16 AIR AND SUNLIGHT 





form water and with nitrogen to form ammonia gas. Both animal 

 and plant life require water for growth and development. 



Depth and Pressure of the Air. We are living at the bottom of 

 a vast ocean of air which has a depth estimated to be from three 

 hundred to five hundred miles. This depth of air exerts an enor- 

 mous pressure upon all objects at sea level. This pressure is nearly 

 fifteen pounds on each square inch of surface or more than a ton 

 to the square foot. On a square rod of land the pressure is two 

 hundred and eighty-nine tons. 



As we go upward this pressure decreases rapidly. In fact, the 

 change is so rapid that we leave about 96 per cent of the entire mass 

 behind us in the first fifteen miles as we go upward. 



In climbing high mountains such as Pikes Peak we soon note 

 this rareness or thinness of the air and we find that breathing be- 

 comes correspondingly difficult. We note also that the air grows 

 colder the higher we go. The soil temperature likewise decreases as 

 the altitude on the mountain side increases, until even in tropical 

 regions frozen ground and perpetual snowdrifts may be found from 

 four to five miles above sea level. From this we learn that the 

 atmosphere performs another important office in keeping the earth 

 warm. The radiant energy of the sun is absorbed in large quanti- 

 ties by the lower and denser layers of atmosphere at the earth's 

 surface, while in the thin air but little if any is absorbed Hence, 

 freezing temperatures are soon reached as we go upward in the 

 atmosphere even in the summer time. Professor Langley states 

 that his experiments at the base and summit of Mt. Whitney led 

 him to believe that had our earth no atmosphere its surface tem- 

 perature, even under the equator at noon, would be at least two 

 hundred degrees below freezing point. 



However, the upper layers of thin air are not without their value 

 to mankind, because they protect the earth from the vast number 

 of meteors or shooting stars which are continually falling into it. 

 These meteors, on account of their high rate of speed and the great 

 amount of friction produced when they reach the air, soon gener- 

 ate enough heat to entirely consume them. Fifty millions or 

 more of them are destroyed in this way every month. 



The Sunlight. The warming as well as the lighting of the earth 

 by the sun is a fact of great importance to us. The source of all 



