COM TRIE DTIONS TO METEOROLOGY. 



43 



these two areas of high pies.snic the winds were feebh- and several centers of local disturbance 

 were formed, attended by some rain. The barometer Cell slowly, and the low center over North 

 Carolina was carried northward and .si)mewhat westward. 



No. 22 was similar to the preceding, except that the rain-fall was very great, and the low 

 center moved rafjidly northward, and the dei)ression at the center of the low area increased. 



The movement of No. 23 towards the southwest, December 12, was apparently due to a 

 similar cause. 



No. 25 may probably be ascribed in part to the same cause. There was a center of low press- 

 ure near Nova Scotia, with a high area near Lake Superior, and a second high area over Green- 

 land. At the same time there was a considerable fall of snow on the north and northwest sides 

 of the low center, and by the joint intiuence of these two causes the low center was carried north- 

 ward and westward. 



The first movement of No. 14 towards the northwest was ap])arently due to a similar cause. 



IV. Sometimes we find an extensive area of low pressure with feeble gradients on its southern 

 side, having northerly winds on its north side and southerly winds on its south side. On the 

 north side the barometer rises and the thermometer falls ; ou the south side the opposite ettects 

 usually take place, but in a less marked degree, and occasionally on the south side the barometer 

 does not fall at all. In either case the center of low pressure is diverted towards the south or 

 southwest, even when no rain-tall, or only a fiill of two or three hundredths of an inch, is reported 

 on that side of the low centei-. This abnormal movement of the low center appears generally to 

 result from the influence of an area of high pressure (or relatively high pressure) prevailing on 

 the north or northeast side, and crowding southward with considerable force. The center of the 

 low area appears to be displaced by the influence of the high area, which tills up the low area ou 

 its northern side, and generally but little change takes place ou its southern side. Table XXII 

 affords several examples of this kind. Table XXVII presents a summary of the leading particu- 

 lars relating to twelve of these low areas. Column 1 gives the number taken from Table XXII j 

 column 2 shows the change of the barometer in twenty-four hours on the northern side of the low 

 center ; column 3 shows the change in the thermometer on the northern side during the same period 5 

 column 4 gives the barometric gradient on the northern side, showing (in decimals of an inch) the 

 change of pressure for a distance of GO nautical miles ; column 5 shows the i^revalent wind on the 

 north side ; and column G shows the magnitude of the high area prevailing within a few hundred 

 miles on the north side of the low center; columns 7-10 show corresponding particulars for the 

 south side of the low center; column 11 shows whether any rainfall was reported on the south 

 side of the low center ; and column 12 shows the change of pressure.which took place at the low 

 center during the period of the western motion. 



Table XXVII. — Changes in areas of low pressure. 



