WUA THER-PR GNOSTICS. 



651 



scale rain falls as a squall drives by. The motion of these areas was 

 found to follow certain laws, so that forecasting weather-changes in 

 advance became a possibility. 



6. That in the temperate zones sometimes, and habitually in the 

 tropics, rain fell without any appreciable change in the isobars, though 

 the wind conformed more regularly to the general law of these lines. 

 This class of rainfall will be called " non-isobaric rain." 



In Fig. 1 we give in a diagrammatic form the broad features only 



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V- cfepress ? /'' \_^ Second^ ^'--S^' '''' 



59.7 



'9-9 



Fio. 1— The Seven Fundamental Shapes of Isobars. 



of the distribution of pressure over the North Atlantic, Europe, and 

 the eastern portions of the United States on February 27, 1865. 

 Coast-lines are omitted, so as not to confuse the eye, so also are lines 

 of latitude and longitude ; but the foot-note at the bottom of the 

 figure represents the equator, and the top of the diagram would be on 

 the Arctic Circle. All pressures of and under 29'9 inches (760 mm.) 

 are shown with dotted lines, so that the eye sees at a glance the broad 

 distribution of high or low pressure. The whole seven fundamental 

 shapes of isobars will be found there. 



Looking at the top of the diagram, we see two nearly circular areas 

 of low pressure, round which the isobars are rather closely packed. 

 Such areas, or rather the configurations of isobars which inclose them, 

 are called " cyclones," from a Greek word meaning a circle, because 

 they are nearly circular, and, as we shall see presently, the wind blows 

 nearly in a circle round their center. Just south of one of the cyclones, 

 the isobar of 29*9 inches (760 mm.) forms a small sort of nearly circu- 

 lar loop, inclosing lower pressure ; this is called a " secondary cyclone," 

 because it is usually secondary or subsidiary to the primary cyclones 

 above described. Farther to the left the same isobar of 29*9 inches 

 bends itself into the shape of the letter V, also inclosing low pressure ; 

 this is called a "V-shaped depression," or, shortly, a "V." Between 

 the two cyclones the isobar of 29*9 inches projects upward, like a wedge 

 or an inverted letter V, but this time incloses high pressure ; this shape 

 of lines is called a " wedge." Below all these we see an oblong area 



