suggested by my father, and is shown in Fig. 5. It consists of alternate
and opposite sectors of dioptric spherical mirror, MM, and of Fresnel's
fixed light apparatus, AA. By the revolution of this composite frame
about the burner, the same immediate alternation of light and darkness
is produced, the first when the front of the fixed panel, and the
second when the back of the mirror, is presented to the eye of the
sailor.
One advantage of the method that I propose is this, that while we are
able to produce a plain intermittent light; an intermittent light of
variable period, ranging from a brief flash to a steady illumination of
half the revolution; and finally, a light combining the immediate
occultation of the intermittent with combination and change of colour,
we can yet preserve comparative lightness in the revolving parts, and
consequent economy in the driving machinery. It must, however, be
noticed, that none of these last methods are applicable to cases where
more than one radiant is employed: for these cases, either my
grandfather's or Mr. Wilson's contrivance must be resorted to.
[Illustration: Fig. 5.]
FOOTNOTE:
[44] Read before the Royal Scottish Society of Arts on 27th March
1871, and awarded the Society's Silver Medal.
ON THE THERMAL INFLUENCE OF FORESTS[45]
The opportunity of an experiment on a comparatively large scale, and
under conditions of comparative isolation, can occur but rarely in such
a science as Meteorology. Hence Mr. Milne Home's proposal for the
plantation of Malta seemed to offer an exceptional opportunity for
progress. Many of the conditions are favourable to the simplicity of the
result; and it seemed natural that, if a searching and systematic series
of observations were to be immediately set afoot, and continued during
the course of the plantation and the growth of the wood, some light
would be thrown on the still doubtful question of the climatic influence
of forests.
Mr. Milne Home expects, as I gather, a threefold result:--1st, an
increased and better regulated supply of available water; 2nd, an
increased rainfall; and, 3rd, a more equable climate, with more
temperate summer heat and winter cold.[46] As to the first of these
expectations, I suppose there can be no doubt that it is justified by
facts; but it may not be unnecessary to guard against any confusion of
the first with the second. Not only does the presence of growing timber
increase and regulate the supply of running and spring water
independently of any change in the amount of rainfall, but as
Boussingault found at Marmato,[47] denudation of forest is sufficient to
decrease that supply, even when the rainfall has increased instead of
diminished in amount. The second and third effects stand apart,
therefore, from any question as to the utility of Mr. Milne Home's
important proposal; they are both, perhaps, worthy of discussion at the
present time, but I wish to confine myself in the present paper to the
examination of the third alone.
A wood, then, may be regarded either as a _superficies_ or as a _solid_;
that is, either as a part of the earth's surface slightly elevated above
the rest, or as a diffused and heterogeneous body displacing a certain
portion of free and mobile atmosphere. It is primarily in the first
character that it attracts our attention, as a radiating and absorbing
surface, exposed to the sun and the currents of the air; such that, if
we imagine a plateau of meadow-land or bare earth raised to the mean
level of the forest's exposed leaf-surface, we shall have an agent
entirely similar in kind, although perhaps widely differing in the
amount of action. Now, by comparing a tract of wood with such a plateau
as we have just supposed, we shall arrive at a clear idea of the
specialities of the former. In the first place, then, the mass of
foliage may be expected to increase the radiating power of each tree.
The upper leaves radiate freely towards the stars and the cold
inter-stellar spaces, while the lower ones radiate to those above and
receive less heat in return; consequently, during the absence of the
sun, each tree cools gradually downward from top to bottom. Hence we
must take into account not merely the area of leaf-surface actually
exposed to the sky, but, to a greater or less extent, the surface of
every leaf in the whole tree or the whole wood. This is evidently a
point in which the action of the forest may be expected to differ from
that of the meadow or naked earth; for though, of course, inferior
strata tend to a certain extent to follow somewhat the same course as
the mass of inferior leaves, they do so to a less degree--conduction,
and the conduction of a very slow conductor, being substituted for
radiation.
