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good representation of the facts, though they have no other existence than as so representing them.

But on all grounds, it is a far higher and more satisfactory process if we can arrive at such a cause as fulfils both Newton's conditions; such as not only affords a key to the phenomena, but is also " true," or is proved to be a real principle existing in nature by other and independent considerations.

Examples in Physics.

1. The varied phenomena of electro-magnetism are all explained by the supposition of a system of currents in directions transverse to the length of the needle. But these currents are not proved to exist by any independent evidence. This, then, is an instance of an hypothetical cause, which yet explains the phenomena.

2. On the other hand, the air is a medium really and independently known to exist; and pulsations are real mechanical effects produced in it. The phenomena of sound are thus referred to a "real cause," which perfectly "explains" them.

3. The explanation of dew before mentioned is also an example of combination of "true" causes.

4. We before alluded to large classes of optical facts which are reducible to the principle of " interference." That rays of light actually possess some inherent property by virtue of which they can so interfere, has been shown by independent experiment, by Arago and Fresnel. Interference, then, is a real effect; and these classes of facts are explained by a " true cause."

The fact of interference itself is again explained perfectly by the theory of waves, propagated in an infinitely rare and elastic medium; also a vast number of other optical facts which have no connexion with interference, and others which have, are all capable of exact explanation by this hypothesis of waves; which, when modified by some peculiar considerations, seems likely to afford a clue to nearly all the most complex phenomena of light. Yet we have no independent proof of the existence of an aether, or the propagation of waves in it. It therefore remains at present an hypothetical cause.

5. Gravitation, or the tendency of matter to fall together with a force proportional directly to the mass and inversely to the square of the distance, is a real thing; we find it independently and experimentally in the attractions which take place within the reach of our investigations; we also find that the extension of the same cause perfectly explains all the movements of the planetary system.

Examples in Geology.

6. The phenomena presented by the actual state and structure of the earth's surface are such as both admit and call for inquiry into the nature of the causes to which they can be referred.

The pursuit of this inquiry soon discloses the evidence and monuments of successive changes which have occurred in the state of the earth's surface. In the attempt to trace these to their causes, sound inductive geology recognises, of course, the same principle of referring to those which are both true, and sufficient to explain the phenomena. We cannot find true causes except in such as are really proved to exist, and found by experience to be in operation. The action of the waters on the land, (whether the continued power of the rivers and ocean, or the occasional force of inundations and torrents,) the subterranean force of earthquakes, and the external operation of volcanoes; the contractions and expansions which must accompany changes in the temperature of any considerable thickness of the earth's crust: these and the like are the real causes to which the sound geologist refers.

The accumulation of soil at the bottom of the waters, the imbedding of animal and vegetable remains in those depositions, the elevation of portions of land out of the sea, are operations really and continually going on. When, therefore, fossil remains of organized beings are found imbedded in rocks bearing also the marks of a similar mode of deposition, we refer to such operations as those just mentioned as true causes to explain the phenomena. And numerous series and successions of such depo

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sits, containing the remains of species now extinct, and, successively in the order of deposition, containing fewer of recent and more of extinct kinds, even to whole genera, classes, and orders of beings, call for the like reference to the continued action of similar causes through periods of countless duration.

And even if difference of opinion arise among geologists as to the rapidity with which such changes may have been effected, yet no sound inquirer refers to causes of different kinds; no one now dreams of the plastic power of nature moulding the semblances of organic remains in her sportive moods; nor of the simultaneous formation of the different strata with all their fossils in one confused mass, from which they subsided at once into their present positions.

The continent of Sweden is shown to have been slowly rising, by elevation in a mass, above the level of the Baltic, by a gradual, insensible movement, unattended by any violence or dislocation; the effect of some enormous subterranean pressure.

Here, then, is a true cause; it is also one which perfectly explains the phenomena presented by numerous other large districts of the earth; which, containing immense deposits of marine shells, must once have formed the bed of the sea, above which they are now elevated; and exhibiting an unbroken level, we infer were elevated gradually, and without disturbance, by similar slowly-acting subterranean forces, as true causes, and sufficient to explain the phenomena.

In the tremendous earthquake which occurred on the coast of Chili in 1835, an eruption of a submarine volcano caused an enormous wave which swept over and entirely desolated a considerable tract of country. The geologist traces the marks of such sudden and violent local inundations in various parts of the earth at remote epochs. Here, then, we have a real cause which explains them. And if, in some instances, the effects appear to have taken place formerly upon a larger scale, still we are not departing from the nature of a real cause in supposing submarine eruptions of greater violence.

Thus, while no difference of opinion exists among rational geologists as to the propriety of attempting to explain the facts solely by reference to "true" causes, questions have, nevertheless, been, and may fairly be, agitated, as to the frequency or intensity with which such causes may have operated in remote periods; and whether we should with greater probability lean to the idea of brief eruptions of enormous violence, or of long-continued action of ordinary energy. Such questions are clearly matter of fair and philosophical discussion. But the important point to be borne in mind is, the distinction between such suppositions as those, and speculations which would refer the effects in question to other agency of a different kind, and which has not any connexion with the legitimate objects of inductive research.

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