692 
THF TROPICAL AGRICULTURIST. [Apr!l i, 1890. 
many years ago in the mountains of Ceylon ; and it 
was afterwards mentioned to me by Sir Samuel Baker, 
who had been equally struck by it. My own experience 
is as follows i — In June 1861, I paid a week's visit to 
the hill sanitarium of Newara Eliya, at an elevation of 
6,200 feet, on the western face of Pedro Talle Galle, the 
highest mouutain in the island. The south-west mon- 
soon was blowing steadily on this face of the range ; 
and during the whole time of my stay it rained, as 
far as I am aware, without an hour's intermission, 
and a dense canopy of cloud enveloped the hill face, 
and never lifted more than a few hundred feet above 
the little valley in which Newara Eliya is built. 
But on leaving the station by the eastern road that 
leads across the crest of the range to Badulla, 
at a distance of five miles one reaches the col 
or dip in the ridge near Hackgalle, and thence the 
road descends some 2,000 feet to a lower table-land 
which stretches away many miies to the east. No 
sooner is this point passed then all rain ceases and 
cloud disappears, and one looks down on the rol'ing 
grassy hills bathed in the sunshine of a tropical sun, 
and swept by the dry westerly wind that descends 
from the mountain ridge. In lit He more than a 
mile one passes from day-long and week-long cloud 
and rain to constant sunshine and a cloudless sky. 
As an almost invariable rule, or at least one with 
few exceptions, ascending air currents are those that 
form cloud and rain, and descending currents are dry 
and bring fine weather. And this holds good what- 
ever may be the immediate cause of these movements 
We may now proceed to consider these greater examples 
to which I have already referred. 
In the great workshop of Nature, in so far at least 
as conoerns our earth, with but few exceptions, all 
movement and all change, even the movements' and 
energies of living things, proceed either directly or 
indirectly from the action of tbe sun. Nowhere is 
this action more direct and more strikingly mani- 
fested than in the movements of the atmosphere. 
Were the sun extinguished, and to become, as perhaps 
it may become long ages hence, a solid cold sphere, 
such as Byron imagined, " wandering darkling in 
eternal space," a few days would suffice to convert 
our mobile and ever-varying atmosphere into a stag- 
nant pall, devoid of vapjur, resting quiescent on a 
lifeless earth, held bound in a more than Arctic frost. 
From such a consummation, despite the supposed 
decaying energy of our sun, we may, however, enter- 
tain a reasonable hope that we are yet far distant. 
Bearing in mind the all-embracing importance of tbe 
sun, let us see how the great movements of the atmo- 
sphere are determind by the way in which the earth 
presents its surface to the solar rays. 
Since the quantity of solar heat received on each 
pa rt of the earth's surface depends on the directness 
or obliquity of his rays— in other words, on the height 
to which the sun ascends in the heavens at noon- 
being greatest where he is directly overhead, as in 
summer in the tropics, it follows that the hottest 
zone of the earth is that in the immediate neigh- 
bourhood of the equator, and the coldest those around 
the poles. 
Did time allow, and were the necessary appliances 
at hand, it would be easy to show you that both 
as a matter of experiment, and also as a deduction 
from physical laws, there must be under such cir- 
cumstances a flow of air from the colder to the warmer 
region in the lower atmosphere, and a return current 
above. And to a certain extent we have these constant 
winds prevailing for about 30° on either side of the 
equator, in the trade-winds, which blow towards the 
equator in the lower atmosphere, and the anti-trades 
blowing in the opposite direction at a great height 
above tbe earth's surface. 
In the neighbourhood of the equator there is a 
zone extending right round the earth in which the 
barometer is lower than either to the north or the south. 
II is duo to the greater heat of the sun, and it is 
towards this that the trade winds blow. It shifts to 
some exteni with the seasons, being more northerly 
in the Slimmer of the northern hemisphere, and more 
Southerly in that of the southern hemisphere { audits 
average position is rather to the north of the equator, 
owing to the fact that there is more land in the 
northern than in the southern hemisphere, and that 
land is more heated by the sun than the ocean. 
