Barometric Wave, November IS*?. 39 



184-5*; viz. "that during fourteen days in November, more 

 or less equally disposed about the middle of the month, the 

 oscillations of the barometer exhibit a remarkably symmetrical 

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

 maximum or highest reading is to a great extent similar to 

 the preceding rise. This rise and fall is not continuous or 

 unbroken ; in three out of four of the occasions on which it has 

 been observed, it has been found to consist of five distinct ele- 

 vations. * * * * At the setting in of the great November wave 

 the barometer is generally low, sometimes below twenty-nine 

 inches. This depression is succeeded by two well-marked 

 undulations, varying from one to two days in duration. The 

 central undulation, which also forms the apex of the great 

 wave, is of larger extent, occupying from three to five days; 

 when this has passed, two smaller undulations corresponding 

 to those at the commencement of the wave, make their appear- 

 ance, and at ^e close of the last the wave terminates," On 

 the last two returns, the great wave presented features that 

 require the preceding type to be somewhat modified. The 

 symmetrical character on both occasions was distinct, but the 

 crown or central apex was not the highest. In 184^6, the wave 

 on the posterior slope, which is bisected by the last vertical 

 line in the engraving, was the highest at the northern and north- 

 western stations ; and on the last return the subordinate waves on 

 both the anterior and posterior slopes were higher than the cen- 

 tral. With this exception, there appears to be nothing to mili- 

 tate ao-ainst the general accuracij of the ti/pe as above expressed. 

 It appears that on all occasions on which the writer has ob- 

 served the great symmetrical wave, it has been completely 

 separated from all^the preceding and succeeding barometric 

 movements, — an iHilividiiality has been attributed to it in con- 

 sequence of this separation ; and the movements between the 

 epochs marking its commencement and termination have more 

 or less been referable to the foregoing type. On most occa- 

 sions the barometric mo^emQUi^ preceding the great wave have 

 occurred at a considerable altitude,-jvhile those sticceeding it 

 have been observed at lower altitudes. 



The individuality of the great symmetrical wave consists 

 more in the character of the barometric movements than eidier 

 in the form, as given by the curve, or the absolute measurement 

 of the atmospheric pressure, as observed by means of the ba- 

 rometer. Both the curve and the altitude of the mercurial 

 colunni vary very considerably, while the symmetrical cha- 

 racter is much more constant. This consists (in most cases) 

 in the initial and terminal minima being very distinct, and the 

 inflexions of the curve as the barometer approaches its terminal 



* Phil. Mag. S. 3. vol. xxix, p. 3o7- 



