such a horst can only have been const.i.tuted of some siliceous rock-material such as we find everywhere rising through the worn-down sediments of the Alps; and the idea that this could retain rigidity under the prevailing temperature conditions, must be dismissed. There is no need to labour this question; the horst cannot have existed. To what, then, is the r.e.t.a.r.dation of the lower parts of the folds, their overthrow, above, to the north, and their _deferlement_, to be ascribed?

A little consideration shows that the very conditions of high temperature and viscosity, which render untenable the hypothesis of a rigid obstacle, suffice to afford a full explanation of the r.e.t.a.r.dation of the roots of the folds. For directed translatory movements cannot be transmitted through a fluid, pressure in which is necessarily hydrostatic, and must be exerted equally in every direction. And this applies, not only to a fluid, but to a body which will yield viscously to an impressed force. There will be a gradation, according as viscosity gives place to rigidity, between the states in which the applied force resolves itself into a purely hydrostatic pressure, and in which it is transmitted through the material as a directed thrust. The nature of the force, in the most general case, of course, has to be considered; whether it is suddenly applied and of brief duration, or steady and long-continued. The latter conditions alone apply to the present case.

It follows from this that, although a tangential force

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or pressure be engendered by a crustal movement occurring to the south, and the resultant effects be transmitted northwards, these stresses can only mechanically affect the rigid parts of the crust into which they are carried. That is to say, they may result in folding and crushing, or horizontally transporting, the upper layers of the Earth's crust; but in the deeper-lying viscous materials they must be resolved into hydrostatic pressure which may act to upheave the overlying covering, but must refuse to transmit the horizontal translatory movements affecting the rigid materials above.

Between the regions in which these two opposing conditions prevail there will be no hard and fast line; but with the downward increase of fluidity there will be a gradual failure of the mechanical conditions and an increase of the hydrostatic.

Thus while the uppermost layers of the crust may be transported to the full amount of the crustal displacement acting from the south (speaking still of the Alps) deeper down there will be a lesser horizontal movement, and still deeper there is no influence to urge the viscous rock-materials in a northerly direction. The consequences of these conditions must be the rec.u.mbence of the folds formed under the crust-stress, and their _deferlement_ towards the north. To see this, we must follow the several stages of development.

The earliest movements, we may suppose, result in flexures of the Jura-Mountain type--that is, in a

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succession of undulations more or less symmetrical. As the orogenic force continues and develops, these undulations give place to folds, the limbs of which are approximately vertical, and the synclinal parts of which become ever more and more depressed into the deeper, and necessarily hotter, underlying materials; the anticlines being probably correspondingly elevated. These events are slowly developed, and the temperature beneath is steadily rising in consequence of the conducted interior heat, and the steady acc.u.mulation of radioactive energy in the sedimentary rocks and in the buried radioactive layer of the Earth. The work expended on the crushed and sheared rock also contributes to the developing temperature. Thus the geotherms must move upwards, and the viscous conditions extend from below; continually diminishing the downward range of the translatory movements progressing in the higher parts. While above the folded sediments are being carried northward, beneath they are becoming anch.o.r.ed in the growing viscosity of the medium. The anticlines will bend over, and the most southerly of the folds will gradually become pushed or bent over those lying to the north.

Finally, the whole upper part of the sheaf will become horizontally rec.u.mbent; and as the uppermost folds will be those experiencing the greatest effects of the continued displacement, the _deferlement_ or overlap must necessarily arise.

We may follow these stages of mountain evolution

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in a diagram (Fig. 9) in which we eliminate intermediate conditions, and regard the early and final stages of development only. In the upper sketch we suppose the lateral compression much developed and the upward movement of the geotherms in progress.

