Imágenes de páginas

satellites upon a planet: and Dr. Herschel gives instances of all these phæ nomena actually completed, or in a train of completion, in different parts of the visible heavens.

Such he submits as his latest opinion of the general construction of the heavens; believing stars, planets, and comets to have originated, and to be still originating, from such a source; the nebulous matter contained in a cubical space seen under an angle of ten degrees demanding a condensation of two trillion and two hundred and eight thousand billion times before it can be so concentrated as to constitute a globe of the diameter and density of our sun.

Some of these masses of light are indistinct and barely visible even by Dr. Herschel's forty feet telescope; and he hence calculates, that if a mass thus traced out contain a cluster of five thousand stars, they must be eleven millions of millions of millions of miles off. M. Huygens entertained an analogous idea; and conceived that there are stars so immensely remote that their light, although travelling at the rate of eleven millions of miles in a minute, and having thus continued to travel from the formation of the earth, or for nearly six thousand years, has not yet reached us.

But this sublime conception is of much earlier origin; and it is due to the magnificence of the Epicurean scheme to state that it is to be found completely developed among its principles. Lucretius has beautifully alluded to it in lines of which I must beg your acceptance of the following feeble translation, the only difference being, that lightning or the electric fluid, is here employed instead of light, at least by Havercamp; for Vossius, in the Leyden edition, gives us light for lightning, reading lumina instead of fulmina.

The poet is speaking of the immensity of space :

The vast whole

What fancied scene can bound? O'er its broad realm,
Immeasur'd, and immeasurably spread,
From age to age resplendent lightnings urge,
In vain, their flight perpetual; distant, still,
And ever distant from the verge of things.
So vast the space or opening space that swells;
Through every part so infinite alike.*


From this immense range of nebulous light Dr. Herschel derives comets as well as stars and planets, believing them, indeed, to be the rudiments of the two latter; and he has especially noticed, as originating from this source, the well remembered comet that so brilliantly, and for so long a period of time, visited our horizon during the close of the year 1811; which he conceives will be converted into a stellar or planetary orb as soon as its luminous matter, and especially that of its enormous tail, shall be sufficiently concentrated for this purpose. This tail he calculated, when at its greatest apparent stretch in October of the same year, at something more than a hundred millions of miles long, and nearly fifteen millions

* Omne quidem vero nihil est quod finiat extra.
Est igitur natura loci, spatiumque profundi,
Quod neque clara suo percurrere fulmina cursu
Perpetuo possint ævi labentia tractu ;
Nec prorsum facere, ut restet minus ire, meando
Usque adeo passim patet ingens copia rebus,
Finibus exemptis, in cunctas undique parteis. †

↑ De Rer. Nat. i. 1000.

broad, though its bright or solid nucleus or planetary body was not supposed to measure more than four hundred and twenty-eight miles. Its perihelion path, or nearest approach to the sun, is stated at a distance of ninety-seven millions of miles, its distance from the earth at ninety-three millions. The comet of 1807 approached the earth within sixty-one millions of miles, or about a third nearer the earth, and that of 1680 within a sixth of its diameter, or as near as 147,000 miles, its tail being of a like length.

There is one comet, however, that we seem to be somewhat better acquainted with than with this that paid us so near a visit, or indeed than with any other, from its having approached us visibly for four times in succession, if not oftener. It was towards the beginning of last century that Mr. Halley was struck with the remark, that the general elements and character of the comets observed in 1531, 1607, and 1682, were nearly the same; whence he concluded that the whole formed but one identical body, that took about seventy-six years to complete its eccentric orbit; and hence, although in consequence of this eccentricity, and its travelling amidst a range of heavenly bodies that are altogether invisible to us, and whose influence seems to bid defiance to calculation, it is difficult to form an estimate of its progress, he ventured to suggest, that it would appear again, making due allowance for these incidents, towards the close of 1758, or the commencement of 1759: and he had the high satisfaction of seeing his prediction verified; the comet passing its perihelion March 12th, 1759, within the limits of the errors of which he thought his results susceptible. It is apparently this comet, which at this last period only excited the curiosity of astronomers and mathematicians, that in 1456, or four revolutions earlier, towards the close of what are called the dark ages, spread such consternation over all Europe, already, indeed, terrified by the rapid successes of the Turkish arms, that Pope Calixtus was induced to compose a prayer for the whole western church, in which both the Turks and the comet were included in one sweeping anathema.

Admitting the truth of Dr. Herschel's hypothesis, as we are now contemplating it, it is possible that some of the lately discovered planets, which are now attendant upon the sun, were formerly comets, whose orbits have for ages been growing progressively more regular, as well as their consti tutional rudiments more dense; and such, indeed, is the opinion of M. Voight, and of various other philosophers on the continent.

