April i, 1890.] THE TROPICAL AGRICULTURIST- 693 
Starting with the well-known fact that the earth re- 
volves on its axis once in the twenty-four hours, let us 
see what will be the consequence, if we suppose a mass 
of any ponderable matter — that is, any substance having 
weight, no matter whether light or heavy — to be sud- 
denly transferred from the equator to latitude 60°. 
As the circumference of the earth at the equator is 
about 24,900 miles, any body whatever, apparently at 
rest at the equator, is carried round the earth's axis at 
the rate of 1,036 miles an hour. But in latitude 60°, 
where the distance from the axis is only half as great 
as at the equator, it is carried round at only half the 
same rate, or 518 miles an hour; and at the pole it 
simply turns round on its own axis. Supposing, then, a 
mass of air to be suddenly transferred from the equator 
to latitude 60°, with the eastward movement that it 
had at the equator, it would be moving twice as fast to 
the east as that part of the earth, and, to any person 
standing on the earth, would be blowing from the west 
with a force far exceeding that of a hurricane. It would 
be moving eastwards 518 miles an hour faster than the 
earth. Indeed, its movement would really be far greater 
than this. In virtue of a mechanical principle known as 
tlie law of the conservation of areas, which means that 
any body revolving round a central point, under the 
influence of a force that pulls it towards that poiut, 
describes equal areas in equal times, instead of only 518 
mile?, it would be revolving round the earth's axis 
1,554 miles an hour faster than that part of the earth. I 
need not, however, specially insist on this point, 
because, as a matter of fact, the air which constitutes 
the anti-trades is not suddenly transferred, but takes 
a day or two to perform its journey, and in the mean- 
time byfar the greater part of its eastward movement 
is lost by friction against the trade-wind which blows 
in the opposite direction underneath it. The point on 
which we have to fix our attention is that, when the 
anti-trades desefnd to earth, they still retain some of 
this eastward movement, and blow, not as south, but 
assouth-west or west-south-west winds. 
On the other hand, the trade-wind, which blows to- 
wards the equator, is coming from a la ! itude where the 
eastwatd movement iB less than at the equator, and its 
own movement eastward is therefore less than that of 
the surface over which it blows. A person, therefore, 
stan ling on the earth, is carried eastward faster than 
the air is moving, and the wind seems to blow against 
him from the north-east. Similarly, to the south of 
the equator, the trade-wind, instead of blowing from 
the south, comes from the south-east. 
Thus, then, we have in both hemispheres a system 
of westerly winds in all higher latitudes than 40°, and 
a system of easterly winds — viz. the trade-winds — 
between about 30° and the equator ; and if the globe 
were either all land or all water, these systems would 
prevail right round the earth. 
Now, it is the pressure of these winds, under the 
influence of centrifugal force, that causes the two 
zones of high barometer in latitudes 30° to 40°, and the 
very low pressure in higher latitudes. It is not difficult 
to understand how this comes about. You are probably 
aware that the earth is not an exact sphere, but what 
is termed an oblate spheroid— that is, it is slightly 
flattened at the poles and protuberant at the equator, 
the difference of the equatorial and polar diameters 
being about 26 miles. It has acquired this form in virtue 
of its rotation on its axis. If you whirl a stone in a 
sliug, the stone has a tendency to fly off at a tangent, 
and, so long as it is retained in the sling, that ten- 
dency is resisted by the tension of the cord. In the 
same way, every object resting on the earth, and the 
substance of the earth itself, has a tendency to fly off 
at a tangent, in consequence of its rotation on its axis, 
and tin- tendency is resisted and overcome by gravity. 
Were the earth not > evolving, its torm, under the 
influence of gravity slope, would be a true sphere. 
If it were to revolving more rapidly than at present, it 
would be still m-TO oblate, flatter at the poles, and 
more bulging in the tropical zone ; if less rapidly, the 
flattening and bulging would lie lees. 
