SIR JOHN HERSCHEL'S LECTURES.*
THIS is a book which, popular as it is, few men could review, in the sense of criticizing it from the point of view of larger know- ledge. From us, at all events, such criticism is impossible ; but the appreciation of the learner is perhaps more useful with regard to books of this nature, than exact estimation by the learned ; for while there are thousands who will be glad to know what they can learn from Sir John Herschel, there are but a few who could judge of the truth of any criticism passed upon him by any one who had any pretence to rank on a level with him in knowledge of the physical sciences.
There are but few scientific men who translate the knowledge of their understanding into the language of the imagination with so much ease and simplicity as Sir John Herschel. With- out any strain of manner, with that facility which seems to imply that he never ascertains any scientific fact without at- tempting, so far as it is possible, to realize what it actually means in some simple, practical illustration, he paints pictureafter picture from the wonderful discoveries made known to us by the study of the physical forces at work on the earth and in the heavens, and of the laws of light and heat, and yet it is never mere pictorial physics ; the motive of every picture is never to astonish, but only to help the learner to realize at once the truth, and also the method of reasoning by which the knowledge of the truth has been attained. Sir John Herschel's whole type of thought is opposed to the dominant school of philosophy, which seeks to get rid of ' cause' altogether, and to speak of nothing but sequences.' •In one of these lectures or essays he avows his absolute disbelief that ' force' can anyhow be got rid of and resolved into mere motion ; and this assumption is really at the bottom of the charm of his philosophical style. He is always trying to show actual pheno- mena in their causes, to give us such a grasp of the scientific facts of the universe as only a man can have who believes that real forces exist behind the changes we see. There is nothing of what is ordinarily called picturesque science in his essays ; though he makes us realize all he tells, it is for the sake of more clearly understanding the operative powers, and not for the sake of dazzling the imagination, that he describes. A purely intellectual kind of vividness marks the style of all these lectures.
The first and one of the most interesting is on earthquakes and volcanoes as a restorative and conservative force in nature. Sir John Herschel shows that the sea by constant friction wears away the land and carries off a great deal of its soil to the ocean bed, thus thickening the superincumbent weight over one part of the crust of the earth and thinning it over another. This increasing inequality of pressure, this added pressure in one part and dimin- ished pressure in another neighbouring part, produces a tendency to crack somewhere near the sea-coast. Whenever such a crack takes place "down goes the land on the heavy side and up on the light side," and by virtue of the interior gaseous pressure the land regains in elevation above the sea what the bottom of the ocean sinks, and wherever there is a volcano or open chimney, quantities of solid matter are vomited forth at it to make up for what falls into the chasm elsewhere. Here is the destructive process :—
"What the sea is doing the rivers are helping it to do. Look at the sand-banks at the mouth of the Thames. What are they but the materials of our island carried out to sea by the stream ? The Ganges carries away from the soil of India, and delivers into the sea, twice as much solid substance weekly as is contained in the Great Pyramid of Egypt. The Irawaddy sweeps off from Burmah 62 cubic feet of earth in every second of time on an average, and there are 86,400 seconds in every day, and 365 days in every year; and so on for the other rivers. What has become of all that great bed of chalk which once covered all the weald of Kent, and formed a continuous mass from Ramsgate and Dover to Beechy Head, running inland to Madamsconrt Hill and Seven Oaks? All clean gone, and swept out into the bosom of the Atlantic, and there forming other chalk-beds. Now, geology assures us, on the most conclusive and undeniable evidence, that ALL our present land, all our continents and islands, have been formed in this way out of the ruins of former ones. The old ones which existed at the beginning of things have all perished, and what we now stand upon has most assuredly been, at one time or other, perhaps many times, the bottom of the sea."
And then Sir John Herschel describes the restorative process,— how the whole coast line of Chili for 100 miles, with the Andes
Familiar latares on &lent* Subjects. By Sir John F. W. Efinookel, K.H. London Strata°.
