OR, PLAIN 



a spheroidal form to the earth, 

 imparts that form in a greater de- 

 gree to the atmosphere, 9. — The 

 atmosphere, in connection with 

 heat, light, vaponrs, and elec- 

 tricity, is the world of beauti- 

 ful and wonderful phenomena. 

 Winds, already mentioned, will 



173. 



be fully explained hereafter. 

 Rainbows, 10, are caused by the 



• 10 



174. _ 



sun's rays falling upon rain drops, 

 and being refracted in passing 

 through them, the refraction con- 

 sisting of unbending or unfolding 

 the coloured elementary rays of 

 which each ray of light consists. 

 Aerial reflections, 11, are caused 

 by states of the atmosphere, in 

 which various currents or strata 



TEACHING. 29 



acquire a considerable difference 

 in temperature and also in den- 

 sity. Thus two layers of air, one 

 being warm and rarefied, the 

 other cold and dense, may acquire 

 a relation to each other such as 

 that of water to air, and objects 

 standing in the rarer medium be 



175. 



reflected in the denser one. The 

 colder air, temporarily resting 

 over the warmer, or the reverse, 

 gives rise to reflections, produc- 

 ing, in some cases, effects so un- 

 usual to our vision, that they 

 become illusions, 11. Pictures of 

 ships and towns inverted, the 

 semblance of water in the midst 

 of burning sands where no water 

 exists, illusive forms of men and 

 animals, and other similar effects, 

 are phenomena of atmospheric 

 refraction and reflection. These 

 effects are variously known as 

 the mirage, the fata morgana, and 

 aerial spectra. The aurora borea- 

 lis, 12, or northern light, is ap- 

 plied to a luminous phenomenon 

 frequently witnessed in the 

 north ; it is called aurora austra- 

 lis when it occurs in the south. 



