1ft ffilows* 81 



there was in the early morning. The expla- 

 nation of this peculiar action of the air is 

 found in the fact that during the day the land 

 is heated much more rapidly on its surface 

 than the water is. 



The radiant energy from the sun is sud- 

 denly arrested at the surface of the earth, 

 which is heated to only a very shallow depth, 

 while in the water it is different ; being trans- 

 parent it is penetrated by the radiant energy 

 to a much greater depth and does not suddenly 

 arrest it, as is the case on land. As the sun 

 rises and the rays strike in a more and more 

 vertical direction the earth becomes rapidly 

 and intensely heated at its surface, and this in 

 turn heats the stratum of air next above it, 

 which is pressing on it with a force of fifteen 

 pounds to the square inch at sea-level. When 

 air is heated it expands, and as it expands it 

 grows lighter. The stratum lying upon the 

 earth as soon as it becomes heated moves up- 

 ward and its place is occupied by the heavier, 

 cooler air that flows in from the sides. We 

 can now see that if there is a strong ascending 

 current of air on the land near the ocean the 

 cooler air from the surface of the ocean will 

 flow in to take the place of the warmer and 

 lighter air that is driven upward, really by 

 the force of gravity which causes the heavier 

 fluid to keep the lowest level. As the earth 

 grows hotter this movement is more and more 



