102 Mr. Webster's experiments respecting dew. 
- We observe, that in hot. weather dew is perceivable long before 
sunset. The laborer feels the moisture on his clothes and on’ the 
grass, while the sun is above the horizon. _ But Iam doubtful wheth- 
er the same phenomenon is observable in the morning, when the tem- 
perature of the air is the same.» On the contrary, I suspect the dew 
evaporates in the morning, with a temperature of the atmosphere, in 
which vapor will condense in the afternoon. In the morning both the 
air and the earth are cool ; and as soon as the sun begins to warm the — 
air and earth, a small degree of evaporation is begun. But when the 
earth has been heated almost to the temperature of blood, the water 
near the surface partakes of the heat. Soon after the warmest part of 
the day is bast, the Aono oh begins to cool, while the earth retains 
the h ca 
a, ae earth emits. a warm vapor; the atmosphere is cooled 
to a; a,pondehser.’ - This is the distillation of nature ;. and thus dew is 
formed. It seems to be the comparative coolness. of the atmosphere, 
which renders the vapor condensable in the afternoon. This is the 
case also in the evening ;_ the earth retaining its heat longer than the 
air, and the warm vapor, constantly ascending into the cooler atmos- 
phere, is speedily condensed... I suspect therefore, that a great por- 
tion of the dew, which is formed during the night, is composed of par- 
ticles, that proceed from the earth during the night. So far then dew 
may be said tovise, and not fall, But it is the vapor in fact, which 
rises, in an imperceptible form, as during the day ; but in the night 
it is spe ene whereas in the day time it continues im- 
perceptible. 
einai Votimideail lead to a solution of the dificulty-be- 
fore mentioned, viz. the appearance of dew on both sides of the upper 
leaves. of plants, while none appeared on either side of the under leaves: 
‘The upper leaves, being exposed to the direct action of the cold. ait, 
