54 THE BOOK OF NATURE STUDY 



school hours must necessarily be limited, such observations may 

 well be prescribed to be done as home work. When this is done 

 the children should be instructed to observe and note down the 

 sequence of colours, the time of persistence of the colours after 

 the disappearance of the sun, the occurrence of colour in the 

 eastern as well as in the western sky, and also to make rough 

 calculations, so far as they can, of the area of the sky over which 

 the sunset colours extend. As the whole series of phenomena 

 which accompanies the setting of the sun is only rarely observ- 

 able on the same evening, a considerable number of observations 

 should be made. The connection between very fine sunsets and 

 clear summer weather, the absence of the beautiful colouring 

 when the sky is heavy with clouds, and the reddish appearance 

 of the sun during periods of evening and morning mist, should 

 all be noticed. If opportunity offers the sunrise appearances, 

 which are generally parallel with those of the sunset, should also 

 be noticed. 



Before, however, the special peculiarities of the sunset sky 

 can be appreciated, attention must be carefully drawn to the 

 appearances seen when the sun is high. By repeated observations 

 make it clear that the purity of the blue of the unclouded sky 

 varies with the amount of moisture in the air, that is with the 

 " fineness " of the weather. More than this, the purity of the 

 blue varies with the degree of elevation of the sun. As it sinks 

 towards evening the sky grows paler ; it is much paler also at 

 the dawn than in the blaze of noonday. Similarly, our winter 

 skies never have the bright blue of summer ; at no period do 

 our skies have the brilliant colouring of more southerly latitudes. 

 By comparing days when the air is loaded with moisture with clear 

 days, by comparing hours or seasons when the sun is low with 

 those when it is high in the heavens, try to get the class to see 

 that the greater the thickness of air traversed, and the more loaded 

 is that air with solid particles, the greater the absorption of light 

 and heat, and therefore the paler the sky, until when it is loaded 

 with clouds the blue darkens to grey or nearly black. 



When speaking of the meaning of the term zenith it is easy to 

 show that the higher up in the sky the sun is the less the thickness 

 of air that its rays have to traverse. Autumnal anticyclones 