that border it, were hoisted at one effort from two to seven feet above its former level on the 19th November, 1822 ; how in 1819 in India the territory of Cutch, for fifty miles long and sixteen broad, was hoisted up ten feet above its former level ; and in 1538 the whole coast of Pozzuoli, near Naples, was reared twenty feet, and remains at that height to this day. In some cases Sir John Herschel shows that the process goes on not by fits and starts, but by gradual and very slow upheaval, as in the case of the floor of the Baltic Sea, which is rising up out of the sea at the rate of two feet per hundred years. Active volcanoes, which are the chimneys by which, in the case of great cracks in the soil of the earth, the imprisoned gases escape, bringing with them quantities of fused solid substances, are almost always, says Sir John Herschel, near the sea-coast,—just because the sea is the power which thins away one part of the cruet of the earth and thickens another, so as to tend to produce a crack :— " Well, now, it is a remarkable fact in the history of volcanos, that there is hardly an instance of an active volcano at any considerable dis- tance from the sea-coast. All the great volcanic chain of the Andes is close to the western coast line of America. Etna is close to the sea ; so is Vesuvius; Teneriffo is very near the African coast ; Mount Erebus is on the edge of the great Antarctic continent. Out of 225 volcanos which are known to have been in actual eruption over the whole earth within the last 150 years, I remember only a single instance of one more than 320 miles from the sea, and even that is on the edge of the Caspian, the largest of all the inland seas—I mean Mount Demawend, in Persia."
To think of earthquakes and volcanoes as a conservative and restoring force is a new conception to some of us; but unquestionably Sir John Herschel does show that they retard the destructive forces of the sea in grinding away the land into fine sand and dust on its own bottom, and does much to thrust out of the sea at one place what has been washed into it at another.
The lectures on the sun, and the comets, and celestial weighingis and measurings are full of still more striking and graphic de- scription. Not, indeed, that they tell us anything that has not often been told before, but that they realize in so simple and forcible a way much that had before been rather abstract figures and general statements, than conceptions representable to the mind's, eye. Take this, for instance, as realizing the actual brightness of the sun :— "Let me say something now of the light of the Ban. The means we have of measuring the intensity of light are not nearly so exact as in the case of heat—but this at least we know—that the most intense lights we can produce artificially, are as nothing compared surface for surface with the sun. The most brilliant and beautiful light which can be artificially produced is that of a ball of quicklime kept violently hot by a flame of mixed ignited oxygen and hydrogen gases playing on its ssiuzrefa:.th:uscunh adobeal4s, wind bnroougmohret nairloe,r3rhat t4wiatilliflatrhnfr tt ha name sun—but if it be held between the eye and the sun, and both so en- feebled by a dark glass as to allow of their being looked at together—it appears as a black spot on the sun or as the black outline of the MO= in an eclipse seen thrown upon it. It has been ascertained by experi- ments which I cannot now describe, that the brightness, the intrinsic splendour, of the surface of such a lime-ball is only 146th part of that of the sun's surface. That is to say, that the sun gives out as muoh light as 146 balls of quicklime each the size of the sun, and each heated, all over its surface in the way I have described, which is the most intense heat we can raise, and in which platina melts like lead."
—and then in a further section Sir John Herschel tells us that the nucleus or kernel of the sun itself, at an immeasurable depth be- neath its intensely luminous photosphere, emits so little light as to appear, in the comparison, quite black, " though that does not prevent its ,being in as vivid a state of fiery glare as a white-hot iron ; when we remember what has been said of the lime light appearing black against the light of the sun's surface. And it is a fact, that when Venus and Mercury pass across the sun, and are seen as round spots on it, they do really appear sensibly blacker than the blacker parts of the spots ;" so that even the kernel of the sun is probably a luminous body, though so much less lumi- nous than its outer envelopes as to seem quite dark in the com-
The chapter on comets is perhaps the most interesting and
romantic in the book. The vivid description which Sir John Herschel gives of the adventures of the different comets, of the sad way they get misled and thrown out of their own individual career by the immense bulk of the planet Jupiter whenever they come too near him,—projected sometimes towards the sun and sometimes away from him, as it may happen,—acted upon without apparent reaction, changed from comets of one period of revolu- tion to comets of a very different period,—is much more interesting than most novels ; and the speculation as to the probable consti- tution of comets with which he concludes, is a dream of quite new philosophical possibilities. But take, first, this exquisite little bit of cometary biography :— " On the 27th February, 1826, Professor Biala, an Austrian astronomer fully studied it was found•by -M. Olansen; another of those indefatigable German computiste, that it revolved in an elliptic orbit in a period of '6 years and $ months. On looking back into the list of comets, it proved to be identical With comets that had been observed in 1772, 1805, and perhaps in 1818. Its. return was accordingly predicted, and the pre- 'diction verified with 'the most striking exactness. And this went on regularly till its appearance (also predieted) in 1846. In that year it was observed as usual, and all seemed to be going