46 Mr. W. R. Birt on the Great Stjmmelrical 



))osterior trough to the apex being 1"824 inch*. The depres- 

 sion of the 7lh of December appears to have been an extra- 

 ordinary depression, somewhat of the character of the depres- 

 sions on December 25, 1810, and 182J. (See Phil. Mag, 

 vol. XXX. p. 18.) The vast magnitude of this wave, its ampli- 

 tude in time, stretching over nine days, fifteen hours and twenty 

 minutes, during the whole of which the barometer was rapidly 

 changing, the acuminated character of its troughs and crest, 

 com.ecled witli the unusual epoch of its posterior trough, will 

 render it a most interesting subject for investigation. The 

 ■phase, as exhibited at London, is so well-marked, that every 

 deviation from it in other localities can be readily ascertained, 

 the depression of the December minimum below that of No- 

 vember clearly and distinctly assigned, and, I apprehend, with 

 such precision, that a chart of deviation from symmetry — 

 regarding perfect symmetry as indicated by the equal altitudes 

 of the two minima — easily constructed, and the rate of pro- 

 gress and direction of motion correctly deduced from the 

 epochs of maximum and minima. I am not aware that an 

 instance has fallen under my notice so suitable for carrying on 

 the inquiry with success, as the wave which transited on the 

 1st of December IS^?. 



In taking up the investigation of this wave with an especial 

 view to the second branch of the inquiry, as noticed above, it 

 must be borne in mind that the curve of the barometric move- 

 ments from November 28 to December 7 is not likely to exhibit 

 a section of an atmospheric wave, properly so called. It has 

 already been determined that "the barometric curve, including 

 a complete rise and fall at any one station, is not the curve 

 resulting from the transit of any one wave; it does not repre- 

 sent \.\\ejbrm of any reality in nature ; but it does represent, 

 and is an exponent of, the effects resulting from the contem- 

 poraneous transits of waves, or systems of vvavesf." The first 

 step is to ascertain the absolute difference of pressure existing 

 between any two or more stations J. Yov this purpose all the 

 observations must be corrected and reduced to the level of the 

 sea, and then the slopes of the individual waves will become 

 apparent, and the directions of the crests and troughs deter- 

 n)ined. From such an investigation, some notion may be 



* The true altitude of this vvave, as determined from the Greenwich 

 observations, appears to have been ] 855 inch ; the epoch of the minimum 

 of the 7th was 2'35 a.m., value 28 .381 ; aad tiie amplitude rather less than 

 here stated. 



t Reports of the British Association for the Advancement of Science, 

 1846, p. 136. 



X Tiie more numerous the stations the more accurate will be the results 

 obtained. 



