CONTRIBUTIONS TO METEOROLOGY. 



39 



Table XXL — Areas of low pressure, Nos. G, 13, 14, and 27. 



By comparing Table XXI with Table XIX, we perceive that iu the ca.sc of the storms which 

 traveled northward the a^ erage barometric oscillation was only two-thirds of that of the storms 

 which traveled southward ; that the average force of the wiud in the former case was less than 

 two thirds of that in the latter case ; that in the latter case the winds which followed the low 

 center were generally from the north, and that 92 per cent, of them were either from the north, 

 northwest, or northeast, while in the former case the winds were very irregular, but southerly 

 winds were somewhat more frequent than northei'ly winds. We perceive, then, that the progress 

 of the storms which traveled southward conformed closely to that of the winds which succeeded 

 the low center, but that in the case of the storms which traveled northward, although there was 

 some tendency towards the same law, this tendency was not strongly marked, and the winds were 

 generally moderative in force. It seems then probable that some other cause or causes than that 

 of the prevalent wind exerted an api)reciable intiueuce in diverting these areas of low pressure 

 towards the north. Among these causes may be mentioned the influence of neighboring areas of 

 high and low pressure, as will be shown hereafter. 



57. Occasionally we And instances in which the storms of the middle latitudes of the United 

 States pursue a course still more abnormal than those shown in Tables XVIII and XX, The 

 Signal Service maps show cases in which the storms of the middle latitudes have pursued for a 

 short time a westerly course. In some of these cases the depression of the barometer was small 

 and there was no well-defined storm center. Sometimes there were two centers of slight depression 

 within a few hundred miles of each other, so that a small change in the force of the winds caused 

 one of the centers to predominate slightly, and thus the center of greatest depression was carried 

 iu an unusual direction. Table XXII shows the most decided cases iu which the centers of low 

 pressure in the middle latitudes of the United States have advanced in a westerly direction. 

 Column 1 gives the reference number; column 2 the date of beginning and end of the westerly 



