76 NATURE 

where there was an excess, and the blue lines where there was 
a deficiency. The general results are that, in each hemisphere, 
pressures are highest in winter, and lowest in summer ; that in 
winter the highest pressures are grouped over the continents, and 
in summer the lowest, and that in winter very low pressures 
prevail in the northern parts of the Atlantic and Pacific oceans 
respectively. The position of the isobars is wholly determined 
by the relative distribution of land and water. 
As regards wind, those places were selected which are favour- 
ably situated for observing its direction, In calculating the 
average direction, the element of force was wholly excluded. 
The years were, so far as possible, the same as those for which 
the mean pressure had been calculated ; but where this was not 
possible, care was taken to see that a good average was in every 
case obtained. The mean direction of the wind for each month 
is shown in the charts by arrows. 
Thus two quite distinct facts were exhibited on the charts, 
viz. lines showing the mean pressure of the atmosphere, and 
arrows showing the prevailing winds at the earth’s surface, each 
being independently arrived at by the summing and averaging 
of observed facts. What relation is there between these two 
classes of facts ? 
1. As regirds regions of Low Pressures : 
In every case where such occur at any season, it is universally 
seen that the relations of the winds to the isobaric lines is exactly 
the same that is illustrated in every storm of wind when the 
winds and pressures are set down in synophic charts.—The 
relation is this ; the wind neither blows round the centre of least 
pressure in circles, nor as tangents to the concentric isobaric 
curves, nor does it blow directly towards thal centre, but it takes 
an intermediate ditection, approaching however more nearly to 
the direction and course of the circular curves than of the radii 
to the centre. The angle is not a right angle but from about 
60° to 80°. Thus the whole system of winds seem to blow vorti- 
cosely in upon the spaces of low pressure. ‘This is the relation 
known as Buy's Ballot Law of the Winds. 
2. As regards regions of High Pressures : 
In all cases where such occur, the winds are seen to flow out 
in every direction. In these cases also, the behaviour of the 
winds differs in no respect from what occurs on particular days 
on which the isobaric lines present the same conditions of 
pressure. The winds flow out of these spaces of high pressure 
in courses exactly the opposite to what takes place when they 
flow in upon spaces of low pressure, and hence such meteorological 
phenomena have been called by Mr, Francis Galton, ‘ Anti- 
cyclones.” ‘This is also, it will be seen, in strict accordance with 
Buy’s Ballot Law. Hence then this broad result is arrived at, viz., 
that the prevailing winds over the globe at all seasons obey Buy’s 
Ballot Law, with reference to the distribution of atmospheric 
pressure. 
The outflow of the air from a region of high pressure, and the 
inflow upon a region of high pressure is reducible to the single 
principle of gravitation, and so marked is this, that if there be 
any other force or forces which put the winds in motion, they 
must be altogether insignificant as compared with gravitation. 
The annual averages of the 115 places distributed over the 
north temperate zone were minutely examined with the view of 
ascertaining how far the commonly-alleged prevalence of equa- 
torial and polar currents is borne out by observation. The result 
of the analysis is this:—There are two maximum directions of 
prevailing winds at the stations of which the S.W. and N.E. at 
Greenwich may be taken as an example. The chief prevailing 
winds of the north temperate zone blow from some point from 
S.S.W. to W. (the true equatorial direction) at 41 per cent. of the 
stations, leaving 59 per cent. at which they are from other points 
of the compass. And the secondary prevailing winds come from 
some point from N.N.E. to E. (the trie polar direction) at 34 
per cent. of the stations or only a thitd of the whole. Hence 
whilst the largest percentage of prevailing winds ate in the 
direction in which truly equatorial and polar winds should blow, 
the percentages from the direction are so large as to preclude 
the supposition of a general flow of the surface winds of 
the temperate zone towards and from the polar régions. 
‘The truth is, it is not the poles of the earth, but the regions of 
high and low pressures which must be regarded as the true poles 
of the winds towards which and from which the great moyements 
of the atmosphere proceed ; and owing to the unequal distribu- 
tion of land and water, the poles of pressure and movements of 
the atmosphere are, as in the case of the poles of temperature, 
very far from being coincident with the North Pole. 



