PKKSinKNTIAl, ADDKKSS Sf-X TIOX A. I7 



As soon as the synoptic daily charts covered a sufifiiciently large 

 area, and extended over even a comparatively short period of 

 time, it became evident that there existed certain definite sys- 

 tems of atmospheric pressure, for the isobars or lines drawn 

 through points of equal pressure afifected day after day certain 

 well-defined forms. An isobaric chart presents most of the 

 features of a hypsometric map. It has, for instance, closed 

 areas of more or less circular isobars with figures decreasing to 

 a central minimum, others with figures increasing to a central 

 maximum ; \'-shaped isobars with figures decreasing or increas?- 

 ing inwards, like the contour lines of valleys or mountain spurs ; 

 and so on. Each system of isobars, as it recurred, was found to 

 be accompanied by the same general features in the weather 

 Storms, for instance, invariably corresponded to a system of 

 closed isobars with low central pressure, whilst continued fine 

 weather corresponded to a system of closed isobars with central 

 liigh pressure. These areas of closed isobars were, therefore, the 

 c-'irliest objects of methodical research. 



The first fact to be well established was that the winds had 

 a clearly defined system of circulation round the closed areas 

 of pressure, clockwise round the areas of high pressure, and 

 counterclockwise round those of low pressure. The same was 

 found to hold for the southern hemisphere with the direction of 

 rotation reversed. This circulation was briefly formulated by 

 F)uys Ballot in the following words: — 



■' Put your l)ack to the wind, and you have the low pressures on 

 your left in the northern hemispliere. and on your right in the southern." 



It is easy to see what importance this law. which we may 

 consider as the fundamental law in modern meteorology, had in 

 his day when steam navigation was still in its infancy. It soon 

 became also evident that the force of the wind^ — as well as its 

 direction — was in close connection with the shape of the isobars. 

 Strong winds almost invariably corresponded to a system of 

 closed i.sobars. and patient investigation gave a rough determina- 

 tion of the relations between the strength of the wind and the 

 closeness of the isobars. These discoveries were the first to be 

 applied to the prediction of storms, for about t86o we already 

 find storm warnings sent to many ports in Europe. 



The general connection of the weather with the pressure 

 systems was. of course, detected at the same time, but progress 

 in this direction has been considerably slower. There are many 

 influences to be taken into account in following the variations 

 of the weather with those of the pressure from day to day. An 

 army of investigators ha.s been at work for the last fifty years 

 in unravelling this tangle. On the whole, it may be said that the 

 weather in general depends on the presence of high and low 

 areas, and its changes on their movement. A low area, in- 

 diflferently called cyclone or depression, is characterised by cloudy 

 and rainy weather. In front of the depression there are no 