This is precisely what happens with the wi st and 
east winds of which we have spoken. West winds are 
revolving faster than the earth, and tend to make 
the atmosphere more protuberant at the equator than 
the solid earth ; hence they press towards the equator, 
io the right of their path in the northern hemisphere, 
and this teadency increases rapidly in high latitudes. 
Easterly winds, on the other hand, tend to render the 
form of the atmosphere more nearly spherical, and 
they, too, press to the right of their path in the 
northern hemisphere or towards the pole. In the south- 
ern hemisphere, for the same reason, both press to the 
left. The result of these two pressures in opposite 
directions is to produce the two zones of high barometer 
in the latitudes in which we find them— viz. between 
the easterly trade-winds and the westerly winds, 
which are the anti-trades that have descended to the 
earth's surface. And the low barometer of higher 
latitudes is produced in like manner by the westerly 
winds pressing away from those regions. 
Thus, then, we find that all this system of winds, 
and the resulting distribution of atmospheric pressure as 
indicated by the barometer, is the result of the sun's 
action in equatorial regions. It is this that gives the 
motive power to the whole system, so far as we have as 
yet traced it, and it is this that produces those great 
inequalities of atmospheric pressure that I have so far 
described. 
It remains now to see how storms are genented by 
these westerly winds. In so far as they retain any 
southing, they are sti:l moving towards the pole in 
the northern hemisphere— that is to fay, they are 
advancing from all sides towards a mere point. Some 
portion of them must therefore be continually turning 
back as the circles of latitude become smaller and 
smaller. But they are now surface-winds, and in order 
so to return they must rise and flow baik as an upper 
current. This they do by forming gre t eddies, or air- 
whirls, in the centre of which the barometer is very 
low, and over which the air ascends, and these great 
air-whirls are the storms of tha temperate zone and of 
our latitudes. It is the ascent and dynamic cooling of 
the air in these great eddies that cause the prolonged, 
rainfall of wet stormy weather. How the eddies ori- 
ginate, or, rather, what particular circumstance causes 
them to originate in one place rather than another, we 
can scarcely say, any more than wo can say how each 
eddy originates in a rapidly .flowing deep river. Some 
very small inequality of pressure probably starts them, 
but, when once formed, they often last for many days, 
and travel some thousands of miles over the earth's 
surface. 
Two such storms are represented on the charts of 
February 1 aud 2, 1883, one on the coast of Labrador, 
the other to the south-west of the British Isles. The 
first of these appears on the chart of January 28, in the 
North Pacific, off the coast of British Columbia. On 
the 29th it had crossed the Rocky Mountains, and was 
traversing the western part of the Hudson's Bay 
Territory. On the 30th it had moved to the south-east, 
and lay just to the west of the Great Lakes, and on the 
31st between Lake Superior and Hudson's Bay. On 
February 1 it had reached the position on the coast of 
Labrador shown in the chart, and on the 2nd had 
moved further to north-east, t>nd lay across Davis's 
Straits, and over the west course of Greenland. After 
this it again changed its course to south-east, and on 
February 4 passed to the north of Scotland, towards 
Denmark, and eventually on to Russia. 
The second storm had originated off the east coast 
of the United States between January 28 and 29, and 
on the following days crossed the Atlantic on a course 
somewhat to north of east, till, or. February 2, it lay 
over England. 
These storms always move in some easterly direction, 
generally between east and north-east, aud often 
several follow in rapid succession on nearly the same 
track. It is this Knowledge that renders it possible 
for the Meteorological Office to issue the daily fore- 
casts that we see in the newspapers. Were it possible 
to obtain telegraphic reports from a few stations out 
in the North Atlantic, these storm warnings could be 
issued with much more certainty, and perhaps longer 
before the arrival of the storm than at present- In 
the case of such storms as that which reached our 
islands on February 2, we often have such warnings 
from America, but their tracks are often more to the 
