A SUGGESTION AS TO THE 'CANALS' OF MARS.
The special characteristics of the numerous lines which intersect the
whole of the equatorial and temperate regions of Mars are, their
straightness combined with their enormous length. It is this which has
led Mr. Lowell to term them 'non-natural features.' Schiaparelli, in his
earlier drawings, showed them curved and of comparatively great width.
Later, he found them to be straight fine lines when seen under the best
conditions, just as Mr. Lowell has always seen them in the pure
atmosphere of his observatory. Both of these observers were at first
doubtful of their reality, but persistent observation continued at many
successive oppositions compelled acceptance of them as actual features
of the planet's disc. So many other observers have now seen them that
the objection of unreality seems no longer valid.
Mr. Lowell urges, however, that their perfect straightness, their
extreme tenuity, their uniformity throughout their whole length, the
dual character of many of them, their relation to the 'oases' and the
form and position of these round black spots, are all proofs of
artificiality and are suggestive of design. And considering that some of
them are actually as long as from Boston to San Francisco, and
relatively to their globe as long as from London to Bombay, his
objection that "no natural phenomena within our knowledge show such
regularity on such a scale" seems, at first, a mighty one.
It is certainly true that we can point to nothing exactly like them
either on the earth or on the moon, and these are the only two planetary
bodies we are in a position to compare with Mars. Yet even these do, I
think, afford us some hints towards an interpretation of the mysterious
lines. But as our knowledge of the internal structure and past history
even of our earth is still imperfect, that of the moon only conjectural,
and that of Mars a perfect blank, it is not perhaps surprising that the
surface-features of the latter do not correspond with those of either of
the others.
_Mr. Pickering's Suggestion._
The best clue to a natural interpretation of the strange features of the
surface of Mars is that suggested by the American astronomer Mr. W.H.
Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the
'canals' of Mars and the rifts as well as the luminous streaks on the
moon are cracks in the volcanic crust, caused by internal stresses due
to the action of the heated interior. These cracks he considers to be
symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's
'oases') because they have originated from them, just as the white
streaks on the moon radiate from the larger craters as centres. He
further supposes that water and carbon-dioxide issue from the interior
into these fissures, and, in conjunction with sunlight, promote the
growth of vegetation. Owing to the very rare atmosphere, the vapours, he
thinks, would not ascend but would roll down the outsides of the
craterlets and along the borders of the canals, thus irrigating the
immediate vicinity and serving to promote the growth of some form of
vegetation which renders the canals and oases visible.[13]
[Footnote 13: _Nature_, vol. 70, p. 536.]
This opinion is especially important because, next to Mr. Lowell, Mr.
Pickering is perhaps the astronomer who has given most attention to Mars
during the last fifteen years. He was for some time at Flagstaff with
Mr. Lowell, and it was he who discovered the oases or craterlets, and
who originated the idea that we did not see the 'canals' themselves but
only the vegetable growth on their borders. He also observed Mars in the
Southern Hemisphere at Arequipa; and he has since made an elaborate
study of the moon by means of a specially constructed telescope of 135
feet focal length, which produced a direct image on photographic plates
nearly 16 inches in diameter.[14]
[Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.]
It is clear therefore that Mr. Lowell's views as to the artificial
nature of the 'canals' of Mars are not accepted by an astronomer of
equal knowledge and still wider experience. Yet Professor Pickering's
alternative view is more a suggestion than an explanation, because there
is no attempt to account for the enormous length and perfect
straightness of the lines on Mars, so different from anything that is
found either on our earth or on the moon. There must evidently be some
great peculiarity of structure or of conditions on Mars to account for
these features, and I shall now attempt to point out what this
peculiarity is and how it may have arisen.
_The Meteoritic Hypothesis._
During the last quarter of a century a considerable change has come over
the opinions of astronomers as regards the probable origin of the Solar
System. The large amount of knowledge of the stellar universe, and
especially of nebulae, of comets and of meteor-streams which we now
possess, together with many other phenomena, such as the constitution of
Saturn's rings, the great number and extent of the minor planets, and
generally of the vast amount of matter in the form of meteor-rings and
meteoric dust in and around our system, have all pointed to a different
origin for the planets and their satellites than that formulated by
Laplace as the Nebular hypothesis.
