282 SUMMER. 



they take place is very much loaded by vapour, so 

 much so, that, thought not so collected into masses 

 as to be visible in a state of haze or fog, it is probably 

 as abundant in quantity within an equal space, and 

 thus forms an invisible mirror, from which the images 

 are reflected. The same thing in principle happens 

 every morning and evening : the refraction of the at- 

 mosphere (and refraction is but a minor kind of reflec- 

 tion) brings the sun before it actually comes to the 

 horizon, retains it after it is actually below, and occa- 

 sions the twilight which both precedes and follows the 

 actual presence of the sun. Those refractive powers 

 are always the greater the more completely that the 

 atmosphere is loaded with moisture, and the more free 

 that it is from agitation by the winds, the action of 

 which prevents the formation of the image, in the same 

 manner that a lake does not repeat the scenery on its 

 banks when the breeze ruffles its suface, or that one 

 cannot see the reflection of one's face in a piece of 

 black broad-cloth or velvet, in the same way as in a 

 smoothly varnished pannel, or a piece of polished 

 marble. 



The formation of these curious images does not take 

 place when the process of evaporation is the most rapid, 

 because the ascent of the particles of water in a state of 

 vapour at such times prevents the formation of the 

 image, by producing a certain tremulous motion in the 

 air, which has much the same effect as wind. Evapo- 

 ration always occasions an indistinctness even in direct 

 vision ; and on those fine summer days when there is a 

 flickering play along the tops of the different elevations, 

 as if there were a spirit walking the earth, of which the 