The object of the present and the preceding lecture has been to submit a sketch of the most obvious properties belonging to MATTER, so as to enable you to obtain a bird's-eye-view of the general phænomena it is capable of assuming, and the general changes it is necessarily sustaining. From the qualities I have placed before you, of passivity, cohesibility, divisibility, and attractions of various kinds, must necessarily result, according to the intensity with which they are called into action, the phænomena of liquidity, viscidity, toughness, elasticity, symmetry of arrangement, solidity, strength, and resilience. But the powers which thus perpetually build up the inorganic world, and to this our survey has been entirely confined, perpetually also destroy it: for the whole, as I have had occasion to observe, is a continued circle of action; a circle most wise, most barmonious, most benevolent: and hence, as one compound substance decays, another springs up in its place, and can only spring up in consequence of such decay.

There is, however, another lesson, if I mistake not, which we may

readily learn from these lectures, however imperfectly delivered, and which is altogether of a moral character: I mean that of humility in regard to our own opinions and attainments; and of complacency, in regard to those of others. After a revolution of six thousand years, during the whole of which period of time the restless ingenuity of man has been incessantly hunting in pursuit of knowledge, what is there in physical philosophy that is thoroughly and perfectly known even at the present moment? and of the little that is thus known, what is there which has been acquired without the clash of controversy, and the warfare of opposing speculations? Truth, indeed, for ever praised be the great Source of Truth, for so eternal and immutable a decree-has at all times issued, and at all times will issue, from the conflict; but while we behold philosophers of the highest reputation, philosophers equally balanced in the endowment of native genius, proved by the great teacher Time to have been alternately mistaken upon points to which they had honestly directed the whole acumen of their intellect, how absurd, how contemptible is the fond confidence of common life! Yet what indeed, when fairly estimated by the survey that has now been briefly taken of the sensible universe,-what is the aggregate opinion, or the aggregate importance of the whole human race! We call ourselves lords of the visible creation: nor ought we at any time, with affected abjection, to degrade or despise the high gift of a rational and immortal existence. Yet, what is the visible creation? by whom peopled? and where are its entrances and outgoings? Turn wherever we will we are equally confounded and overpowered: the little and the great are alike beyond our comprehension. If we take the microscope it unfolds to us, as I observed in our last lecture, living beings probably endowed with as complex and perfect a structure as the whale or the elephant, so minute that a million of millions of them do not occupy a bulk larger than a common grain of sand. If we exchange the microscope for the telescope, we behold man himself reduced to a comparative scale of almost infinitely smaller dimension, fixed to a minute planet that is scarcely perceptible throughout the vast extent of the solar system; while this system itself forms but an insensible point in the multitudinous marshallings of groups of worlds upon groups of worlds, above, below, and on every side of us, that spread through all the immensity of space, and in sublime, though silent harmony declare the glory of God, and show forth his handy-work.



THERE are some subjects on which the philosopher is obliged to exercise nearly as much imagination as the poet; for it is the only faculty by which he can expatiate upon them. Such is a great part of the magnificent study upon which we have touched in our preceding lectures.-Space, immensity, infinity, pure incorporeal intelligence, matter created out of nothing, innumerable systems of worlds, and innumerable orders of beings ;-where is the mind strong enough to grapple with such ideas as these? They at once entice and overwhelm us. Reason copes with them till she is exhausted, and then gives us over to conjecture. Hence, as we have already seen,

[ocr errors]

invention at times takes the place of induction, and the man of wisdom has his dream as well as the man of fancy.

Let us descend from such magnificent flights: let us quit the possible for the actual; and equally incapable of following up the fugitive material of which the visible universe consists, into its elementary principles and collective mass, let us examine it as far as we are able, in the general laws, structure, and phænomena it exhibits in the solid substance of the globe on which we tread.

It is this inquiry that constitutes the science of GEOLOGY, a brief outline of which is intended as a study for the present lecture ;-a science than which few are of more importance, but which is only at present in its infancy, and of course almost entirely indebted for its existence to the unwearied assiduity and discoveries of modern times.

The direct object of geology is, to unfold the solid substance of the earth-to discover by what causes its several parts have been either arranged or disorganized—and from what operations have originated the general stratification of its materials, the inequalities of its surface, and the vast variety of bodies that enter into its make.