We come next, however, to a second point of difference. In the case of
the meadow, the chilled air continues to lie upon the surface, the
grass, as Humboldt says, remaining all night submerged in the stratum of
lowest temperature; while in the case of trees, the coldest air is
continually passing down to the space underneath the boughs, or what we
may perhaps term the crypt of the forest. Here it is that the
consideration of any piece of woodland conceived as a solid comes
naturally in; for this solid contains a portion of the atmosphere,
partially cut off from the rest, more or less excluded from the
influence of wind, and lying upon a soil that is screened all day from
isolation by the impending mass of foliage. In this way (and chiefly, I
think, from the exclusion of winds), we have underneath the radiating
leaf-surface a stratum of comparatively stagnant air, protected from
many sudden variations of temperature, and tending only slowly to bring
itself into equilibrium with the more general changes that take place in
the free atmosphere.
Over and above what has been mentioned, thermal effects have been
attributed to the vital activity of the leaves in the transudation of
water, and even to the respiration and circulation of living wood. The
whole actual amount of thermal influence, however, is so small that I
may rest satisfied with mere mention. If these actions have any effect
at all, it must be practically insensible; and the others that I have
already stated are not only sufficient validly to account for all the
observed differences, but would lead naturally to the expectation of
differences very much larger and better marked. To these observations I
proceed at once. Experience has been acquired upon the following three
points:--1, The relation between the temperature of the trunk of a tree
and the temperature of the surrounding atmosphere; 2, The relation
between the temperature of the air under a wood and the temperature of
the air outside; and, 3, The relation between the temperature of the air
above a wood and the temperature of the air above cleared land.
As to the first question, there are several independent series of
observations; and I may remark in passing, what applies to all, that
allowance must be made throughout for some factor of specific heat. The
results were as follows:--The seasonal and monthly means in the tree and
in the air were not sensibly different. The variations in the tree, in
M. Becquerel's own observations, appear as considerably less than a
fourth of those in the atmosphere, and he has calculated, from
observations made at Geneva between 1796 and 1798, that the variations
in the tree were less than a fifth of those in the air; but the tree in
this case, besides being of a different species, was seven or eight
inches thicker than the one experimented on by himself.[48] The
variations in the tree, therefore, are always less than those in the
air, the ratio between the two depending apparently on the thickness of
the tree in question and the rapidity with which the variations followed
upon one another. The times of the maxima, moreover, were widely
different: in the air, the maximum occurs at 2 P.M. in winter, and at 3
P.M. in summer; in the tree, it occurs in winter at 6 P.M., and in
summer between 10 and 11 P.M. At nine in the morning in the month of
June, the temperatures of the tree and of the air had come to an
equilibrium. A similar difference of progression is visible in the
means, which differ most in spring and autumn, and tend to equalise
themselves in winter and in summer. But it appears most strikingly in
the case of variations somewhat longer in period than the daily ranges.
The following temperatures occurred during M. Becquerel's observations
in the Jardin des Plantes:--
Date. Temperature of Temperature in
the Air. the Tree.
1859. Dec. 15, 26.78 deg. 32 deg.
" 16, 19.76 deg. 32 deg.
" 17, 17.78 deg. 31.46 deg.
" 18, 13.28 deg. 30.56 deg.
" 19, 12.02 deg. 28.40 deg.
" 20, 12.54 deg. 25.34 deg.
" 21, 38.30 deg. 27.86 deg.
" 22, 43.34 deg. 30.92 deg.
" 23, 44.06 deg. 31.46 deg.