It is now seen more clearly than at any earlier period, that most of the
planets possess special characteristics which distinguish them from one
another, and that such an origin as Laplace suggested--the slow cooling
and contraction of one vast sun-mist or nebula, besides presenting
inherent difficulties--many think them impossibilities--in itself does
not afford an adequate explanation of these peculiarities. Hence has
arisen what is termed the Meteoritic theory, which has been ably
advocated for many years by Sir Norman Lockyer, and with some
unimportant modifications is now becoming widely accepted. Briefly, this
theory is, that the planets have been formed by the slow aggregation of
solid particles around centres of greatest condensation; but as many of
my readers may be altogether unacquainted with it, I will here give a
very clear statement of what it is, from Professor J.W. Gregory's
presidential address to the Geological Section of the British
Association of the present year. He began by saying that these modern
views were of far more practical use to men of science than that of
Laplace, and that they give us a history of the world consistent with
the actual records of geology. He then continues:
"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae,
comets, and many so-called stars consist of swarms of meteorites which,
though normally cold and dark, are heated by repeated collisions, and so
become luminous. They may even be volatilised into glowing meteoric
vapour; but in time this heat is dissipated, and the force of gravity
condenses a meteoritic swarm into a single globe. 'Some of the swarms
are,' says Lockyer, 'truly members of the solar system,' and some of
these travel round the sun in nearly circular orbits, like planets. They
may be regarded as infinitesimal planets, and so Chamberlain calls them
'planetismals.'
"The planetismal theory is a development of the meteoritic theory, and
presents it in an especially attractive guise. It regards meteorites as
very sparsely distributed through space, and gravity as powerless to
collect them into dense groups. So it assigns the parentage of the solar
system to a spiral nebula composed of planetismals, and the planets as
formed from knots in the nebula, where many planetismals had been
concentrated near the intersections of their orbits. These groups of
meteorites, already as dense as a swarm of bees, were then packed closer
by the influence of gravity, and the contracting mass was heated by the
pressure, even above the normal melting-point of the material, which was
kept rigid by the weight of the overlying layers."
Now, adopting this theory as the last word of science upon the subject
of the origin of planets, we see that it affords immense scope for
diversity in results depending on the total _amount_ of matter available
within the range of attraction of an incipient planetary mass, and the
_rates_ at which this matter becomes available. By a special combination
of these two quantities (which have almost certainly been different for
each planet) I think we may be able to throw some light upon the
structure and physical features of Mars.
_The Probable Mode of Origin of Mars._
This planet, lying between two of much greater mass, has evidently had
less material from which to be formed by aggregation; and if we
assume--as in the absence of evidence to the contrary we have a right to
do--that its beginnings were not much later (or earlier) than those of
the earth, then its smaller size shows that it has in all probability
aggregated very much more slowly. But the internal heat acquired by a
planet while forming in this manner will depend upon the rate at which
it aggregates and the velocity with which the planetismals' fall into
it, and this velocity will increase with its mass and consequent force
of gravity. In the early stages of a planet's growth it will probably
remain cold, the small amount of heat produced by each impact being lost
by radiation before the next one occurs; and with a small and slowly
aggregating planet this condition will prevail till it approaches its
full size. Then only will its gravitative force be sufficient to cause
incoming matter to fall upon it with so powerful an impact as to produce
intense heat. Further, the compressive force of a small planet will be a
less effective heat-producing agency than in the case of a larger one.
The earth we know has acquired a large amount of internal heat, probably
sufficient to liquefy its whole interior; but Mars has only one-ninth
part the mass of the earth, and it is quite possible, and even probable,
that its comparatively small attractive force would never have liquefied
or even permanently heated the more central portions of its mass. This
being admitted, I suggest the following course of events as quite
possible, and not even improbable, in the case of this planet. During
the whole of its early growth, and till it acquired nearly its present
diameter, its rate of aggregation was so slow that the planetismals
falling upon it, though they might have been heated and even partially
liquefied by the impact, were never in such quantity as to produce any
considerable heating effect on the whole mass, and each local rise of
temperature was soon lost by radiation. The planet thus grew as a solid
and cold mass, compacted together by the impact of the incoming matter
as well as by its slowly increasing gravitative force. But when it had
attained to within perhaps 100, perhaps 50 miles, or less, of its
present diameter, a great change occurred in the opportunity for further
growth. Some large and dense swarm of meteorites, perhaps containing a
number of bodies of the size of the asteroids, came within the range of
the sun's attraction and were drawn by it into an orbit which crossed
that of Mars at such a small angle that the planet was able at each
revolution to capture a considerable number of them. The result might
then be that, as in the case of the earth, the continuous inpour of the
fresh matter first heated, and later on liquefied the greater part of it
as well perhaps as a thin layer of the planet's original surface; so
that when in due course the whole of the meteor-swarm had been captured,
Mars had acquired its present mass, but would consist of an intensely
heated, and either liquid or plastic thin outer shell resting upon a
cold and solid interior.
