1 82 ELEMENTARY SCIENCE 



and rise, and then, as it condenses, to flow away toward 

 the poles. This means that there gets to be actually less 

 air per cubic foot over the hot-belt than there is over the 

 regions north and south. So, in accordance with the law 

 of diffusion, or as we say, to equalize the pressure, cooler 

 air comes flowing in (at the bottom of the atmosphere) 

 from north and south. We may call such Sowings of air 

 "convection currents" if we like, but they are more com- 

 monly called winds, the winds which flow toward the 

 equator for the cause just indicated being called the trade- 

 winds. They were so called originally because they were 

 of great assistance to the trade carried on in sailing vessels, 

 but they might well have been so called because they are 

 part of a trade of air between one region and another. 



Have you not noticed that a long spell of hot, dry weather 

 is almost sure to be "broken up" by the coming of wind 

 and rain ? In this case, too, a sort of " trade " occurs. The 

 longer the hot spell continues, the lower the pressure tends 

 to become until at last it is as though the atmosphere 

 could "stand it no longer," and winds, usually bringing 

 rain, rush in to balance the pressure again. 



We have noted that air temperature has a direct effect 

 upon air-pressure, and that both have great effects upon 

 the wind and the weather. Differences in pressure cause 

 air movements, air movements are wind, and the winds 

 bring the rain or snow. So we see in studying weather a 

 whole chain of causes and effects. We also see, in this 

 connection, the need the weather bureau has for making 

 diagrams to indicate each day the states of temperature 

 and pressure all over the country. For only by means of 

 such facts as these "weather-maps" present, can the prob- 

 able weather of the following day or week be foretold. 



