THE BEASON WHY. 



' I do set mj- bow in the cloud and it shall be for a token of a covenant between 

 me and the earth." 



482. Wliy do we see the sun 

 sun-set ? 



before sun-rise, and offer 



Because of the refractive effects of the atmosphere. Rays of 

 light, passing obliquely from the sun through the air to the earth, 

 are refracted three or four times by the varying density of the 

 medium. Each refraction bends the rays towards the perpendicular ; 

 and hence we see the sun before it rises and after it sets, 



Fig. 18. DIAGRAM EXHIBITING THE BEFEACTION OP THE SUN'S EATS IN 

 PASSING THEOUGH THE ATMOSPHEEB. 



483. Why do figures, viewed through tlie hot air proceeding 

 from furnaces, and from lime-kilns, appear distorted and 

 tremulous ? 



Because the ever varying density of the air which is flying away 

 in hot currents, and succeeded by cold, constantly changes the 

 refractive power of the medium through which the figures are 

 viewed. 



484. Why do the stars twinkle ? 



Because their light reaches us through variously heated and 

 moving currents of air. In this case the earth is the Jciln, and 

 the stars the object that is viewed through the refractive medium. 



485. Why does much twinJcling of the stars foretell lad 

 weather ? 



Because it denotes that there are various aerial currents of 

 different temperatures and densities, producing atmospheric 

 disturbance. 