The size and position of the two recently discovered satellites of Mars,
which are believed to be not more than ten miles in diameter, the more
remote revolving around its primary very little slower than the planet
rotates, while the nearer one, which is considerably less distant from
the planet's surface than its own antipodes and revolves around it more
than three times during the Martian day, may perhaps be looked upon as
the remnants of the great meteor-swarm which completed the Martian
development, and which are perhaps themselves destined at some distant
period to fall into the planet. Should future astronomers witness the
phenomenon the effect produced upon its surface would be full of
instruction.
As the result of such an origin as that suggested, Mars would possess a
structure which, in the essential feature of heat-distribution, would be
the very opposite of that which is believed to characterise the earth,
yet it might have been produced by a very slight modification of the
same process. This peculiar heat-distribution, together with a much
smaller mass and gravitative force, would lead to a very different
development of the surface and an altogether diverse geological history
from ours, which has throughout been profoundly influenced by its heated
interior, its vast supply of water, and the continuous physical and
chemical reactions between the interior and the crust.
These reactions have, in our case, been of substantially the same
nature, and very nearly of the same degree of intensity throughout the
whole vast eons of geological time, and they have resulted in a
wonderfully complex succession of rock-formations--volcanic, plutonic,
and sedimentary--more or less intermingled throughout the whole series,
here remaining horizontal as when first deposited, there upheaved or
depressed, fractured or crushed, inclined or contorted; denuded by rain
and rivers with the assistance of heat and cold, of frost and ice, in an
unceasing series of changes, so that however varied the surface may be,
with hill and dale, plains and uplands, mountain ranges and deep
intervening valleys, these are as nothing to the diversities of interior
structure, as exhibited in the sides of every alpine valley or
precipitous escarpment, and made known to us by the work of the miner
and the well-borer in every part of the world.
_Structural Straight Lines on the Earth._
The great characteristic of the earth, both on its surface and in its
interior, is thus seen to be extreme diversity both of form and
structure, and this is further intensified by the varied texture,
constitution, hardness, and density of the various rocks and debris of
which it is composed. It is therefore not surprising that, with such a
complex outer crust, we should nowhere find examples of those
geometrical forms and almost world-wide straight lines that give such a
remarkable, and as Mr. Lowell maintains, 'non-natural' character to the
surface of Mars, but which, as it seems to me, of themselves afford
_prima facie_ evidence of a corresponding simplicity and uniformity in
its internal structure.
Yet we are not ourselves by any means devoid of 'straight lines'
structurally produced, in spite of every obstacle of diversity of form
and texture, of softness and hardness, of lamination or crystallisation,
which are adverse to such developments. Examples of these are the
numerous 'faults' which occur in the harder rocks, and which often
extend for great distances in almost perfect straight lines. In our own
country we have the Tyneside and Craven faults in the North of England,
which are 30 miles long and often 20 yards wide; but even more striking
is the great Cleveland Dyke--a wall of volcanic rock dipping slightly
towards the south, but sometimes being almost vertical, and stretching
across the country, over hill and dale, in an almost perfect straight
line from a point on the coast ten miles north of Scarborough, in a
west-by-north direction, passing about two miles south of Stockton and
terminating about six miles north-by-east of Barnard Castle, a distance
of very nearly 60 miles. The great fault between the Highlands and
Lowlands of Scotland extends across the country from Stonehaven to near
Helensburgh, a distance of 120 miles; and there are very many more of
less importance.
