12. The surface of a leaf dew-covered
Dew has frequently been quoted as “A shower from heaven,” but this is not literally correct. True, it appears rather mysteriously from a clear sky, and upon a still, cloudless night covers thickly every blade of grass and plant life with seeming raindrops, and that frequently where rain clouds rarely appear, and the rain seldom falls. In such climates, where a rainfall is rare, it is certainly a most beneficial and wise provision, for it gathers upon all herbage and vegetation, in sparkling, refreshing profusion; while it avoids instinctively all barren rocky formations and all things which could not be benefited by its grateful cooling, moisture. Also, in cold, damp climates, where the air is continually saturated with moisture, and where an additional amount is not required, the gathering clouds and the dampness of the chilly atmosphere prevent a radiation of heat from the earth, and the dew never falls in such climates.
There are three requisites which appear to be essential for the formation of the dew: First, that the air should be moist; second, that the surface upon which it falls shall be cold, and third, that the sky be clear.
Of course the atmosphere always contains a greater amount of moisture after a rainfall, when the air has been greatly cooled. Evaporation is then continually going on among all objects lying near the surface of the earth. Blades of grass and all plants near the ground gradually cool and assume a lower temperature after sunset; they are preparing for the fall of the dew.
It has been remarked that certain plants possess greater powers of radiating heat and of expelling moisture through evaporative process than others; upon such plants the dew deposit is always more profuse, while those plants possessing less powers of radiation and evaporation, collect little dew.
13. Dew caught and held upon down of plant stem
14. Dew upon the down of a leaf