226 RADIATION. 



A. Because the air will always hold 

 in solution a certain quantity of vapour, 

 (which varies according to its tempera- 

 ture) : and when the air is not saturated 

 with vapour, it may be condensed without 

 parting with it. 



Q,. Why are there EVER FOGS at night ? 



A. If the air be pretty well saturated 

 with vapour during the day, as soon as 

 its capacity for holding vapour is lessened 

 by the cold night, it deposits some of the 

 superabundant vapour in the form of dew 

 or fog. 



Q. Why is therevery OFTEN afog over MARSHES 

 and RIVERS at night-time ? 



A. The air of marshes is almost 

 always near saturation; and, therefore, 

 the least depression of temperature, will 

 compel it to relinquish some part of its 

 moisture in dew or fog. 



Q. What is the DIFFERENCE between DEW and 



RAIN? 



A. In dew, the condensation is made 

 near the earth's surface : 



In rain, the drops fall from a consid- 

 erable height ; but the cause of both is 