A moment's comparison of the two columns will make the principle
apparent. The temperature of the air falls nearly fifteen degrees in
five days; the temperature of the tree, sluggishly following, falls in
the same time less than four degrees. Between the 19th and the 20th the
temperature of the air has changed its direction of motion, and risen
nearly a degree; but the temperature of the tree persists in its former
course, and continues to fall nearly three degrees farther. On the 21st
there comes a sudden increase of heat, a sudden thaw; the temperature of
the air rises twenty-five and a half degrees; the change at last reaches
the tree, but only raises its temperature by less than three degrees;
and even two days afterwards, when the air is already twelve degrees
above freezing point, the tree is still half a degree below it. Take,
again, the following case:--
Date Temperature of Temperature in
the Air. the Tree.
1859. July 13, 84.92 deg. 76.28 deg.
" 14, 82.58 deg. 78.62 deg.
" 15, 80.42 deg. 77.72 deg.
" 16, 79.88 deg. 78.44 deg.
" 17, 73.22 deg. 75.92 deg.
" 18, 68.54 deg. 74.30 deg.
" 19, 65.66 deg. 70.70 deg.
The same order reappears. From the 13th to the 19th the temperature of
the air steadily falls, while the temperature of the tree continues
apparently to follow the course of previous variations, and does not
really begin to fall, is not really affected by the ebb of heat, until
the 17th, three days at least after it had been operating in the
air.[49] Hence we may conclude that all variations of the temperature
of the air, whatever be their period, from twenty-four hours up to
twelve months, are followed in the same manner by variations in the
temperature of the tree; and that those in the tree are always less in
amount and considerably slower of occurrence than those in the air. This
_thermal sluggishness_, so to speak, seems capable of explaining all the
phenomena of the case without any hypothetical vital power of resisting
temperatures below the freezing point, such as is hinted at even by
Becquerel.
Reaumur, indeed, is said to have observed temperatures in slender trees
nearly thirty degrees higher than the temperature of the air in the sun;
but we are not informed as to the conditions under which this
observation was made, and it is therefore impossible to assign to it its
proper value. The sap of the ice-plant is said to be materially colder
than the surrounding atmosphere; and there are several other somewhat
incongruous facts, which tend, at first sight, to favour the view of
some inherent power of resistance in some plants to high temperatures,
and in others to low temperatures.[50] But such a supposition seems in
the meantime to be gratuitous. Keeping in view the thermal
redispositions, which must be greatly favoured by the ascent of the sap,
and the difference between the condition as to temperature of such parts
as the root, the heart of the trunk, and the extreme foliage, and never
forgetting the unknown factor of specific heat, we may still regard it
as possible to account for all anomalies without the aid of any such
hypothesis. We may, therefore, I think, disregard small exceptions, and
state the result as follows:--
If, after every rise or fall, the temperature of the air remained
stationary for a length of time proportional to the amount of the
change, it seems probable--setting aside all question of vital
heat--that the temperature of the tree would always finally equalise
itself with the new temperature of the air, and that the range in tree
and atmosphere would thus become the same. This pause, however, does not
occur: the variations follow each other without interval; and the
slow-conducting wood is never allowed enough time to overtake the rapid
changes of the more sensitive air. Hence, so far as we can see at
present, trees appear to be simply bad conductors, and to have no more
influence upon the temperature of their surroundings than is fully
accounted for by the consequent tardiness of their thermal variations.
