mid several of their singular Appearances. 297 



perly so called, then the greens and yellows, and continuinsf 

 thus to the red : hence the yellouiih, orange, and red co- 

 lours, under which the clouds apj)cai-. This period of 

 shades, namely, the eveniiig, manifests itself graduaily in 

 proportion as the sun approaches the horizon. Terresirial 

 objects, tlie part of the air near the sun, and even that Ju- 

 niinarv itself, are tinned with tlie same shades. When his 

 ravs can be received on a prism, it is seen that the ravs 

 really absorbed correspond to tlie general coloration of llie 

 moment. 



In consequence of the successi^'c increase and density of 

 the vapours traversed by tlie light, cKjuds dinirently placed 

 must at the same instant be tinged of ditllrent colours. 

 The highest maybe white, while the rest, at a less elevation, 

 will be yellow, and others still lower will be proportionally 

 redder. At an eo,ual elevation, the most distant iVom the 

 point where the sun sets w ill incline to red, and the nearest 

 to yellow. 



One may then see on bodies naturally white, blue or 

 green shadows, as Buftou and other jihllosophers have re- 

 marked. They are only, as has been already said, tlic 

 cflect of the contrast of the actual colour of tlic illuniinaled 

 and the obscure part. 



Contrasts may also render complex the colour of the 

 clouds; for example, when a great portion of the heavens 

 shows a blue colour. There are some the colour of v. hich 

 arises merely from this cause, and some are observed some- 

 times during the day bv those who are on a high mountain, 

 or in any other situation which secures the eye from ti>e too 

 strong direct or reverberated action of the solar light; but 

 in this case the clouds have only a yellow isb. shade, exactly 

 of the complementary colour ot the sky-blue. 



It is under a similar colour that the moon is sometimes 

 seen, when she is very high, a little before or after the sua 

 has passed the horizon. It seems, also, that she a])pears 

 thus, or even altogether white, when there exist at the san)e 

 time in tlie atmosphere clouds variously coloured by thu 

 vapours of the east or the west. By this concourse of cir- 

 cumstances we have a new proofof the diflerence of the 

 causes to which these colorations arc owing. 



Wc must remark, in the last place, that liv the Irregu- 

 larity of terrestrial localities, and the sti'le of the atnl(^- 

 sphere, these phaenomeaa may be concealed, or subject to 

 different interruptions. In our climates, the coloration of 

 (.he clouds for the most part does not attain to its utmost 

 term. On certain eVcnings, however, if the skv is very se- 

 rene 