The dotted line may be a.s.sumed to be a geotherm having a temperature of viscosity. If the conditions here shown persist

{Fig. 9}

indefinitely, there is no doubt that the only further developments possible are the continued crushing of the sediments and the bodily displacement of the whole ma.s.s to the north. The second figure is intended to show in what manner these results are evaded. The geotherm of viscosity has risen. All above it is affected mechanically by the continuing stress, and borne northwards in varying

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degree depending upon the rigidity. The folds have been overthrown and drawn out; those which lay originally most to the south have become the uppermost; and, experiencing the maximum amount of displacement, overlap those lying beneath. There has also been a certain amount of upthrow owing to the hydrostatic pressure. This last-mentioned element of the phenomena is of highly indeterminate character, for we know not the limits to which the hydrostatic pressure may be transmitted, and where it may most readily find relief. While, according to some of the published sections, the uplifting force would seem to have influenced the final results of the orogenic movements, a discussion of its effects would not be profitable.

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OTHER MINDS THAN OURS?

IN the year 1610 Galileo, looking through his telescope then newly perfected by his own hands, discovered that the planet Jupiter was attended by a train of tiny stars which went round and round him just as the moon goes round the Earth.

It was a revelation too great to be credited by mankind. It was opposed to the doctrine of the centrality of the Earth, for it suggested that other worlds const.i.tuted like ours might exist in the heavens.

Some said it was a mere optic illusion; others that he who looked through such a tube did it at the peril of his soul--it was but a delusion of Satan. Galileo converted a few of the unbelievers who had the courage to look through his telescope. To the others he said, he hoped they would see those moons on their way to heaven.

Old as this story is it has never lost its pathos or its teaching.

The spirit which a.s.sailed Galileo's discoveries and which finally was potent to overshadow his declining years, closed in former days the mouths of those who asked the question written at the head of this lecture: "Are we to believe that there are other minds than ours?"

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Today we consider the question in a very different spirit. Few would regard it as either foolish or improper. Its intense interest would be admitted by all, and but for the limitations closing our way on every side it would, doubtless, attract the most earnest investigation. Even on the mere balance of judgment between the probable and the improbable, we have little to go on.

We know nothing definitely as to the conditions under which life may originate: whether these are such as to be rare almost to impossibility, or common almost to certainty. Only within narrow limits of temperature and in presence of certain of the elements, can life like ours exist, and outside these conditions life, if such there be, must be different from ours. Once originated it is so const.i.tuted as to a.s.sail the energies around it and to advance from less to greater. Do we know more than these vague facts?

Yes, we have in our experience one other fact and one involving much.

We know that our world is very old; that life has been for many millions of years upon it; and that Man as a thinking being is but of yesterday. Here is then a condition to be fulfilled. To every world is physically a.s.signed a limit to the period during which it is habitable according to our knowledge of life and its necessities. This limit pa.s.sed and rationality missed, the chance for that world is gone for ever, and other minds than ours a.s.suredly will not from it contemplate the universe. Looking at our own world we see that the tree of life has,

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indeed, branched, leaved and, possibly, budded many times; it never bloomed but once.

All difficulties dissolve and speculations become needless under one condition only: that in which rationality may be inferred directly or indirectly by our observations on some sister world in s.p.a.ce, This is just the evidence which in recent years has been claimed as derived from a study of the surface of Mars. To that planet our hope of such evidence is restricted. Our survey in all other directions is barred by insurmountable difficulties.

Unless some meteoric record reached our Earth, revelationary of intelligence on a perished world, our only hope of obtaining such evidence rests on the observation of Mars' surface features. To this subject we confine our attention in what follows.

The observations made during recent years upon the surface features of Mars have, excusably enough, given rise to sensational reports. We must consider under what circ.u.mstances these observations have been made.

Mars comes into particularly favourable conditions for observation every fifteen years. It is true that every two years and two months we overtake him in his...o...b..t and he is then in "opposition." That is, the Earth is between him and the sun: he is therefore in the opposite part of the heavens to the sun. Now Mars' orbit is very excentric, sometimes he is 139 million miles from the sun, and sometimes he as as much as 154 million miles from the sun. The Earth's...o...b..t is, by comparison, almost

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a circle. Evidently if we pa.s.s him when he is nearest to the sun we see him at his best; not only because he is then nearest to us, but because he is then also most brightly lit. In such favourable oppositions we are within 35 million miles of him; if Mars was in aphelion we would pa.s.s him at a distance of 61 million miles. Opposition occurs under the most favourable circ.u.mstances every fifteen years. There was one in 1862, another in 1877, one in 1892, and so on.