Observations bearing on the second of the three points have been made by
Becquerel in France, by La Cour in Jutland and Iceland, and by Rivoli at
Posen. The results are perfectly congruous. Becquerel's observations[51]
were made under wood, and about a hundred yards outside in open ground,
at three stations in the district of Montargis, Loiret. There was a
difference of more than one degree Fahrenheit between the mean annual
temperatures in favour of the open ground. The mean summer temperature
in the wood was from two to three degrees lower than the mean summer
temperature outside. The mean maxima in the wood were also lower than
those without by a little more than two degrees. Herr La Cour[52] found
the daily range consistently smaller inside the wood than outside. As
far as regards the mean winter temperatures, there is an excess in
favour of the forest, but so trifling in amount as to be unworthy of
much consideration. Libri found that the minimum winter temperatures
were not sensibly lower at Florence, after the Apennines had been
denuded of forest, than they had been before.[53] The disheartening
contradictoriness of his observations on this subject led Herr Rivoli to
the following ingenious and satisfactory comparison.[54] Arranging his
results according to the wind that blew on the day of observation, he
set against each other the variation of the temperature under wood from
that without, and the variation of the temperature of the wind from the
local mean for the month:--
+-------------------------------------------------------------+
| Wind. | N. | N.E.| E. | S.E.| S. | S.W.| W. | N.W.|
| |-----+-----+-----+-----+-----+-----+-----+-----|
|Var. in Wood |+0.60|+0.26|+0.26|+0.04|-0.04|-0.20|+0.16|+0.07|
|Var. in Wind |-0.30|-2.60|-3.30|-1.20|+1.00|+1.30|+1.00|+1.00|
+-------------------------------------------------------------+
From this curious comparison, it becomes apparent that the variations of
the difference in question depend upon the amount of variations of
temperature which take place in the free air, and on the slowness with
which such changes are communicated to the stagnant atmosphere of woods;
in other words, as Herr Rivoli boldly formulates it, a forest is simply
a bad conductor. But this is precisely the same conclusion as we have
already arrived at with regard to individual trees; and in Herr Rivoli's
table, what we see is just another case of what we saw in M.
Becquerel's--the different progression of temperatures. It must be
obvious, however, that the thermal condition of a single tree must be
different in many ways from that of a combination of trees and more or
less stagnant air, such as we call a forest. And accordingly we find, in
the case of the latter, the following new feature: The mean yearly
temperature of woods is lower than the mean yearly temperature of free
air, while they are decidedly colder in summer, and very little, if at
all, warmer in winter. Hence, on the whole, forests are colder than
cleared lands. But this is just what might have been expected from the
amount of evaporation, the continued descent of cold air, and its
stagnation in the close and sunless crypt of a forest; and one can only
wonder here, as elsewhere, that the resultant difference is so
insignificant and doubtful.
We come now to the third point in question, the thermal influence of
woods upon the air above them. It will be remembered that we have seen
reason to believe their effect to be similar to that of certain other
surfaces, except in so far as it may be altered, in the case of the
forest, by the greater extent of effective radiating area, and by the
possibility of generating a descending cold current as well as an
ascending hot one. M. Becquerel is (so far as I can learn) the only
observer who has taken up the elucidation of this subject. He placed his
thermometers at three points:[55] A and B were both about seventy feet
above the surface of the ground; but A was at the summit of a chestnut
tree, while B was in the free air, fifty feet away from the other. C was
four or five feet above the ground, with a northern exposure; there was
also a fourth station to the south, at the same level as this last, but
its readings are very seldom referred to. After several years of
observation, the mean temperature at A was found to be between one and
two degrees higher than that at B. The order of progression of
differences is as instructive here as in the two former investigations.
The maximum difference in favour of station A occurred between three and
five in the afternoon, later or sooner according as there had been more
or less sunshine, and ranged sometimes as high as seven degrees. After
this the difference kept declining until sunrise, when there was often a
difference of a degree, or a degree and a half, upon the other side. On
cloudy days the difference tended to a minimum. During a rainy month of
April, for example, the difference in favour of station A was less than
half a degree; the first fifteen days of May following, however, were
sunny, and the difference rose to more than a degree and a half.[56] It
will be observed that I have omitted up to the present point all mention
of station C. I do so because M. Becquerel's language leaves it doubtful
whether the observations made at this station are logically comparable
with those made at the other two. If the end in view were to compare
the progression of temperatures above the earth, above a tree, and in
free air, removed from all such radiative and absorptive influences, it
is plain that all three should have been equally exposed to the sun or
kept equally in shadow. As the observations were made, they give us no
notion of the relative action of earth-surface and forest-surface upon
the temperature of the contiguous atmosphere; and this, as it seems to
me, was just the _crux_ of the problem. So far, however, as they go,
they seem to justify the view that all these actions are the same in
kind, however they may differ in degree. We find the forest heating the
air during the day, and heating it more or less according as there has
been more or less sunshine for it to absorb, and we find it also
chilling it during the night; both of which are actions common to any
radiating surface, and would be produced, if with differences of amount
and time, by any other such surface raised to the mean level of the
exposed foliage.
