_The Constancy and Variety of Climates._
It is possible to conceive arrangements of our system, according
to which all parts of the earth might have the same, or nearly
the same, climate. If, for example, we suppose the earth to be a
flat disk, or flat ring, like the ring of Saturn, revolving in its
own plane as that does, each part of both the flat surfaces would
have the same exposure to the sun, and the same temperature, so
far as the sun’s effect is concerned. There is no obvious reason
why a planet of such a form might not be occupied by animals and
vegetables, as well as our present earth; and on this supposition
the climate would be every where the same, and the whole surface
might be covered with life, without the necessity of there being
any difference in the kind of inhabitants belonging to different
parts.
Again, it is possible to conceive arrangements according to which
no part of our planet should have any steady climate. This may
probably be the case with a comet. If we suppose such a body,
revolving round the sun in a very oblong ellipse, to be of small
size and of a very high temperature, and therefore to cool rapidly;
and if we suppose it also to be surrounded by a large atmosphere,
composed of various gases; there would, on the surface of such a
body, be no average climate or seasons for each place. The years,
if we give this name to the intervals of time occupied by its
successive revolutions, would be entirely unlike one another. The
greatest heat of one year might be cool compared with the greatest
cold of a preceding one. The greatest heats and colds might succeed
each other at intervals perpetually unequal. The atmosphere might
be perpetually changing its composition by the condensation of
some of its constituent gases. In the operations of the elements,
all would be incessant and rapid change, without recurrence or
compensation. We cannot say that organized beings could not be
fitted for such a habitation; but if they were, the adaptation must
be made by means of a constitution quite different from that of
almost all organized beings known to us.
The state of things upon the earth, in its present condition, is
very different from both these suppositions. The climate of the
same place, notwithstanding perpetual and apparently irregular
change, possesses a remarkable steadiness. And, though in different
places the annual succession of appearances in the earth and
heavens, is, in some of its main characters, the same, the result
of these influences in the average climate is very different.
Now, to this remarkable constitution of the earth as to climate,
the constitution of the animal and vegetable world is precisely
adapted. The differences of different climates are provided for by
the existence of entirely different classes of plants and animals
in different countries. The constancy of climate at the same place
is a necessary condition of the prosperity of each species there
fixed.
We shall illustrate, by a few details, these characteristics in the
constitution of inorganic and of organic nature, with the view of
fixing the reader’s attention upon the correspondence of the two.
1. The succession and alternation, at any given place, of heat
and cold, rain and sunshine, wind and calm, and other atmospheric
changes, appears at first sight to be extremely irregular, and not
subject to any law. It is, however, easy to see, with a little
attention, that there is a certain degree of constancy in the
average weather and seasons of each place, though the particular
facts of which these generalities are made up seem to be out of the
reach of fixed laws. And when we apply any numerical measure to
these particular occurrences, and take the average of the numbers
thus observed, we generally find a remarkably close correspondence
in the numbers belonging to the whole, or to analogous portions of
successive years. This will be found to apply to the measures given
by the thermometer, the barometer, the hygrometer, the rain gauge,
and similar instruments. Thus it is found that very hot summers,
or very cold winters, raise or depress the mean annual temperature
very little above or below the general standard.
The heat may be expressed by degrees of the thermometer; the
temperature of the day is estimated by this measure taken at
a certain period of the day, which is found by experience to
correspond with the daily average; and the mean annual temperature
will then be the average of all the heights of the thermometer for
every day in the year.
The mean annual temperature of London, thus measured, is about
50 degrees 4-10ths. The frost of the year 1788 was so severe that
the Thames was passable on the ice; the mean temperature of that
year was 50 degrees 6-10ths, being within a small fraction a degree
of the standard. In 1796, when the greatest cold ever observed in
London occurred, the mean temperature of the year was 50 degrees
1-10th, which is likewise within a fraction of a degree of the
standard. In the severe winter of 1813-14, when the Thames, Tyne,
and other large rivers in England were completely frozen over, the
mean temperature of the two years was 49 degrees, being little more
than a degree below the standard. And in the year 1808, when the
summer was so hot that the temperature in London was as high as 93½
degrees, the mean heat of the year was 50½, which is about that of
the standard.
The same numerical indications of the constancy of climate at the
same place might be collected from the records of other instruments
of the kind above-mentioned.
We shall, hereafter, consider some of the very complex agencies by
which this steadiness is produced; and shall endeavour to point
out intentional adaptations to this object. But we may, in the
meantime, observe how this property of the atmospheric changes is
made subservient to a further object.
To this constancy of the climates of each place, the structure
of plants is adapted; almost all vegetables require a particular
mean temperature of the year, or of some season of the year; a
particular degree of moisture, and similar conditions. This will
be seen by observing that the range of most plants as to climate
is very limited. A vegetable which flourishes where the mean
temperature is 55 degrees, would pine and wither when removed to a
region where the average is 50 degrees. If, therefore, the average
at each place were to vary as much as this, our plants with their
present constitutions would suffer, languish, and soon die.
2. It will be readily understood that the same mode of measurement
by which we learn the constancy of climate at the same place,
serves to show us the variety which belongs to different places.
While the variations of the same region vanish when we take the
averages even of moderate periods, those of distant countries are
fixed and perpetual; and stand out more clear and distinct, the
longer is the interval for which we measure their operation.
In the way of measuring already described, the mean temperature
of Petersburg is 39 degrees, of Rome 60, of Cairo 72. Such
observations as these, and others of the same kind, have been made
at various places, collected and recorded; and in this way the
surface of the earth can be divided by boundary lines into various
strips, according to these physical differences. Thus, the zones
which take in all the places having the same or nearly the same
mean annual temperature, have been called _isothermal_ zones. These
zones run nearly parallel to the equator, but not exactly, for,
in Europe, they bend to the north in going eastward. In the same
manner, the lines passing through all places which have an equal
temperature for the summer or the winter half of the year, have
been called respectively _isotheral_ and _isochimal_ lines. These
do not coincide with the isothermal lines, for a place may have
the same temperature as another, though its summer be hotter and
its winter colder, as is the case of Pekin compared with London.
In the same way we might conceive lines drawn according to the
conditions of clouds, rain, wind, and the like circumstances, if we
had observations enough to enable us to lay down such lines. The
course of vegetation depends upon the combined influence of all
such conditions; and the lines which bound the spread of particular
vegetable productions do not, in most cases, coincide with any of
the separate meteorological boundaries above spoken of. Thus, the
northern limit of vineyards runs through France, in a direction
very nearly north-east and south-west, while the line of equal
temperature is nearly east and west. And the spontaneous growth or
advantageous cultivation of other plants, is in like manner bounded
by lines of which the course depends upon very complex causes, but
of which the position is generally precise and fixed.