- -* . ie) en 
q 4q 
[WVov. 24, 1870 
The causes which bring about an unequal distribution of the 
mass of the earth’s atmosphere are chiefly two—viz., the tem- 
perature principally, and secondarily the moisture of the atmo- 
sphere, their relations to the geographical distribution of land and 
water. The charts show that where there is an excess of tem- 
perature, as in the interior of Asia in summer, and where there is 
a relative excess of moisture, as in the belt of calms and in the 
north of the Atlantic in winter, and where there is an excess of 
cold, as in the interior of Asia in winter, and where there is 4 
deficiency of moisture—there atmospheric pressure is ie 
Hence where pressures are low there the air is specifically light, 
and where pressures are high there the air is heavy relatively to 
that of surrounding regions, 
Further, since vast columns of air are poured by the pre- 
vailing winds into those regions where pressures are low without 
increasing that pressure, we must suppose that there is an outflow 
from these regions through upper currents, and this inference is 
all the more inevitable, seeing that the specifically light air resting 
over these regions supplies the conditions of an ascending current. 
Again, since vast volumes of air are driven off from the regions 
of high pressure without diminishing the pressure, it must be 
inferred that the higa pressure is maintained by accessions poured 
in upon it by the upper currents, and this inference is the more 
certain because in such regions the air is relatively heavier than 
in surrounding regions, thus supplying the conditions of a de- 
scending current. 
It is evident that the currents from the regions of low pressure 
will continue to ascend as long as their pressure is less than that 
in regions surrounding them a/ #2 same height, but that as soon 
as they reach a height where the pressure is less, towards afid 
over that region will they flow as upper currents of the atmo- 
sphere. ‘The courses of these upper currents will be directed 
towards those regions where the air is coldest and driest near the 
surface of the earth, because being thereby densest, the great 
mass of the air will condense in the lower beds, thus leaving 
less air or a diminished pressure in the upper regions. Thus the 
high winter pressure of Asia will be maintained by air being 
poured upon it by upper currents from the regions of low pressure 
in the north of the Atlantic, the north of the Pacific, and the 
Equatorial regions of the south. . 
From these considerations the following Zheory of the Mowe- 
ments of the Larth’s Atmosphere necessarily follows. The winds 
on the surface of the earth are known from the isobaric lines, the 
direction being from regions of high towards regions of low 
pressures, subject to the changes produced in the direction of the 
currents by the earth’s rotation; and the upper currents of the 
atmosphere may be inferred from the isobaric lines taken reversely 
togetlier with the isothermal lines taken directly. In other 
words, the regions of lowest pressure, by giving the ascending 
currents, point out the sources or fountains wlience the upper 
currents flow ; and the isothermal lines, by showing where, on 
account of the low temperature, the greatest portion of the air is 
condensed in the lower beds and so diminishing the pressure in 
the upper beds, point out the regions towards and over which the 
upper currents diffuse themselves. , , 
In this discussion of the prevailing winds and mean atmo- 
spheric pressure over the globe, there is contained the first 
approximate answer to the two questions :— 
1. What are the motions of the earth’s atmosphere? 
2. What are the causes of these motions ? 
It has been shown by Dr. Balfour Stewart* that these ques- 
tions form the fitst two stages in the development of meteorology: 
Considering the importance of the subjects of this diseussion 
which enter deeply into Physical Geography, Geology, the 
Science of Navigation; and the General Physics of the globe; 
itis to be hoped that observation will Le made and turned to 
account in rectifying the isobaric lines and filling up the blanks 
of the winds over the ocean, and portions of Africa and South 
America, data from these regions being in this as well as every 
similar discussion to a large extent Wanting. ALEX. BUCHAN 


SOCIETIES AND ACADEMIES 
LONDON \ tod f 
Zoological Society, November 15.—Prof, W. lH. Flower 
F.R.S., V.P., in the chair. An eighth letter was read 
Mr. W. H. Hudson, contaihing further observations on the 
Ornithology of Buenos Ayres.—Mr. Sclater exhibited a specimen 
* See NaTuRE of December 2, 1869; p.129. 
Se eee I ens tein ae on. 
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