272 WELLS'S NATURAL PHILOSOPHY. 



cess, continuing throughout the night, maintaLos the surface of the water and 

 the air at nearly the same temperature. 



Although dew does not appear upon ships in mid-ocean, it is freelr depos- 

 ited on the same vessels arriving in the vicinity of land. Thus, navigatora 

 who proceed from the Straits of Sunda to the Coromandel coast, know tliat 

 they are near the end of the voyage when they perceive the ropes, sails, and 

 other objects placed on the deck become moistened with dew during the 

 night 



The exposed parts of the human body are never covered with dew, because 

 fii3 vital temperature, varying from 9G° to 9S° F., effectually prevents a loss 

 of heat sufficient for its deposition. 



Dew is produced most copiously in tropical countries, because there is in 

 such latitudes the greatest difference between the temperature of the day and 

 that of the night. The development of vegetation is also greatest in tropica) 

 countries, and a great part of the nocturnal cooUng is due to the leaves which 

 present to the sky an immense number of thin bodies, having large surface, 

 well adapted to radiate heat. 



Dew rarely falls upon the small islands of the Pacific; the reason is, that 

 the air over the vast ocean in which these islands are situated, preserves a 

 nearly unrform temperature day and night. The islands are comparatively 

 of small extent, and the stratum of air cooled by the contact of the sod is 

 warmed by mixing with the air that is constantly reaching it from the sea. 

 This prevents a depression of temperature in the air sufficient to cause a depo- 

 eition of dew. 



What is frost? 606. Frost is frozGn dew. 



"VVhen the temperature of the body upon which the dew is 

 deposited sinks below 32° F., the moisture freezes and assumes a sohd form, 

 constituting what is called "frost." 



Shrubs and low plants are more liable to be injured by frost than trees of 

 a greater elevation, since the air contiguous to the surface of the ground is the 

 most reduced in temperature. 



Why does a ^^ exceedingly thin covering of muslin, 

 pro'tect'^oTSs matting, etc., will prevent the deposition of 

 froS?^*^"^ "'■ dew or frost upon an object, since it prevents 

 the radiation of heat, and a consequent cool- 

 ing suflScient to occasion the production of either dew or 

 frost. 



Fig. 224, in which the arrows indicate the movements of heat, and the 

 numerals the temperatures of the earth and air under diflerent circumstances, 

 will render the explanations of the phenomena of dew and frost more in- 

 telligible. 



The figures in the middle/jf the diagram represent the temperature of the air 

 at a distance from the surface of the earth ; the figures in the margin, the 

 temperature of the air adjoining the surface of the earth ; the figures below 



