To a poetical mind there is no exercise more agreeable than that of
tracing in the economy of Nature certain trains of causes and effects
that seem to represent her as a kind benefactor, aiming to promote the
happiness of all creatures. While we treat of the beauty of trees and of
their capacity to afford shelter, shade, and salubrity, it is pleasant,
while continuing our observations, to find no end to the advantages that
flow from them. We have studied them as the beautifiers of landscape, as
the sources of vitality and salubrity in the atmosphere, as our shade in
summer and our shelter in winter; as the cause of equability, both of
temperature and of moisture. We may also discover in them and their
branches an infinite number of lightning-rods, presenting millions of
points both for the discharge and the absorption of electricity. Trees
differ from other plants in this respect only by presenting their points
at a greater elevation, where they can act more immediately upon the
clouds.
Trees, especially in dense assemblages, may therefore, in frequent
instances, be the immediate occasion of showers, by conducting to the
earth the electric fluid of the clouds, and inducing that non-electric
state which precedes the discharge of rain. This seems to be effected by
electric disorganization. An _organized_ cloud is an aggregation of
vaporous particles, which are suspended in the atmosphere and held in a
state of _union without contact_. Being in a similarly electrified
condition, they are kept separate by that law of electricity which
causes two pith-balls, suspended by threads, when similarly electrified,
to repel each other at certain distances. All those clouds that show a
definite and organized arrangement, and resemble feathers or lace, are
charged with electricity. As they accumulate they lose their symmetrical
arrangement, but do not mix, until some object, charged with opposite
electricity, comes near them and draws from the mass its electric fluid,
when the vaporous particles, losing their mutual repulsion, immediately
coalesce and descend in rain.
To illustrate the action of trees in producing showers, we will suppose
a dense electric cloud to be passing over a dry plain containing only a
few trees. Not meeting with any conducting objects of appreciable force
on its journey, it remains suspended in the heavens until it reaches
either a large collection of water, or encounters a forest, over which,
as over a lake, there rests always, in calm weather, a stratum of
invisible moisture, which is a powerful conducting agent. The trees,
with their numerous vegetable points, and the vapor that overspreads
them, combine their force in drawing down the electric fluid from the
cloud passing over, causing the whole mass to descend in showers. The
damp stratum of air which, in still weather, rests upon the surface of
every large sheet of water, being a powerful conductor, serves to
explain a phenomenon often observed in a dry season near the coast. A
dense electric cloud is seen to pass over our heads, without shedding a
drop of rain, until it reaches the ocean, when the humid air above the
waves, acting as a conductor, causes the cloud to part with its electric
fluid and to fall in copious showers at the same moment.
Occasionally a similar cloud, after rising in the west about thirty
degrees, will be turned from its direct course, and repelled by the dry,
heated atmosphere resting on the plain, and, attracted by the invisible
cloud of moisture that hovers over the river valley, is seen to take the
course of the river in its journey toward the sea. Hence it is notorious
that in a very dry time the rivers obtain more showers than the plains,
and the wooded mountainous regions more than the open and level country.
And we may regard it as a happy accident in the economy of nature, that
trees should be the most serviceable in nearly all other respects,
hardly less than as electric agents, upon those situations which are of
the least value for the purposes of agriculture. Their branches on lofty
ridges and elevations, extending near the level of the lower clouds, are
like so many lightning-rods on the buildings of an elevated city, and
exert a powerful influence in conducting the electric fluid from an
overcharged atmospheric stratum, and preventing, in some degree, those
accumulations that produce thunder-storms. Nature employs this grand
vegetable apparatus as one of the means of preserving that equilibrium,
both of moisture and electricity, which cannot be greatly disturbed
without dangerous commotions.
I have said nothing of trees as a protection from lightning; but there
are many curious facts and superstitions on record in relation to this
point. “When a thunder-storm threatened,” as Suetonius relates,
“Tiberius never failed to wear a crown of laurel-leaves, impressed with
the belief that lightning never touched the leaves of this tree.” The
general opinion that certain trees are exempt from the stroke of
lightning is very ancient. It probably originated in some religious
ideas of their sanctity, and men in more enlightened times have
endeavored to explain it by philosophy, instead of rejecting it as
fable. It was affirmed by Hugh Maxwell, an American writer, that
lightning often strikes the elm, the chestnut, the oak, the pine, and
less frequently the ash; but it always evades the beech, the birch, and
the maple. Captain Dibdin remarks, in a letter to Alexander Wilson, that
in the forests of Virginia the pines, though taller than the oaks, were
less frequently injured by lightning, and considers them pretty secure
when growing among oaks. These accounts by different writers are too
various and contradictory to be of much value in aiding us to discover
the truth. It is probable that the partial exemption of certain trees
from the stroke of lightning, if any such accounts be true, depends on
their size and shape. A tall tree in an assemblage would be more exposed
than the others. It may also be supposed that if a tree has a regular
ramification, smooth and straight branches and trunk, it is better
formed for a conductor, and that it would be more liable to receive a
charge of the fluid. But all these opinions are probably of the same
character with those respecting the antipathy of serpents for certain
trees,—traditionary notions which are hardly worthy of investigation.
The opinion of the ancients concerning the immunity of the laurel was
probably derived from their idea of its sanctity as the tree which was
dedicated to Apollo. At the present day there exists in Italy a similar
notion concerning the white grapevine. Some of the peasantry of that
country are accustomed to twining its branches around the head and waist
as a protection from a thunder-stroke.
Trees are generally believed to protect a house adjoining them from
lightning; on the contrary, it is known that men and animals seeking
refuge under a tree in an open plain are in greater danger than outside
of it. The lightning is therefore probably conducted by the water
passing down on the surface of the branches and trunk; for if the tree
itself were the conductor, the lightning would pass through the trunk
into the ground, and, like a lightning-rod, act as a protection to
objects near, but not in contact with it. Dr. Franklin thought the
safest place a few yards distant from a tree, and a little outside of
its widest spread. It is unsafe to stand under the drip of a tree, which
might convey to the person an electric charge. It was the opinion of M.
Arago, that trees overtopping houses at small distances cannot be
regarded as affording sure protection, like a properly adjusted
lightning-rod; but he admitted that when a storm passes over a forest it
is decidedly enfeebled. The forest certainly diminishes the power of a
thunderbolt. The security derived from trees attaches principally to
large assemblages. Though a house may receive but little protection from
a few tall trees standing near it, it is not to be denied that a village
or hamlet is rendered more secure by adjoining woods.