170 SCIENCE OF COMMON THINGS. 



No dew on the ocean. No dew falls on the human body. 



Although dew dcx s not appear upon ships at a great distance from land, 

 it is freely deposited on the same vessels arriving in the vicinity of terra 

 firma. Thus, navigators who proceed from the Straits of Sunda to the 

 Coromandel coast, know that 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. 



1112 Why does not dew form upon the surface of water ? 



Because whenever the aqueous particles at the sur- 

 face are cooled, they become heavier than those below 

 uiein, and sink, while warmer arid lighter particles rise 

 to the top. These, in their turn, become heavier, and 

 descend; and the process, continuing throughout the 

 night, maintains the surface of the water and the air at 

 nearly the same temperature. 



1113 Is the temperature at which dew is deposited from the air always 

 the same ? 



No ; when the air is saturated with moisture, a slight 

 reduction of temperature occasions a deposition of dew; 

 but when the air is very dry, a greater reduction of 

 temperature is necessary to condense its vapor. 



1114 Why are the exposed parts of the human body never covered with 

 deiv? 



Because the vital heat, varying from 96 to 98 Fah- 

 renheit, effectually prevents such a loss of warmth as is 

 necessary to its production. 



1115 In what countries are the dews most copious and abundant ? 



In tropical climates. 



1116 What is the reason of this f 



Because in those countries there is the greatest dif- 

 ference between the temperature of the day and that 

 of the night. 



The development of vegetation is greatest in tropical countries, and a 

 great part of the nocturnal cooling is due to the leaves, which present to 

 the sky ?.T\ immense number of thin bodies, having large surface, well 

 adapted to -radiate heat. 



