150 SCIENTIFIC AGRICULTURE. 



been supposed to come from volcanoes in the moon : but there 

 is still great obscurity hanging about the whole subject 



COLOR OF THE SKY. 



The general color of the unclouded sky is azure or blue: 

 this is explained on the supposition that the particles of the 

 atmosphere, when illuminated, reflect mostly the blue rays. 

 Whenever the prevailing color of the sky is anything but a 

 pure blue, it is discolored by smoke, vapor and clouds; the 

 more dense these clouds, the nearer the color -of the sky 

 approaches to black. The deep red of the morning and 

 evening sky is explained by supposing the atmosphere permits 

 only the red and yellow rays to pass, and reflects the blue 

 rays. [Muller.] 



The fiery red of morning is caused by an excess of moisture, 

 which, notwithstanding the tendency of the sun's rays to 

 disperse it, forms clouds in the atmosphere, and hence indi- 

 cates rain. A gray sky at morning and a red sky at evening, 

 on the contrary, foretell fine weather. The various other 

 beautiful hues which tint the sky and fringe the massive 

 clouds, so as to produce all the varied gorgeous drapery of 

 the heavens, are caused by the absorption, refraction and 

 reflection of the different rays of the solar spectrum. 



TWILIGHT. 



Twilight is the diminished light of day, which is seen from 

 the setting of the sun on its sinking below the western hori- 

 zon, till the last faint gleaming of day has disappeared. The 

 time at which twilight begins and ends, is altogether arbi- 

 trary, and must depend very much on the acuteness of the 

 vision of the observer. It has been said to commence at the 

 moment of sunset, and terminate when the first small stars 

 are visible. Twilight is short in countries having a pure sky : 

 in Chili, it lasts only a few minutes. In high latitudes it is 