Much more extensive are some of the great continental dislocations,
often forming valleys of considerable width and length. The Upper Rhine
flows in one of these great valleys of subsidence for about 180 miles,
from Mulhausen to Frankfort, in a generally straight line, though
modified by denudation. Vaster still is the valley of the Jordan through
the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the
Gulf of Akaba, believed to form one vast geological depression or
fracture extending in a straight line for 400 miles.
Thousands of such faults, dykes, or depressions exist in every part of
the world, all believed to be due to the gradual shrinking of the heated
interior to which the solid crust has to accommodate itself, and they
are especially interesting and instructive for our present purpose as
showing the tendency of such fractures of solid rock-material to extend
to great lengths in straight lines, notwithstanding the extreme
irregularity both in the surface contour as well as in the internal
structures of the varied deposits and formations through which they
pass.
_Probable Origin of the Surface-features of Mars._
Returning now to Mars, let us consider the probable course of events
from the point at which we left it. The heat produced by impact and
condensation would be likely to release gases which had been in
combination with some of the solid matter, or perhaps been itself in a
solid state due to intense cold, and these, escaping outwards to the
surface, would produce on a small scale a certain amount of upheaval and
volcanic disturbance; and as an outer crust rapidly formed, a number of
vents might remain as craters or craterlets in a moderate state of
activity. Owing to the comparatively small force of gravity, the outer
crust would become scoriaceous and more or less permeated by the gases,
which would continue to escape through it, and this would facilitate the
cooling of the whole of the heated outer crust, and allow it to become
rather densely compacted. When the greater portion of the gases had thus
escaped to the outer surface and assisted to form a scanty atmosphere,
such as now exists, there would be no more internal disturbance and the
cooling of the heated outer coating would steadily progress, resulting
at last in a slightly heated, and later in a cold layer of moderate
thickness and great general uniformity. Owing to the absence of rain and
rivers, denudation such as we experience would be unknown, though the
superficial scoriaceous crust might be partially broken up by expansion
and contraction, and suffer a certain amount of atmospheric erosion.
The final result of this mode of aggregation would be, that the planet
would consist of an outer layer of moderate thickness as compared with
the central mass, which outer layer would have cooled from a highly
heated state to a temperature considerably below the freezing-point, and
this would have been all the time _contracting upon a previously cold,
and therefore non-contracting nucleus._ The result would be that very
early in the process great superficial tensions would be produced, which
could only be relieved by cracks or fissures, which would initiate at
points of weakness--probably at the craterlets already referred to--from
which they would radiate in several directions. Each crack thus formed
near the surface would, as cooling progressed, develop in length and
depth; and owing to the general uniformity of the material, and possibly
some amount of crystalline structure due to slow and continuous cooling
down to a very low temperature, the cracks would tend to run on in
straight lines and to extend vertically downwards, which two
circumstances would necessarily result in their forming portions of
'great circles' on the planet's surface--the two great facts which Mr.
Lowell appeals to as being especially 'non-natural.'
_Symmetry of Basaltic Columns._
We have however one quite natural fact on our earth which serves to
illustrate one of these two features, the direction of the downward
fissure. This is, the comparatively common phenomenon of basaltic
columns and 'Giant's Causeways.' The wonderful regularity of these, and
especially the not unfrequent upright pillars in serried ranks, as in
the palisades of the Hudson river, must have always impressed observers
with their appearance of artificiality. Yet they are undoubtedly the
result of the very slow cooling and contraction of melted rocks under
compression by strata _below and above them_, so that, when once
solidified, the mass was held in position and the tension produced by
contraction could only be relieved by numerous very small cracks at
short distances from each other in every direction, resulting in five,
six, or seven-sided polygons, with sides only a few inches long. This
contraction began of course at the coolest surface, generally the upper
one; and observation of these columns in various positions has
established the rule that their direction lengthways _is always at right
angles to the cooling surface_, and thus, whenever this surface was
horizontal, the columns became almost exactly vertical.
_How this applies to Mars._
One of the features of the surface of Mars that Mr. Lowell describes
with much confidence is, that it is wonderfully uniform and level, which
of course it would be if it had once been in a liquid or plastic state,
and not much disturbed since by volcanic or other internal movements.
