ON ATMOSPHERIC WAVES. 123 
elements of this normal wave freed from all extraneous cireumstances. At 
Munich we possess barometric records every hour for the three years ; these 
are reduced to the freezing temperature and accompanied with observations 
from which the tension of the vapour may be obtained. It will be.necessary 
for the three periods embraced in the diagram to express the barometric al- 
titudes in English inches, and reduce them to the level of the sea. When 
so reduced the vapour pressure must be deducted, leaving the gaseous pres- 
sure only, and this must be further corrected for the diurnal and annual 
variations of the gaseous pressure; we shall thus obtain three curves repre- 
senting the variations of gaseous pressure, the causes of which we are seeking, 
and the mean of these three curves will to a certain extent be freed from those 
indentations which appear to result from the passage of secondary waves. 
The same process must be adopted with respect to the observations at 
Prague, Brussels. and Greenwich ; when this is accomplished we shall obtain 
four normal curves, the comparison of which will be highly instructive and 
important. 
The curve of 1842 is tinted for the purpose of indicating the prevalent 
wind during the period occupied by one coloured portion. There does not 
appear much apparent relation between the colours and the flexures of the 
curve. Two points, however, claim our especial attention,—the change in 
the direction of the wind to nearly the opposite point, on the transit of the 
crest,—and the calms intervening between that and other changes nearly of a 
similar character. N.E. winds are coloured blue, S.W. pink, and S.E. green, 
The direction has been obtained from the Greenwich observations. 
Sir John Herschel has shown in his ‘ Report on the Reduction of Meteoro- 
logical Observations’ (Report, 1843, p. 99), that there must be a close and 
purely dynamical connexion between the advancing form of the wave and the 
molecular movement of the air; the character of the molecular movement will 
greatly depend on the order of the wave. In the absence of data for deter- 
mining the precise characters of the waves under consideration, it may not be 
uninteresting to offer a few remarks on the two points to which our atten- 
tion has been directed :—1st. The calm preceding the reversion of wind on the 
transit of the crest, Nov. 18th, 1842. A very casual comparison of the direc- 
tion of the wind at several stations marked on the area, shown in Plate XLII. 
(Report, 1844), indicates that the molecular movement was directed towards 
the point of least pressure, a result to be expected, and perfectly in accordance 
with the beautiful deductions of Col. Sabine (see his Report on the Meteor- 
ology of Toronto, Report, 1844). Now in the case of a large wave stretch- 
ing over an extensive area, the anterior and posterior troughs would mark out 
parallel or nearly parallel lines of least pressure; the molecular movement 
would be strongest in these troughs, and directed towards them from each 
side; at stations removed from them the force of the wind would be greatly 
diminished, and at the intervening crest it would be so small as to be inap- 
preciable ; but however small it might be, upon the crest passing any station, 
the direction of the wind at that station would be reversed, and it would in- 
crease in intensity until the transit of the posterior trough. In this manner 
it is apprehended that the reversion of the wind, and the calm preceding it, 
Noy. 18th, 1842, are explained. The Greenwich observations offer a fine 
illustration of the increase of intensity. November 19th, 6 and 8 hours, the 
‘anemometer recorded a pressure of 2 to 4lbs. to the square foot 30 hours 
after transit. Qnd. The remaining calms in the diagram may be explained 
n the same way, but the synchronous traversing of different systems of waves 
masks the effects and prevents the relations between the wind and the advan- 
