44 Mr. W. R. Birt on the Great Barometric Wave. 



in my Report on Atmospheric Waves, presented to the British 



Association in 184-6''. '' First, the determination ofthephases 

 of the larger undulations, with the smaller secondary waves 

 superposed on their slopes, forming the ti/pes of the various 

 seasons of the year; and second, the absolute extent of each 

 atmospheric wave in space, the direction of its crest, its am- 

 plitude in miles, the altitude of its crest above, and the depres- 

 sion of its troughs below the surface of general repose of the 

 atmosphere, the })lace of its formation, the manner in which 

 it is propagated, the precise direction and extent of its motion, 

 the force with which it is translated from place to place, and 

 the locality of its final extinction." 



In addition to the determination of the type of the middle 

 portion of November, now generally known as the great sym- 

 metrical barometric wave of November, we are, I apprehend, 

 on the eve of ascertaining the type for November during its 

 whole extent. I have now before me a chart of the barometric 

 movements at Halifax during ihe last November; and if I 

 mistake not, the characters of the entire oscillations closely 

 agree with those of preceding Novembers. The annual de- 

 pression on or about the 28th f is most distinctly marked ; and 

 the opposite character, that of a high barometer at the com- 

 mencement of the month, is as distinct. 1842 presented us 

 with an elevated mercurial column at the commencement of 

 November: the same order of things occurred in 1845 and 

 1846; and there is a high probability that the movements 

 preceding the great wave occur at a much greater altitude 

 than those succeeding it. Figs. 4 and 5 are curves of the 

 annual depression on or about the 28lh, as it passed London 

 in 1847 and 1798. In 1798 the epoch was a day earlier. 

 The horizontal line in fig. 4 represents an altitude of 30000, 

 in fig. 5 its value is 29*500. It is worthy of remark that this 

 depression in 1847 was succeeded by an enormous wave, 

 which passed its maximum on the night between the 1st 

 and 2nd of December, value 30'397. This maximum forms 

 the December boundary of the depression of the 28th, see 

 fig. 4. Upon the passage of this maximum the barometer 

 coumienced falling rapidly, and continued thus falling, with 

 one or two interruptions, until early on the morning of 

 the 7th, when at 5'35 a.m. the reading of the barometer at 

 London was 28-.573. Thus the depressions of the 28th of 

 November and the 7th of December form the anterior and 

 posterior troughs of this imn)ense wave, its altitude from the 



* Reports of the British Association for the Ailvuncement of Science, 

 1846, p. 163. 



t Phil. Mag. .S. 3. vol. xxix. p. 359. 