The result would be that cracks formed by contraction of the hardened
outer crust would be vertical; and, in a generally uniform material at a
very uniform temperature, these cracks would continue almost
indefinitely in straight lines. The hardened and contracting surface
being free to move laterally on account of there being a more heated and
plastic layer below it, the cracks once initiated above would
continually widen at the surface as they penetrated deeper and deeper
into the slightly heated substratum. Now, as basalt begins to soften at
about 1400° F. and the surface of Mars has cooled to at least the
freezing-point--perhaps very much below it--the contraction would be so
great that if the fissures produced were 500 miles apart they might be
three miles wide at the surface, and, if only 100 miles apart, then
about two-thirds of a mile wide.[15] But as the production of the
fissures might have occupied perhaps millions of years, a considerable
amount of atmospheric denudation would result, however slowly it acted.
Expansion and contraction would wear away the edges and sides of the
fissures, fill up many of them with the debris, and widen them at the
surfaces to perhaps double their original size.[16]
[Footnote 15: The coefficient of contraction of basalt is 0.000006 for
1° F., which would lead to the results given here.]
[Footnote 16: Mr. W.H. Pickering observed clouds on Mars 15 miles high;
these are the 'projections' seen on the terminator when the planet is
partially illuminated. They were at first thought to be mountains; but
during the opposition of 1894, more than 400 of them were seen at
Flagstaff during nine months' observation. Usually they are of rare
occurrence. They are seen to change in form and position from day to
day, and Mr. Lowell is strongly of opinion that they are dust-storms,
not what we term clouds. They were mostly about 13 miles high,
indicating considerable aerial disturbance on the planet, and therefore
capable of producing proportional surface denudation.]
_Suggested Explanation of the 'Oases.'_
The numerous round dots seen upon the 'canals,' and especially at points
from which several canals radiate and where they intersect--termed
'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained
in two ways. Those from which several canals radiate may be true craters
from which the gases imprisoned in the heated surface layers have
gradually escaped. They would be situated at points of weakness in the
crust, and become centres from which cracks would start during
contraction. Those dots which occur at the crossing of two straight
canals or cracks may have originated from the fact that at such
intersections there would be four sharply-projecting angles, which,
being exposed to the influence of alternate heat and cold (during day
and night) on the two opposite surfaces, would inevitably in time become
fractured and crumbled away, resulting in the formation of a roughly
circular chasm which would become partly filled up by the debris. Those
formed by cracks radiating from craterlets would also be subject to the
same process of rounding off to an even greater extent; and thus would
be produced the 'oases' of various sizes up to 50 miles or more in
diameter recorded by Mr. Lowell and other observers.
_Probable Function of the Great Fissures._
Mr. Pickering, as we have seen, supposes that these fissures give out
the gases which, overflowing on each side, favour the growth of the
supposed vegetation which renders the course of the canals visible, and
this no doubt may have been the case during the remote periods when
these cracks gave access to the heated portions of the surface layer.
But it seems more probable that Mars has now cooled down to the almost
uniform mean temperature it derives from solar heat, and that the
fissures--now for the most part broad shallow valleys--serve merely as
channels along which the liquids and heavy gases derived from the
melting of the polar snows naturally flow, and, owing to their nearly
level surfaces, overflow to a certain distance on each side of them.
_Suggested Origin of the Blue Patches._
These heavy gases, mainly perhaps, as has been often suggested,
carbon-dioxide, would, when in large quantity and of considerable depth,
reflect a good deal of light, and, being almost inevitably dust-laden,
might produce that blue tinge adjacent to the melting snow-caps which
Mr. Lowell has erroneously assumed to be itself a proof of the presence
of liquid water. Just as the blue of our sky is undoubtedly due to
reflection from the ultra-minute dust particles in our higher
atmosphere, similar particles brought down by the 'snow' from the higher
Martian atmosphere might produce the blue tinge in the great volumes of
heavy gas produced by its evaporation or liquefaction.
It may be noted that Mr. Lowell objects to the carbon-dioxide theory of
the formation of the snow-caps, that this gas at low pressures does not
liquefy, but passes at once from the solid to the gaseous state, and
that only water remains liquid sufficiently long to produce the blue
colour' which plays so large a part in his argument for the mild climate
essential for an inhabited planet. But this argument, as I have already
shown, is valueless. For only very deep water can possibly show a blue
colour by reflected light, while a dust-laden atmosphere--especially
with a layer of very dense gas at the bottom of it, as would be the case
with the newly evaporated carbon-dioxide from the diminishing snow-cap
--would provide the very conditions likely to produce this blue tinge of
colour.