This simple wind system of the trades and anti-trades 
does not extend right round the earth, nor beyond 30° or 
40° of latitude in either hemisphere. Were the earth's 
surface uniformly land or uniformly water, there pro- 
bably would be a system of trade-winds all round the 
globe, blowing from both hemispheres towards the 
equator ; but even in that case they would not extend 
much, if at all, beyond thtir preseut limits. In the 
first place, every great mass of land seta up an inde- 
pendent system of air currents, since the land is 
hotter than the ocean in the summer, and colder in the 
winter. Iu the summer, therefore, there is a tendency 
to an indraught of air from the sea to the land in the 
lower atmosphere, and an out-flow above, and in the 
winter the opposite; and this tendency modifies or 
interrupts the system of the trades and anti-trades; 
We have this tendency shown most distinctly in the 
monsoons of South-Eastern Asia, where, both in the 
India and China seas, a south-west wind in the summer 
takes the place which in the absence of the Asiatic 
e mtinent would be held by a north-east trade-wind. 
An it is only in the winter that a north-east wind 
blows, and this is then termed the north-east monsoon. 
In the seoond place, as I have said, the system 
of trade- winds could not in any case extend far beyond 
their present limits in latitude, owing to the f«ct 
tha f . the earth is a sphere and not a cylinder. Let 
us fix our attention for a moment on the anti-lrades 
— the upper winds which blow from the equator to- 
wards the poles. The equator, from which tht-y start, 
is a circle about 24,900 miles in circumference ; tbe 
poles are mere points, and, therefore, the whole of 
the air that blows towards the poles must turn back 
in any case before it reaches the pole, and must begin to 
turn back before it has gone very far on its journey. 
And, as a fact, a great part of it does turn back between 
30° and 40° of latitude, which I have already mentioned 
as being the limit of the trade-winds. A part of the 
remainder descends to the earth's surface, and sweeps 
the Northern Atlantic and the North Pacific as a south- 
west wind. 
On the chart which represents the average distribu- 
tion of atmospheric pressure in January, there are two 
somewhat interrupted zones of high pressure over 
the ocean in these latitudes. These mark the regions 
in which the anti-trades descend to the earth's surface, 
and from which the trade-winds start. Over the 
ocean in all higher latitudes, both in the northern 
and southern hemispheres, the barometer is low — for 
the most part, indeed, much lower than over the 
equator ; and the region intervening between the zones 
of high pressure and the seat of lowest pressure is 
that of predominant south-west, or at all events 
westerly, winds. Since our islands are situated on the 
border of this region of low pressure, south-west are 
our prevailing winds. 
But now two questions arise : first, Why are these 
winds westerly, and not simply south winds ? and 
second, How is it that the barometer is so low over 
the North Atlantic and North Pacific Oceans, and also 
in the southern hemisphere ih high latitudes, seeing 
that in these latitudes, at least in winter, the sun's 
heat is so much less than at the tropics? The chart 
represents the state of things in midwinter of the 
northern hemisphere, and yet everywhere to the north 
of latitude 40° the deep blue tint indicates that the 
pressure is lower than even in the southern tropic, 
where the sun shines vertically overhead. Clearly 
this low pressure must be due to some other cause 
than the warmth of the air. 
The explanation of this remarkable distribution of 
the atmospheric pressure, of the existence of two zones 
of high pressure in latitudes 30° to 40°, an 1 of very 
low pressure in higher latitudes, except in so far as 
they are modified by the alternations of land and 
water, was first given by the American ptnsicist, 
Prof. Ferrel. Its full demonstration is to be obtained 
only from the consideration of somewhat recondite 
mechanical laws, but a general idea of the causes oper- 
ating may be gathered from very simple considerations, 
which may be demonstrated with a terrestrial globe. 
