
ON THE REDUCTION OF METEOROLOGICAL OBSERVATIONS, 99 
Before concluding this report it may not be irrelevant to add a few words 
on the subject of the winds generally as connected with barometrical move- 
ments, In the mode in which I have been accustomed to consider the sub- 
ject, the winds may be divided into two classes,—winds of translation and 
winds of oscillation ; nearly in the way in which the movements of the ocean 
may be divided into oceanic currents and tide-streams, and these again (re~ 
garded as the result of oscillatory movements) may be referred to the general 
laws which regulate the molecular movements of water in contact with the 
bottom of the sea, when under the influence of undulatory agitation, The 
trade-winds and greater aerial currents of a similar character, to which the 
name of monsoons is given, are winds of translation. They have a distinct ba- 
rometrical origin, in the diminution of pressure in approaching the equator, 
caused by the expansion of the equatorial atmosphere and the overflowing of 
the upper strata outwards towards the poles. But this cause is not oscilla- 
tory but permanent. Monsoons also arise in the same way ; from local heat- 
ing and cooling periodically renewed, it is true; but in long periods of six 
months in duration, so as to give rise to steady currents. With such winds 
the present research has little connexion, except in so far as their powerful 
influence mixes with and masks the effect of the other class of winds,—those 
which arise from barometric oscillation, and which are connected with such 
oscillation in a more direct and intimate manner. The oscillations themselyes 
perhaps take their rise in local and temporary causes prevailing over great 
areas simultaneously, the principal no doubt depending on the prevalence of 
cloud or clear sky, rain, or dryness over great tracts for several days or weeks 
in succession, But once produced, and an extensive atmospheric undulatio2 
once propagated, a wind or system of winds dependent on such undulation ne- 
cessarily arises also. Every wave-like movement in a fluid (see Weber’s ‘ Wel- 
lenlehre’) consists of two distinct things, an advancing form and a molecular 
movement, which latter consists in a two-fold motion of each particle, vertical 
and horizontal. Laying aside at present the consideration of the vertical 
movement (which belongs only to the strata not in contact with the ground, 
and with which probably many interesting particulars respecting the forma- 
tion and dispersion of clouds, the precipitation of rain, and the generation of 
lightning are connected), those strata which are in immediate contact with 
the ground, in obedience to the general laws of fluid movement, have their 
vertical movement zero and their horizontal a maximum, and for the most 
part (in periodical waves) alternately progressive and retrogressive. 
Now, the advancing form is indicated to us by the barometer, the mole- 
eular movement by the wind, and between these two phznomena there sub- 
sists of necessity a close and purely dynamical connexion. And it would 
be no small meteorological discovery if, by the study of the characters and 
progress of barometrical fluctuations, we could either make out any law of 
the greater ones which would enable us even roughly to predict them, or any 
peculiarity in their physiognomy by which we could recognize them in their 
earlier stages, as by this we might possibly be led to the prediction of great 
storms. 
Everybody must have remarked the sudden reversion of wind which often 
accompanies short and brisk storms accompanied with thunder and lightning 
do not now speak of the great so-called “revolving” gales). This phe- 
nomenon I consider as yuite a case in point. But the horizontal motion of 
a particle of air in contact with the earth’s surface need not be a straight 
line or even a returning curve. It would be the former only in the case of a 
straight, cylindriform wave once passing or periodically repeated ; it would 
be the latter in the case of an oscillatory movement revolving round a node 
H2 
