90 SCIENCE OF COMMON THINGS. 



When fogs occur. When vapor forms clouds and when fogs. 



about equal in temperature with the surface of the 

 earth itself; for which reason the fog is not condensed 

 into dew, but remains floating above the earth as a 

 thick cloud. 



57*5 This fog seems to rise higher and higher, and yet remains quite as 

 dense below as at first : explain the cause of this ? 



The air resting on the earth is first chilled, and chills 

 the air resting on it ; the air which touches this new 

 layer of fog being also condensed, layer is added' to, 

 layer ; and thus the fog seems to be rising, when (in 

 fact) it is only deepening. 



57*B Why are there not fogs every night f 



Because the air will always hold in solution a cer- 

 tain quantity of vapor (which varies according to its 

 temperature) ; and, when the air is not saturated, it 

 may be cooled without parting with its vapor. 



57"? Wften do fogs occur at night f 



When the air is saturated with vapor during the 

 day. When this is the case, it deposits some of its 

 superabundant moisture in the form of dew or fog as 

 soon as its capacity for holding vapor is lessened by 

 the cold night. 



578 Why is there very often a fog over marshes and rivers at night- 

 time ? 



Because the air of marshes is almost always near 

 saturation and therefore the least depression of tem- 

 perature will compel it to relinquish some of its moist- 

 ure in the form of dew or fog. 



57*9 Why does vapor sometimes form into clouds, and sometimes rest 

 upon the earth as mist or fog 1 



This depends on the temperature of the air. "When 

 the surface of the earth is warmer than the air, the 

 vapor of the earth (being condensed by the chill air) 

 becomes mist or fog. But, when the air is warmer 

 than the earth, the vapor rises through the air, and be- 

 comes cloud. 



58O If cold air produces fog, why is it not foggy on a frosty morning f 



1, Because less vapor is formed on a, frosty day ; 



