. He found that 

 while there would be a heavy dew on the grass 

 outside there was little or none within the in- 

 closure. These experiments were conducted 

 in various ways and closely watched to see 

 that none of the phenomena were in any way 

 connected with falling rain. It has been de- 

 termined that substances like grass and green 



