36 REPORT—1846, 
wise circulated*, In accordance with these instructions, about thirty sets of inter- 
esting and valuable observations had been made; the stations extending in one 
direction from the west of Ireland to Heligoland, and in the other from the Scilly 
Isles to the Orkneys. These observations Mr. Birt had subjected to a very careful 
comparison, especially those made at his own residence near London, with those 
which he made in the autumn of 1842 at Leicester Square. The result of this com- 
parison was such as clearly to show that there was a most striking coincidence between 
the barometric movements of October and November 1845, and those of a portion of 
September, October and November 1842. So close did this coincidence appear to 
the author, that during the period from October 1 to November 21 in 1845, the baro- 
metric movements of October 23 to 26 were the only oscillations that appeared to 
have no corresponding movements in 1842. It appeared that the great wave com- 
menced in 1845, near midnight, between the 6th and 7th, that it culminated on the 
14th, and terminated on the 21st; during the 10% days previous to the setting in of 
the wave, the movements in 1842 and 1845 were almostidentical. Mr. Birt observed 
that in 1845 the great wave, in all its essential features, was very distinctly marked ; 
that it was completely separated from all the preceding barometric movements, and 
that the individuality that was thus given to it, induced the strong belief that we haye 
obtained the type of the barometric oscillations during the middle portion of No- 
vember. This type he proposed to express in the following language :— 
“‘ 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 symme- 
trical character, that is to say, the fail 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 elevations. The complete 
rise and fall has been termed the great symmetrical barometric wave of November, 
and as such has been considered to result from the transit of a large wave, but there 
is great reason to believe that while it may be due to the transit of a normal wave 
of about fourteen days’ amplitude, it also exhibits the transits of five secondary super- 
posed waves. At the setting in of the great November wave the barometer is gene- 
rally Jow, sometimes below 29 inches. This depression is succeeded by éwo 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 and longer duration, occu- 
pying from three to five days; when this has passed, two smaller undulations, corre- 
sponding to those at the commencement of the wave, make their appearance, and at 
the close of the last the wave terminates.” 
Mr. Birt exhibited curves of observations that had been made during November at 
Dublin, from 1829 to 1845 inclusive, which he had received from Capt. Larcom of 
the Royal Engineers. From these curves, it appeared that the great wave had been 
observed at Dublin in twelve out of seventeen years, and that with two exceptions in 
eleven years of distinct and well-marked transits of the great wave, the epochs of the 
maxima were confined to five days, near the middle of the month, namely from the 
12th to the 17th+. 
The author then proceeded to notice the comparison he had instituted between the 
curves he had obtained from various stations, and exhibited curves from twelve stations 
in Ireland, England and Heligoland. From a consideration of these curves (which 
were so arranged as to show the departure from symmetry in certain directions), he 
argued that while the posterior slope of a wave of considerable magnitude was passing 
off towards the E.N.E., the front of another was approaching from the N.W., and that 
it was the interference of the two that produced the symmetrical arrangement of the 
curves. In that portion of the area covered by the advancing wave the barometer 
rose; in that covered by the receding wave it fell; while in that in which the two 
waves interfered, the atmosphere as regarded these waves was quiescent, and the 
smaller secondary waves passed on uninfluenced by them. He also showed that 
* These instructions, with a short notice of the wave, were published in the Atheneum 
of Sept. 6, 1845. No. 932, pp. 880, 881. 
+ While exhibiting these curves, the author invited the attention of the Section to a very 
remarkable and apparently constant depression of the mercurial column which occurred about 
the 28th of November. It had been observed in fifteen out of the seventeen years’ observa~ 
tions, and appeared to be unconnected with the great wave. 
ie 
