442 



NATURE S TEACHINGS. 



weight. Let, for example, any one measure the surface of his 

 own hand, reduce it to square inches, add together fifteen 

 pounds for every square inch, and he will then appreciate the 

 weight of the atmospheric ocean in which we live. On an 

 average, every human being endures a pressure of some ninety 

 thousand pounds. 



This ocean is in perpetual movement, sometimes violently, 

 which we call storm ; sometimes gently, which we call breeze ; 

 and sometimes very gently, which we call calm. There are 

 air-spouts as well as water-spouts ; and, in fact, the water-spout 

 is nothing but a continuance of the air-spout, as is shown by 

 the moving sand-columns of the desert. Whatever may be 

 the character of the winds, as we call this movement, the air is 

 never for a moment still ; and, indeed, were it to be still for any 

 time, the whole human race would perish. 



How winds are caused we shall see by the aid of the 

 diagram on the left-hand side of the illustration. 



AIR-CURKENTS. 



VENTILATION OP MINES. 



The original cause is the sun. His rays fall upon the earth, 

 heating it, and so by radiation heating the air. JNow, as has 

 been remarked, heated air will cause a heavy balloon to float 

 through ordinary air, and to carry up a considerable amount of 

 dead weight besides ; consequently the heated air must ascend, 

 while cool and heavier air rushes in to take its place, and thus 

 the currents are produced. Were the earth set straight 

 upright, the currents would invariably run in one direction ; 

 but, as it is tilted on one side, the needful variety is obtained, 

 and we find the winds blowing from all parts of the compass. 



The principle, therefore, of all winds is, that heat expands, 



