^ 



310 



ON ATMOSPHERIC WAVES 



[Sect. X, 



for extra observations at certain seasons : of these^ three 

 only have yet been ascertained — the type for the middle of 

 Novem])er — the annual depression on or about the 28th of 

 November — and the annual elevation on or about the 25th 

 of December. The enunciation of the first is as under : 



" That during fourteen days in November, more or less 



- ^ 



equally disposed about the middle of the month, the oscil- 

 lations of the barometer exhibit a remarkably symmetrical 

 character, that is to say, the fall succeeding the transit of 

 the maximum or the highest reading is to a great extent 

 similar to the preceding rise. This rise and fall is not 

 coiitimious or unbroken ; in some cases it consists o^five, 

 in others of three distinct elevations. The complete rise 

 and fall has been termed the great symmetrical baro- 

 metric wave of November. At its setting in the baro- 

 meter is generally low, sometimes below twenty-nine 

 inches. This depression is generally succeeded by two 

 \^ ell-marked undulations, varying from one to two days m 

 duration. The central undulation, which also forms the 

 apex of the gretit wave, is of larger extent, occupying 

 from three to five days ; when this has passed, two smaller 

 imdulations corresponding to those at the commencement 

 of the wave make their appearance, and at the close of 

 the last the w^ave terminates." With but sHght excep- 

 tions, the observations of eight successive years have 

 confirmed the senera 



correctness of this type. On two 



occasions the central apex has not been the highest, 

 and these deviations, with others of a minor character, 

 form the exceptions alluded to. This type only has 

 reference to London and the south-eastern parts of 

 England ; proceeding westw^ard, north-westward, and 



) 



^ 



\ 



*^^ 