To recapitulate:
1st. We find that single trees appear to act simply as bad conductors.
2nd. We find that woods, regarded as solids, are, on the whole, slightly
lower in temperature than the free air which they have displaced, and
that they tend slowly to adapt themselves to the various thermal changes
that take place without them.
3rd. We find forests regarded as surfaces acting like any other part of
the earth's surface, probably with more or less difference in amount and
progression, which we still lack the information necessary to estimate.
All this done, I am afraid that there can be little doubt that the more
general climatic investigations will be long and vexatious. Even in
South America, with extremely favourable conditions, the result is far
from being definite. Glancing over the table published by M. Becquerel
in his book on climates, from the observations of Humboldt, Hall,
Boussingault, and others, it becomes evident, I think, that nothing can
be founded upon the comparisons therein instituted; that all reasoning,
in the present state of our information, is premature and unreliable.
Strong statements have certainly been made; and particular cases lend
themselves to the formation of hasty judgments. "From the Bay of Cupica
to the Gulf of Guayaquil," says M. Boussingault, "the country is covered
with immense forest and traversed by numerous rivers; it rains there
almost ceaselessly; and the mean temperature of this moist district
scarcely reaches 78.8 deg. F.... At Payta commence the sandy deserts of
Priura and Sechura; to the constant humidity of Choco succeeds almost at
once an extreme of dryness; and the mean temperature of the coast
increases at the same time by 1.8 deg. F."[57] Even in this selected
favourable instance it might be argued that the part performed in the
change by the presence or absence of forest was comparatively small;
there seems to have been, at the same time, an entire change of soil;
and, in our present ignorance, it would be difficult to say by how much
this of itself is able to affect the climate. Moreover, it is possible
that the humidity of the one district is due to other causes besides the
presence of wood, or even that the presence of wood is itself only an
effect of some more general difference or combination of differences. Be
that as it may, however, we have only to look a little longer at the
table before referred to, to see how little weight can be laid on such
special instances. Let us take five stations, all in this very district
of Choco. Hacquita is eight hundred and twenty feet above Novita, and
their mean temperatures are the same. Alto de Mombu, again, is five
hundred feet higher than Hacquita, and the mean temperature has here
fallen nearly two degrees. Go up another five hundred feet to Tambo de
la Orquita, and again we find no fall in the mean temperature. Go up
some five hundred further to Chami, and there is a fall in the mean
temperature of nearly six degrees. Such numbers are evidently quite
untrustworthy; and hence we may judge how much confidence can be placed
in any generalisation from these South American mean temperatures.
The question is probably considered too simply--too much to the neglect
of concurrent influences. Until we know, for example, somewhat more of
the comparative radiant powers of different soils, we cannot expect any
very definite result. A change of temperature would certainly be
effected by the plantation of such a marshy district as the Sologne,
because, if nothing else were done, the roots might pierce the
impenetrable subsoil, allow the surface-water to drain itself off, and
thus dry the country. But might not the change be quite different if the
soil planted were a shifting sand, which, _fixed_ by the roots of the
trees, would become gradually covered with a vegetable earth, and be
thus changed from dry to wet? Again, the complication and conflict of
effects arises, not only from the soil, vegetation, and geographical
position of the place of the experiment itself, but from the
distribution of similar or different conditions in its immediate
neighbourhood, and probably to great distances on every side. A forest,
for example, as we know from Herr Rivoli's comparison, would exercise a
perfectly different influence in a cold country subject to warm winds,
and in a warm country subject to cold winds; so that our question might
meet with different solutions even on the east and west coasts of Great
Britain.