WEATHER MAPS 251 



feet at Baton Rouge, and 21.5 feet at New Orleans. The results agreed 

 remarkably with the predictions, although the predictions were for 

 floods 1 to 1 .5 feet higher than any previous record. Thus warned, the 

 people set to work to strengthen the banks and levees, to remove their 

 cattle and their families to places of safety, and to get all freight away 

 from the wharves. It is estimated that property to the value of 

 $16,180,000 was saved by these warnings. The cost of the river and 

 flood service for the entire country during 1912 was $80,000, or only 

 % of 1 P er cen t of the value of the property saved in this one flood. 



In a similar way the Bureau gives warnings of coming frost, so that 

 whatever is possible may be done to save crops. When the weather 

 observer notes that a cold wave is coming, he sends word to the prin- 

 cipal city of his region. From this as a center, warning is sent by tele- 

 gram and telephone, by messenger, and by great signs placed on trains, 

 to all the producers of that region. 



The warning which the Weather Bureau gives to shipping is also of 

 great importance, as we should realize if we could count the many 

 thousands of ships, large and small, that annually enter and leave our 

 ports on the Atlantic, the Pacific, the Gulf, and the Great Lakes. 

 Within 45 minutes after a storm is predicted, the warning is placed in 

 the hands of every ship captain in every lake and ocean port. Storm 

 signals are also put up in all ports. As a result of the Weather Service, 

 the losses due to storms on the Great Lakes have fallen off two thirds. 



281. Weather Maps. In the preceding section we 

 have learned how useful the weather service is. Let us 

 now see how "weather maps" are made. In the first 

 place, all the weather stations having the same barometer 

 height are joined by solid lines, as in Fig. 224, A. These 

 lines therefore show equal pressures, and are called isobars, 

 from isos, "equal," and barus, "heavy." If we examine 

 the map we shall see that the isobars curVe, in irregularly 

 circular lines, about areas of low and of high pressure. 

 The pressure increases from the centers of the "lows" to 



