SCIENTIFIC TRACTS. 



NUMBER III. 



THE ATMOSPHERE. 



THE first thing which arrests our attention in looking 

 into the atmosphere, is the blue color of the sky. Philoso- 

 phy has called upon the science of Optics for an ex- 

 planation of this effect, and learns that it is caused by 

 the resistance presented by the air to the different rays 

 of light. It is easily shown that the light which comes 

 from the sun, is composed of seven distinct colors, 

 whose blended effect is the whiteness with which we 

 are familiar. These colors consist of red, orange, yel- 

 low, green, blue, indigo and violet. The blue ray is 

 supposed to be one of the most difficult of transmission, 

 and is the most readily absorbed by the atmosphere. 

 The red rays, on the contrary, readily pass through the 

 air, while the blue are almost lost. To this absorption 

 of the latter, we are indebted for the bright azure which 

 tinges distant mountains. 



As we ascend into the atmosphere, the deepness of 

 its color diminishes, and to the traveller on the Alps or 

 the Andes, the sky appears quite black. 



It is to I he ready passage of the red ray, that the 

 diver is indebted for his vision in deep water. The red 

 rays penetrate the sea, while the blue are reflected 

 from its surface. From the reflective power of the at- 

 mosphere, we derive the delightful twilight before the 

 sun rises, and after he has passed from our view. 



The word Pneumatics is derived from the Greek 

 word pneuma, which signifies spirit, that being one of 

 the names by which the air was called by the ancients. 

 This term is applied to that branch of natural philoso- 

 phy which treats of the mechanical properties of the 



