126 REPORT—1846. 
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 superposed waves 
of a similar character to those riding on the wave of seventeen days’ interval, 
Oct. 1 to 19 (VII.). The great November wave consequently possesses a com- 
pound character : at its setting-in the barometer is generally /ow, sometimes 
below twenty-nine inches. ‘This depression is succeeded by éwo well-marked 
undulations, varying from one to two days in duration. Thecentral 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, cor- 
responding to those at the commencement of the wave, make their appearance, 
and at the close of the last the wave terminates.” This was the order of 
things in 1842, 1843 and 1845. The smaller undulations in these instances 
were not identical, that is, they did not occur on the same points of the wave 
in each case; but the two preceding and the two succeeding undulations to 
the larger or central one were well-marked ; the physiognomy of the wave was 
readily recognized. 
The wave of 1844 exhibited a striking departure from this type in two re- 
markable particulars ; the epoch of transit and compound form of wave. The 
epoch was considerably earlier than in 1842, 1843 or 1845, namely Oct. 27; 
and the compound form consisted only of ¢hree instead of five undulations. 
The symmetry however was very apparent. This departure fromthe November 
type may probably be connected with the earlier occurrence of the wave; 
future observations will doubtless make us acquainted with its cause. 
XII. Capt. Larcom of the Royal Engineers has most obligingly forwarded 
me, in addition to the observations made during the months of Oct., Nov. and 
Dec. 1845, curves of the barometric undulations observed at Dublin during 
the Novembers of 1829 to 1845 inclusive. These curves are so admirably cal- 
culated to confirm or disprove the views advanced in XI., that I avail myself of 
his permission to lay them before you; and I beg to acknowledge the great 
obligations I am under to that officer for the valuable assistance he has rendered 
me inthisinquiry, both with respect to the immediate subject now under discus- 
sion (the great wave), and the return of the other extensive undulations before 
alluded to, which are admirably illustrated by the curves with which he has 
furnished me, and which I have much pleasure in submitting to the Association. 
Review of the essential features of the Great Symmetrical Barometric Wave, 
as exhibited in aseries of Curves representing the Barometric Undulations 
as observed at Dublin (Mount Joy, Ordnance Survey Office, Phoenix Park) 
during the Novembers of 1829 to 1845 inclusive. 
[It may be well to notice, previous to proceeding with this review, that 
Dublin is not constantly situated in the line of greatest symmetry. In 1842 
it appeared to form one of the points in the line, but the observations of Nov. 
1845 have shown that this line is not stable. The line of greatest symmetry 
appeared on that occasion to coincide to a great extent with the southern 
coast of England, so that Dublin was thrown to the north of it. It is pro- 
bable this line has a sort of oscillatory motion, and this may to a great ex- 
tent explain the nodal character of Brussels asa barometric station, the fixed 
point in the line being not far removed from that city. The departure from 
symmetry on many of the returns of the great wave at Dublin, may readily 
be accounted for by the line of greatest symmetry being considerably removed 
from thence. ] 
1829. The great wave very distinct; the two anterior undulations well- 
marked, the two posterior not so distinct. 
