_The Mass of the Earth._
We shall now consider the adaptation which may, as we conceive, be
traced in the amount of some of the quantities which determine the
course of events in the organic world; and especially in the amount
of the _forces_ which are in action. The life of vegetables and
animals implies a constant motion of their fluid parts, and this
motion must be produced by forces which urge or draw the particles
of the fluids. The positions of the parts of vegetables are also
the result of the flexibility and elasticity of their substance;
the voluntary motions of animals are produced by the tension of the
muscles. But in all those cases, the effect really produced depends
upon the force of gravity also; and in order that the motions and
positions may be such as answer their purpose, the forces which
produce them must have a due proportion to the force of gravity.
In human works, if, for instance, we have a fluid to raise, or
a weight to move, some calculation is requisite, in order to
determine the power which we must use, relatively to the work which
is to be done: we have a mechanical problem to solve, in order that
we may adjust the one to the other. And the same adjustment, the
same result of a comparison of quantities, manifests itself in the
relation which the forces of the organic world bear to the force of
gravity.
The force of gravity might, so far as we can judge, have been
different from what it now is. It depends upon the mass of the
earth; and this mass is one of the elements of the solar system,
which is not determined by any cosmical necessity of which we are
aware. The masses of the several planets are very different, and
do not appear to follow any determinate rule, except that upon
the whole those nearer to the sun appear to be smaller, and those
nearer the outskirts of the system to be larger. We cannot see any
thing which would have prevented either the size or the density of
the earth from being different, to a very great extent, from what
they are.
Now, it will be very obvious that if the intensity of gravity
were to be much increased, or much diminished, if every object
were to become twice as heavy or only half as heavy as it now is,
all the forces, both of involuntary and voluntary motion which
produce the present orderly and suitable results by being properly
proportioned to the resistance which they experience, would be
thrown off their balance; they would produce motions too quick or
too slow, wrong positions, jerks and stops, instead of steady,
well conducted movements. The universe would be like a machine ill
regulated; every thing would go wrong; repeated collisions and a
rapid disorganization must be the consequence. We will, however,
attempt to illustrate one or two of the cases in which this would
take place, by pointing out forces which act in the organic world,
and which are adjusted to the force of gravity.
1. The first instance we shall take, is the force manifested by
the ascent of the sap in vegetables. It appears by a multitude
of indisputable experiments, (among the rest, those of Hales,
Mirbel, and Dutrochet,) that all plants imbibe moisture by their
roots, and _pump it up_, by some internal force, into every part
of their frame, distributing it into every leaf. It will be easily
conceived that this operation must require a very considerable
mechanical force; for the fluid must be sustained as if it were a
single column reaching to the top of the tree. The division into
minute parts and distribution through small vessels does not at all
diminish the total force requisite to raise it. If, for instance,
the tree be thirty-three feet high, the pressure must be fifteen
pounds upon every square inch in the section of the vessels of
the bottom in order merely to support the sap. And it is not only
supported, but propelled upwards with great force, so as to supply
the constant evaporation of the leaves. The pumping power of the
tree must, therefore, be very considerable.
That this power is great, has been confirmed by various curious
experiments, especially by those of Hales. He measured the force
with which the stems and branches of trees draw the fluid from
below, and push it upwards. He found, for instance, that a vine in
the _bleeding_ season could push up its sap in a glass tube to the
height of twenty-one feet above the stump of an amputated branch.
The force which produces this effect is part of the economy of the
vegetable world; and it is clear that the due operation of the
force depends upon its being rightly proportioned to the force of
gravity. The weight of the fluid must be counterbalanced, and an
excess of force must exist to produce the motion upwards. In the
common course of vegetable life, the rate of ascent of the sap is
regulated, on the one hand, by the upward pressure of the vegetable
power, and on the other, by the amount of the gravity of the fluid,
along with the other resistances, which are to be overcome. If,
therefore, we suppose gravity to increase, the rapidity of this
vegetable circulation will diminish, and the rate at which this
function proceeds, will not correspond either to the course of the
seasons, or the other physiological processes with which this has
to co-operate. We might easily conceive such an increase of gravity
as would stop the vital movements of the plant in a very short
time. In like manner, a diminution of the gravity of the vegetable
juices would accelerate the rising of the sap, and would, probably,
hurry and overload the leaves and other organs, so as to interfere
with their due operation. Some injurious change, at least, would
take place.
Here, then, we have the forces of the minutest parts of vegetables
adjusted to the magnitude of the whole mass of the earth on which
they exist. There is no apparent connexion between the quantity of
matter of the earth, and the force of imbibition of the roots of a
vine, or the force of propulsion of the vessels of its branches.
Yet, these things have such a proportion as the well being of the
vine requires. How is this to be accounted for, but by supposing
that the circumstances under which the vine was to grow, were
attended to in devising its structure?
We have not here pretended to decide whether this force of
propulsion of vegetables is mechanical or not, because the argument
is the same for our purpose on either supposition. Some very
curious experiments have recently been made, (by M. Dutrochet,)
which are supposed to show that the force is mechanical; that when
two different fluids are separated by a thin membrane, a force
which M. Dutrochet calls _endosmose_ urges one fluid through the
membrane: and that the roots of plants are provided with small
vesicles which act the part of such a membrane. M. Poisson has
further attempted to show that this force of _endosmose_ may be
considered as a particular modification of capillary action. If
these views be true, we have here two mechanical forces, capillary
action and gravity, which are adjusted to each other in the manner
precisely suited to the welfare of vegetables.