In pursuing this investigation, many difficulties occur to us. The bare surface, or mere crust of the earth's structure, is the whole we are capable of boring into, or of acquiring a knowledge of, even by the deepest clefts of volcanoes, or the deepest bottoms of different seas. It is not often, however, that we have the power of examining either seas or volcanoes so low as to their bottom. The inhabitable part of the globe bears but a small proportion to the uninhabitable, and the civilized an almost infinitely smaller proportion still. Hence our experience must be extremely limited; a thousand facts may be readily conceived to be unfolded that we are incapable of accounting for; and, at the same time, a variety of contradictory hypotheses to be formed, with a view of accounting for them.

So far as the superficies of the earth has been laid open to us by ravines, rivers, mines, earthquakes, and other causes, we find it composed of a multitude of stony masses, sometimes simple, or consisting of a single mineral substance, as limestone, serpentine, or quartz; but more frequently compound, or constructed of two or more simple materials, variously intermixed and united; as granite, which is a composition of quartz, felspar, and mica; and sienite, which is a composition of felspar and hornblend. These stony masses or rocks are numerous, and they appear to be laid one over the other, so that a rock of one kind of stone is covered by a rock of another kind, and this second by a third kind, and so on, in many instances, for a very considerable number of times in succession. In this superposition of rocks it is easily observable that their situation is not arbitrary. Every stratum occupies a determinate place; so that they follow each other in regular order from the deepest part of the earth's crust, which has been examined, to the very surface. Thus there are two things respecting rocks which claim our peculiar attention-their composition and their relative situation. And independently of the rocks thus considered as constituting almost the whole of the earth's crust, there are other masses of fossil materials that must be likewise minutely studied; which traverse rocks in a different direction, and are known by the name of veins; as if the rocks had been split asunder in different places from top to bottom, and the chasms had been afterwards filled up with the matter which constitutes the vein. And hence the VEINS which intersect rocks are as much

entitled to our attention as the STRUCTURE and SITUATION of the rocks themselves.

Rocks, as to their STRUCTURE, may be contemplated under two divisions, simple and compound.

The simple division is, however, rather a speculative than a practical contemplation. It is possible that rocks, and of immense magnitude, may exist in parts of the globe we are not acquainted with, that are perfectly simple and unmixed in their structure; but it is seldom, perhaps never, that they have been actually found in such a state, at least to any considerable extent.

It is only under a compound form, therefore, or as composed of more than one mineral substance, that rocks are to be contemplated in our present survey of the subject; and in this form we meet with them of two kinds: CEMENTED, or composed of grains, or nodules, agglutinated by a cement, as sandstone and breccia or pudding-stone; and AGGREGATED, OF composed of parts connected without a cement, as granite and gneiss. The component parts of the cemented rocks are often very multifarious; those of granite and gneiss much less so, consisting chiefly of felspar, mica, and quartz, with garnets, shorl, or hornblend occasionally intermixed with the mass. The granite that forms the flagstones of Westminster Bridge are supposed to have been brought from Dartmoor; and, like the rest of the Dartmoor granite, is remarkable for the length of its crystals of felspar, which in some instances are not less than four inches.

The aggregate rocks, like the cemented, are sometimes found of an indeterminate, but more generally of a determinate or regular form; and it is the office of that branch of mineralogy to which M. Werner has given the name of oryctognosy, to distinguish and describe them by these peculiarities. This is a branch into which I cannot plunge, for it would lead us from that general view of the science to which our present course of study is directed, into a detailed analysis. Those who are desirous of pursuing it in this line of developement may consult with great advantage Professor Jameson's System of Mineralogy, or M. Brogniart's "Traité Elémentaire," or M. Cuvier's Essay on the Theory of the Earth, prefixed to his Fossil Remains. I can only observe, at present, that the total number of rocky masses, or different kinds of rocks, whether simple or compound, which have been hitherto observed, amount to about sixty; of which the principal seem to be the eight following: granite, gneiss, hornblend, limestone, wacke, basalt, quartz, and clay.

Let us next pass on, then, to consider their RELATIVE SITUATION. Of the different rocks thus glanced at, and placed over each other, the whole crust of the earth is composed, to the greatest depth that the industry of man has been able to penetrate; and I have already observed, that, with respect to each other, they occupy a determinate situation which holds invariably in every part of the globe. Thus, limestone, excepting under particular circumstances hereafter to be explained, is no where found under granite, but always above it. This general view of the subject may indeed induce a supposition that every separate layer which constitutes a part of the earth's surface is extended round the entire globe, and wrapped about the central nucleus, like the coats of an onion; the kind of rock that is always lowest, or nearest the centre, uniformly supporting a second kind, and this second kind a third, and so on. Now, though the different kinds or layers of rocks do not in reality extend round the earth in this uninterrupted manner-though partly from the inequality of the nucleus on which they rest, partly from their

« AnteriorContinuar »