PNEUMATICS. 77 



air reflecting the blue rays more copiously than any 

 other. The higher the observer is placed, the deeper 

 the blue becomes ; consequently, at a certain height, 

 the blue will disappear altogether, and the sky be- 

 come black ; that is to say, will reflect no light at all. 

 The colour becomes always lighter in proportion to 

 the quantity of vapour mixed in the air ; hence it is 

 obviously caused by the vapour. 



Another Explanation. 



The colour of the sky depends on the quantity of 

 opaque vapour in the air. The less vapour there is, 

 the darker is the colour of the sky. If the air were 

 entirely free from opaque vapour the sky would ap- 

 pear black. The particles of opaque vapour in the 

 sky reflect chiefly the blue rays, and thus give rise 

 to the blue colour of the sky. The colour of the sky 

 is darker at the zenith than at the horizon, because 

 the quantity of vapour through which the eye looks 

 at the horizon is greater than at the zenith. 