2. As another instance of adaptation between the force of gravity
and forces which exist in the vegetable world, we may take the
positions of flowers. Some flowers grow with the hollow of their
cup upwards: others “hang the pensive head,” and turn the opening
downwards. Now of these “nodding flowers,” as Linnæus calls
them, he observes that they are such as have their pistil longer
than the stamens; and, in consequence of this position, the dust
from the anthers which are at the ends of the stamens can fall
upon the stigma or extremity of the pistil; which process is
requisite for making the flower fertile. He gives as instances the
flowers _campanula_, _leucoium_, _galanthus_, _fritillaria_. Other
botanists have remarked that the position changes at different
periods of the flower’s progress. The pistil of the Euphorbia
(which is a little globe or germen on a slender stalk) grows
upright at first, and is taller than the stamens: at the period
suited to its fecundation, the stalk bends under the weight of
the ball at its extremity, so as to depress the germen below the
stamens; after this it again becomes erect, the globe being now a
fruit filled with fertile seeds.
The positions in all these cases depend upon the length and
flexibility of the stalk which supports the flower, or in the
case of the Euphorbia, the germen. It is clear that a very slight
alteration in the force of gravity, or in the stiffness of the
stalk, would entirely alter the position of the flower cup, and
thus make the continuation of the species impossible. We have
therefore here a little mechanical contrivance, which would have
been frustrated if the proper intensity of gravity had not been
assumed in the reckoning. An earth greater or smaller, denser or
rarer than the one on which we live, would require a change in the
structure and strength of the footstalks of all the little flowers
that hang their heads under our hedges. There is something curious
in thus considering the whole mass of the earth from pole to pole,
and from circumference to centre, as employed in keeping a snowdrop
in the position most suited to the promotion of its vegetable
health.
It would be easy to mention many other parts of the economy of
vegetable life, which depend for their use on their adaptation to
the force of gravity. Such are the forces and conditions which
determine the positions of leaves and of branches. Such again
those parts of the vegetable constitution which have reference to
the pressure of the atmosphere; for differences in this pressure
appear to exercise a powerful influence on the functions of plants,
and to require differences of structure. But we pass over these
considerations. The slightest attention to the relations of natural
objects will show that the subject is inexhaustible; and all that
we can or need do is to give a few examples, such as may show the
nature of the impression which the examination of the universe
produces.
3. Another instance of the adjustment of organic structure to the
force of gravity may be pointed out in the muscular powers of
animals. If the force of gravity were increased in any considerable
proportion at the surface of the earth, it is manifest that all
the swiftness, and strength, and grace of animal motions must
disappear. If, for instance, the earth were as large as Jupiter,
gravity would be eleven times what it is, the lightness of the
fawn, the speed of the hare, the spring of the tiger, could no
longer exist with the existing muscular powers of those animals;
for man to lift himself upright, or to crawl from place to place,
would be a labour slower and more painful than the motions of the
sloth. The density and pressure of the air too would be increased
to an intolerable extent, and the operation of respiration, and
others, which depend upon these mechanical properties, would be
rendered laborious, ineffectual, and probably impossible.
If, on the other hand, the force of gravity were much lessened,
inconveniences of an opposite kind would occur. The air would be
too thin to breathe; the weight of our bodies, and of all the
substances surrounding us, would become too slight to resist the
perpetually occurring causes of derangement and unsteadiness: we
should feel a want of ballast in our movements.
It has sometimes been maintained by fanciful theorists that the
earth is merely a shell, and that the central parts are hollow.
All the reasons we can collect appear to be in favour of its being
a solid mass, considerably denser than any known rock. If this be
so, and if we suppose the interior to be at any time scooped out,
so as to leave only such a shell as the above mentioned speculators
have asserted, we should not be left in ignorance of the change,
though the appearance of the surface might remain the same. We
should discover the want of the usual force of gravity, by the
instability of all about us. Things would not lie where we placed
them, but would slide away with the slightest push. We should have
a difficulty in standing or walking, something like what we have
on ship-board when the deck is inclined; and we should stagger
helplessly through an atmosphere thinner than that which oppresses
the respiration of the traveller on the tops of the highest
mountains.
We see therefore that those dark and unknown central portions of
the earth, which are placed far beyond the reach of the miner and
the geologist, and of which man will probably never know anything
directly, are not to be considered as quite disconnected with us,
as deposits of useless lumber without effect or purpose. We feel
their influence on every step we take and on every breath we draw;
and the powers we possess, and the comforts we enjoy would be
unprofitable to us, if they had not been prepared with a reference
to those as well as to the near and visible portions of the earth’s
mass.
The arbitrary quantity, therefore, of which we have been treating,
the intensity of the force of gravity, appears to have been taken
account of, in establishing the laws of those forces by which
the processes of vegetable and animal life are carried on. And
this leads us inevitably, we conceive, to the belief of a supreme
contriving mind, by which these laws were thus devised and thus
established.