ON ATMOSPHERIC WAVES. 125 
during the seventeen-day undulation of Oct. 1 to 19, 1845, having their re- 
spective maxima on the 5th, 10th, and 14th. ‘Taking the same vertical or- 
dinates in each curve, we have the epochs of the troughs of the first superposed 
waves nearly similar but separated by an interval of civil reckoning of four- 
teen and a half days; thatis, the gentle undulation of the last half of September 
occurred a fortnight later, and the superposed waves indicating the disturbed 
state of the atmosphere, and characterizing the period of greater barometric os- 
cillation, came rolling on a fortnight earlier ; the two coinciding and producing 
the compound curve really observed. The first of these superposed waves 
being about half the size of the corresponding wave in 1842, passed its maxi- 
mum about a day and a half earlier, and a small wave succeeding it brought 
the minimum on the same vertical line as that of 1842. Ina similar manner 
the largest superposed wave in 1845 culminated at a later period of the normal 
wave than the smaller wave of 1842. In consequence of these different rela- 
tions of the superposed waves of 1842 and 1845, the two apices were much 
nearer in 1842 than in 1845. 
From these remarks it appears that, taking the barometric movements from 
Sept. 14, 1842 to Nov. 25 of the same year, containing two undulations of 
seventeen days’ interval, and comparing them with those from Oct.1 to Nov. 
21 of 1845, only one undulation of seventeen days’ interval was observed in 
the latter year, namely from Oct. 1 to 19; that this undulation was not of the 
gentle flowing character manifested by that from Sept. 14 to Oct. 1, 1842, 
but was interrupted by the same number of superposed waves as that from 
Oct. 14 to 31, 1842; and that this state of things was brought about by the 
later occurrence of the normal wave, and the earlier occurrence of the super- 
posed waves. Of the two seventeen-day undulations of 1842 the first (Sept. 
14 to Oct. 1) returned in 1845. 
VIII. In addition to the absence of the second seventeen-day interval, Oct. 
14 to 31, 1842, in the observations of 1845; the preceding movements, Oct. 6 
to 13, 1842 (the barometer attaining a considerable altitude), were not ob- 
served in 1845. 
IX. During the period from Oct. 1 to Nov. 21 in 1845, the barometric 
movements of Oct. 23 to 26 were the only oscillations that appeared to have 
no corresponding movements in 1842. 
X. The distinctness with which the great wave commenced in 1842 and 
1845, and the breaking off of the exact similarity between the curve of the 
' preceding ten and a half days which had been so closely maintained just as 
the wave commenced in 1845, exhibit this interesting phenomenon in all its 
individuality, and completely separate it from all the preceding barometric 
_ movements. ; 
XI. The individuality which is thus given to the great wave, the distinct- 
ness of its essential features, the close resemblance of its curves in 1842, 1843 
_ and 1845, and the closer relations existing between those of 1842 and 1845, 
induce the strong belief that we have obtained the ¢ype of the barometric 
_ oscillations during the middle portion of November. ‘This type I propose 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 
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 
i 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 
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