When Mars is 35 million miles off and we apply a telescope magnifying 1,000 diameters, we see him as if placed 35,000 miles off. This would be seven times nearer than we see the moon with the naked eye. As Mars has a diameter about twice as great as that of the moon, at such a distance he would look fourteen times the diameter of the moon. Granting favourable conditions of atmosphere much should be seen.

But these are just the conditions of atmosphere of which most of the European observatories cannot boast. It is to the honour of Schiaparelli, of Milan, that under comparatively unfavourable conditions and with a small instrument, he so far outstripped his contemporaries in the observation of the features of Mars that those contemporaries received much of his early discoveries with scepticism. Light and dark outlines and patches on the planet's surface had indeed been mapped by others, and even a couple of the ca.n.a.ls sighted; but at the opposition of 1877 Schiaparelli first mapped any considerable

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number of the celebrated "ca.n.a.ls" and showed that these const.i.tuted an extraordinary and characteristic feature of the planet's geography. He called them "ca.n.a.li," meaning thereby "channels." It is remarkable indeed that a mistranslation appears really responsible for the initiation of the idea that these features are ca.n.a.ls.

In 1882 Schiaparelli startled the astronomical world by declaring that he saw some of the ca.n.a.ls double--that is appearing as two parallel lines. As these lines span the planet's surface for distances of many thousands of miles the announcement naturally gave rise to much surprise and, as I have said, to much scepticism. But he resolutely stuck to his statement. Here is his map of 1882. It is sufficiently startling.

In 1892 he drew a new map. It adds a little to the former map, but the doubling was not so well seen. It is just the strangest feature about this doubling that at times it is conspicuous, at times invisible. A line which is distinctly seen as a single line at one time, a few weeks later will appear distinctly to consist of two parallel lines; like railway tracks, but tracks perhaps 200 miles apart and up to 3,000 or even 4,000 miles in length.

Many speculations were, of course, made to account for the origin of such features. No known surface peculiarity on the Earth or moon at all resembles these features. The moon's surface as you know is cracked and

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streaked. But the cracks are what we generally find cracks to be--either aimless, wandering lines, or, if radiating from a centre, then lines which contract in width as they leave the point of rupture. Where will we find cracks accurately parallel to one another sweeping round a planet's face with steady curvature for, 4,000 miles, and crossing each other as if quite unhampered by one another's presence? If the phenomenon on Mars be due to cracks they imply a uniformity in thickness and strength of crust, a h.o.m.ogeneity, quite beyond all antic.i.p.ation.

We will afterwards see that the course of the lines is itself further opposed to the theory that haphazard cracking of the crust of the planet is responsible for the lines. It was also suggested that the surface of the planet was covered with ice and that these were cracks in the ice. This theory has even greater difficulties than the last to contend with. Rivers have been suggested. A glance at our own maps at once disposes of this hypothesis. Rivers wander just as cracks do and parallel rivers like parallel cracks are unknown.

In time the many suggestions were put aside. One only remained.

That the lines are actually the work of intelligence; actually are ca.n.a.ls, artificially made, constructed for irrigation purposes on a scale of which we can hardly form any conception based on our own earthly engineering structures.

During the opposition of 1894, Percival Lowell, along with A. E.

Dougla.s.s, and W. H. Pickering,

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observed the planet from the summit of a mountain in Arizona, using an 18-inch refracting telescope and every resource of delicate measurement and spectroscopy. So superb a climate favoured them that for ten months the planet was kept under continual observation. Over 900 drawings were made and not only were Schiaparelli's channels confirmed, but they added 116 to his 79, on that portion of the planet visible at that opposition.