FIRE AND ITS USES 157 



Southern. The course of both trades and westerlies is further 

 made irregular by the irregularity of the distribution of land 

 and water. Still they are sufficiently regular to be of much 

 importance in commerce, and were much more so in the days of 

 sailing vessels. 



In addition to these general air movements from temperate 

 regions toward the Equator and poles along the earth's surface 

 and in the reverse direction high up in the air, there are local 

 winds produced as variations in heat and moisture develop local 

 areas of high and low pressure. The winds blow along the earth's 

 surface from the high-pressure areas to the low-pressure. Daily 

 reports of atmospheric pressure are sent from many stations all 

 over the country to the Weather Bureau at Washington so that 

 with a knowledge of the location of high- and low-pressure areas, 

 the country over, the probable direction of the wind at any 

 locality can be predicted. If the difference in pressure between 

 adjacent high and low areas is very great, the winds will be 

 strong; severe blows can be foretold in time to warn vessels 

 and persons interested in such forecasts. 



When moist air is rising into the upper atmosphere which is 

 cool, the moisture will be condensed to form clouds, and if the 

 rising air is very moist, the condensation produces rain. The 

 air coming into a low-pressure area from the south is usually 

 warm and moist; therefore clouds and showers may be expected 

 on the south side of a low-pressure area. On the other hand, the 

 air coming in from the north is cool and dry, and since it grows 

 warm as it moves southward it can take up additional moisture. 

 On the north side of a low-pressure area fair weather may be 

 expected. Having reports from many stations on the humidity 

 of the air as well as on temperature and pressure, the Weather 

 Bureau embodies these in the daily weather map on the basis of 

 which the predictions are made (Figs. 62 and 63, pp. 158, 159). 



Improved industrial processes offer very many illustrations 

 of the way in which our knowledge of fire and its methods of 

 control have contributed to the advance of civilization and the 



