AQUEOUS ATMOSPHERIC PHENOMENA. 



481 



portion of its hoard of aqueous particles. On nights that are perfectly cloudless, there- 

 fore, the deposition of dew is greater than when the sky is partially screened ; on those 

 that are both cloudy and windy there is none whatever formed ; but a gentle motion in 

 the air on a clear night is favourable to its production in the greatest copiousness, by 

 bringing fresh portions of the atmosphere, laden with moisture, into contact with the 

 colder bodies at the surface. The theory of the dewing process at once explains the ra- 

 tionale of the practice adopted by gardeners, to protect tender plants and fruit trees in 

 blossom on clear nights from cold, by spreading over them a mat or awning. The cover- 

 ing performs the same office as the clouds. It returns the heat radiated from the plants 

 to them, and thus a temperature is preserved which prevents refrigeration. It is often 

 observable that substances exposed at night to the action of the same circumstances, 

 differ greatly in the amount of dew deposited on them, some being thickly coated with its 

 pearls, while others are without a single globule. This arises frem the varying capacity 

 of bodies for the radiation of heat. Light downy substances part with it more freely than 

 the solids ; so that the former cool down to the dew-point, or that degree of the thermo- 

 meter at which its disengagement from the atmosphere commences, while the latter, 

 remaining above it, receive no deposition. The following enumeration has been given of 

 the substances which dew, in its formation, has been remarked more particularly to 

 cling to : 



Threads of the gossamer 



Swansdown 



Fine raw silk 



Fine unwrought cotton. 



Flax, wool, grass, hair 



Low plants and vegetables, both dead and living 



Glass 



Animal substances. 



As the temperature of substances must be reduced below that of the atmosphere in 

 order to the formation of dew, it is never observed, in temperate climates, upon the naked 

 parts of a living and healthy human body. 



The substances that show a marked inaptitude to receive dew, are 



Rocks 



Bare earth 



Considerable masses of water. 



The metals, in the following order : 

 Platina, least inaptitude. 

 Iron, steel, zinc, and lead, the next. 

 Gold, silver, copper, and tin, the greatest. 



Dr. Stocke recorded the following results as to the relative quantity of dew deposited 

 upon a variety of bodies : 



Glass 



Polished brass - 

 Rough brass 

 Rough iron, tinned 



Smooth 

 Rusty 

 Quicksilver 

 Smooth tin 

 Rough lead 



do. 

 do. 



Much dew. 

 Very little. 

 A little more. 

 Very much. 

 Scarcely any. 

 None. 

 None. 

 None. 

 Much. 



Polished lead - 



Silver - 



Silver, gilded 



Blue porcelain - 



A stone slab 



Basket of Indian cane - 



Smooth white oak plank 



Ditto black - 



Ditto fir plank - 



- A little 



- None. 



- None. 



- None. 

 Much. 



- A little. 

 Very much. 

 Much less. 



- Little. 



In opposition to the moisture of dew, that of mists is deposited upon all substances 

 exposed to it alike ; and another distinction is, that the moisture of mists exists previous 

 to deposition in a visible state, and is produced quite independent of the bodies that 

 receive it. 



A preceding statement, that in temperate climates dew is never observed upon the 

 naked parts of a living and healthy human body, is not true of tropical countries, where, 

 after the high temperature of the day, under a perfectly clear sky, the earth radiates its 