It may be considered a support to this view that carbonic-acid gas
becomes liquid at--140° F. and solid at--162° F., temperatures far
higher than we should expect to prevail in the polar and north temperate
regions of Mars during a considerable part of the year, but such as
might be reached there during the summer solstice when the `snows' so
rapidly disappear, to be re-formed a few months later.
_The Double Canals._
The curious phenomena of the 'double canals' are undoubtedly the most
difficult to explain satisfactorily on any theory that has yet been
suggested. They vary in distance apart from about 100 to 400 miles. In
many cases they appear perfectly parallel, and Mr. Lowell gives us the
impression that they are almost always so. But his maps show, in some
cases, decided differences of width at the two extremities, indicating
considerable want of parallelism. A few of the curved canals are also
double.
There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or
starting points, of the Euphrates and Phison, two widely separated
double canals diverging at an angle of about 40° from the same two
oases, so that the two inner canals cross each other. Now this suggests
two wide bands of weakness in the planet's crust radiating probably from
within the dark tract called the 'Mare Icarium,' and that some
widespread volcanic outburst initiated diverging cracks on either side
of these bands. Something of this kind may have been the cause of most
of the double canals, or they may have been started from two or more
craterlets not far apart, the direction being at first decided by some
local peculiarity of structure; and where begun continuing in straight
lines owing to homogeneity or uniform density of material. This is very
vague, but the phenomena are so remarkable, and so very imperfectly
known at present, that nothing but suggestion can be attempted.
_Concluding Remarks on the 'Canals.'_
In this somewhat detailed exposition of a possible, and, I hope, a
probable explanation of the surface-features of Mars, I have
endeavoured to be guided by known facts or accepted theories both
astronomical and geological. I think I may claim to have shown that
there are some analogous features of terrestrial rock-structure to
serve as guides towards a natural and intelligible explanation of the
strange geometric markings discovered during the last thirty years, and
which have raised this planet from comparative obscurity into a position
of the very first rank both in astronomical and popular interest.
This wide-spread interest is very largely due to Mr. Lowell's devotion
to its study, both in seeking out so admirable a position as regards
altitude and climate, and in establishing there a first-class
observatory; and also in bringing his discoveries before the public in
connection with a theory so startling as to compel attention. I venture
to think that his merit as one of our first astronomical observers will
in no way be diminished by the rejection of his theory, and the
substitution of one more in accordance with the actually observed facts.
APPENDIX.
_A Suggested Experiment to Illustrate the 'Canals' of Mars._
If my explanation of the 'canals' should be substantially correct--that
is, if they were produced by the contraction of a heated outward crust
upon a cold, and therefore non-contracting interior, the result of such
a condition might be shown experimentally.
Several baked clay balls might be formed to serve as cores, say of 8 to
10 inches in diameter. These being fixed within moulds of say half an
inch to an inch greater diameter, the outer layer would be formed by
pouring in some suitable heated liquid material, and releasing it from
the mould as soon as consolidation occurs, so that it may cool rapidly
from the _outside._ Some kinds of impure glass, or the brittle metals
bismuth or antimony or alloys of these might be used, in order to see
what form the resulting fractures would take. It would be well to have
several duplicates of each ball, and, as soon as tension through
contraction manifests itself, to try the effect of firing very small
charges of small shot to ascertain whether such impacts would start
radiating fractures. When taken from the moulds, the balls should be
suspended in a slight current of air, and kept rotating, to reproduce
the planetary condition as nearly as possible.
The exact size and material of the cores, the thickness of the heated
outer crust, the material best suited to show fracture by contraction,
and the details of their treatment, might be modified in various ways as
suggested by the results first obtained. Such a series of experiments
would probably throw further light on the physical conditions which have
produced the gigantic system of fissures or channels we see upon the
surface of Mars, though it would not, of course, prove that such
conditions actually existed there. In such a speculative matter we can
only be guided by probabilities, based upon whatever evidence is
available.