REFECTION AND REFRACTION. 283 



motion could be seen but not the form, the outlines of 

 objects are much worse defined, and small and distant 

 ones are much less distinctly seen, than when the air 

 ceases to take up moisture. Thus vision becomes a 

 sort of weather glass ; and if in the course of fine 

 summer weather distant objects and the distant horizon 

 become more than usually distinct, if that does not 

 obviously depend upon some local cause, it is one of 

 the most unerring signs of rain. 



Now that which occasions the " looming" and reflec- 

 tions of objects in the air over the estuary is a similar 

 state of the atmosphere, and indicates that the upward 

 motion in evaporation is suspended ; only that the 

 cause is local, it would be followed by rain ; sometimes 

 it is so ; and very often it is immediately followed by 

 fog, the real mirror that gives out the images of the 

 ships, is in fact, a fog in the act of forming, although 

 it has not attained sufficient density to diminish very 

 much the transparency of the atmosphere. The land 

 begins to be heated, which puts a stop to the land 

 wind, and also alters its temperature ; this change 

 takes place first near the surface of the earth, and the 

 union of the cold sea wind and the warm land one, 

 renders the whole mass less capable of suspending hu- 

 midity, and fog is formed. That fog does not, how- 

 ever, attain the consistency of rain, neither does it 

 always moisten the surface of the ground, because the 

 means by which it is formed are confined to the spot 

 where the two currents of air meet, and to the time 

 during which the general motion is changing from 

 seaward to landward. 