on quietly and com- 'fortably, when, beheld! suddenly on 'the 13th of January it split into two distinct comets ! witeh•with a head and coma• and a little nucleus of its own. There is sem° little contradiction about the exact date. -Lieutenant Maury, of the United States' Observatory of Washington, '.1*orted officially on the nth having seen it double on the 13th, but Pro- fessor Wichmann, who sad) it datble on the 15th, avers that he had a ..good view of it en 'the llth, and:remarked nothing particular in its ap- pearance. Be that as it may, the comet from a single became a double o ne. What domestic troubles caused the secession it is impossible to conjecture, but the two receded farther and farther" from each other up to a certain moderate distance, with some-degree of mutual comsenni- cation and a very odd interchange of light—one day one head being brighter, and another the 'other--till they Seem to have agreed finally to part company. The oddest part of the story, however, is yet to come. The year 1852' brought round the time for their reappearance, and behold ! there they both were, at about the same distance from each o ther, and both visible in one telescope. The orbit of this comet very nearly indeed intersects that of the earth on the place which the earth -occupies on the 30th of 'November. If ever the earth is to be swal- lowed np by a comet, Or to swallow up one, it will be on or about that day of the year. In the year 1832 we missed it by a month. The head of the comet enveloped that point of our orbit, but this happened on the 29th of October, so that we escaped that time. Had a meeting taken place, from what we know of comets, it is most probable that no harm would have happened, and that nobody would have known any- thing about it. It would appear that we are happily relieved from the 'dread of such a collision. It is now (February, 1866) overdue! Its orbit has been recomputed and an ephemeris calculated. Astronomers have been eagerly looking out for its reappearance for the last two months, when, according to all former experience, it ought to have been - conapicuously visible,,but 'without success, giving rise to the strangest theories. At all events, it seems to have fairly disappeared,' and that without any such excuse as in the case of Lexell's, the preponderant attraction of some great planet. Can it have come into contact or exceedingly close approach to some asteroid as yet undiscovered ; or, peradventure, plunged into and 'got beiVildered among the ring of meteorolites, which astronomers more than suspect ?"
-Here is a comet dividing, -as, it is said, worms-cut in tircrwill -do, into two quite independent comets, which sail -as consorts for a -few years in the-sky, return at the right moment still in company, 'and then, 'when they are expected back once again,'plunge
into in- visibility MAI they had both gone- down together in a squall of the celestial firmament. Singular' and most fascinating in its suggm-
• Lion of philosophic *etas is Sir John Herschel's final speculation its to-the sun's probable analysis of the matter of which comets• are composed into two components,—one, matter on which the sun exercises an attractive force-as it does on 'the material of all our planets,—ancl the other, matter composing a' great part of what is called the tail of the comets, on which the sun's influence is not 'attractive, but repulsive, and which recedes further -and further into space as the head' of the comet approaches the'sun. ' Sir John Herschel suggests that, just as St. Clair Deville has Shown that the chemical affinity between the oxygen and hydrogen of which water consists is so -much weakened by a very high temperature, -that " the mere difference of difficulty in traversing an earthenware tube suffices to set them free of one another," so the action of the _sun'S heat might weaken sufficiently the atomic bond ' of • union between that portion of the cemetery matter which the sun attracts and that portion which it repels, that at every return to the neighbourhood of the sun a good deal of the matter liable to • repulsion by the sun should be cast off into space, and the rest more and more contracted till it settles down into the comparatively hard-nucleus of a, planet. The plausibility of this theory is that those comets which have very little or no tails, like Encke's comet, always contract after passing round the sun (passing through „perihelion), and expand again as they -.get to a distance. Sir John Herschel suggests that at each visit this temporary evapora- tion, as it were, of a. portion of their bulk, weakens its bond of union with the nucleus of the comet, till at last it is cast off wholly into space. What a train of speculation the mere exist once of elements in cometarymatter liable to repulsion from the sun, instead of attraction to it, suggests I -If any such elements of matter linger bound up in ordinary planetary or (say) terrestrial matter, they might be set free by some future change,-and if the bodies of rational beings could ever be made of such matter, they would, instead of being confined, as -all bodies we know are, to the earth and solar .system by the law of gravitation, be, by the very force of repulsion, projected into universes beyond the solar universe to which we belong. At present the highest idea we have of physical impossibility is of corporeal frames getting beyond the attraction-of the earth, and still more, of the solar Josephstadt, discovered a Ssaltll' comet. When its motions were care- central force ; but if there be a- sort Of matter imprisonable in gratitating matter, and yet alio separable from it, which, when separate, is simply repelled by the:Suir,-=then there is a kind of inaterial -frame which might (conceivably) benitide in thiS System, and yet made by the law of its nature totravel out of it.
We have noted but one or two points-in a-bOok Of a most pro- found and romantic scientific charin. 'We leave our readers to find outiruntederable•Otlititi•fOr th-enisefves.