SIR JOHN HERSCHEL'S LECTURES.

New Zealand Herald, Volume IV, Issue 1075, 25 April 1867, Page 6

SIR JOHN HERSCHEL'S LECTURES.

[From the Spectator, "November 21.] FainiHitr .TLcc/itrcs on Scientific Subjects. By Sir Jihn 1?. W. Hersciiel, Knt, [London, Straliau.] This is a book which, popular as it is, feu- men could review, in the sense of criticizing it fiom the point of view of larger knowledge. From us at all events, such criticism is impossible ; but the appreciation of the learner pciliaps more useful with regard to books of this natuic, than exact estimation by the learned ; for while there are thousands who will be glad to know what thov can learn from Sir John Herschel, there are'but a few who could judge the truth of any criticism passed upon him by any one who had any pretence to rank 011 a level with him m knowledge of the physical sciences. There are but few scientific men who translate the knowledge of their understanding into the language of the imagination lth so much ease and simplicity as Sir. John -> terschel. Without any strain of manner, witli that facility which seems to imply that li? never ascertains anv scientific fact without .attempting, so far as it is"possible, to realise what it actually means in some simple, practical* illustration, he paints picture after piclurc 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 evorr picture is never to astonish., but only to help the learner to realise at once the truth, and also the method of reasoning by which the knowledge of the truth has been attained. Sir John Hevschel's whole type of thought is opposed to the dominant school ot philosophy, which seeks to get rid ot " cause altogethci, and to speak of nothing but " sequences. 11l one of these lectures or essays he avows his absolute disbelief that " force" can anyhow be <*ot 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 phenomena in their causes, to give us such a grasp ot the scientific facts ot the universe as only a man can nave who believes that real forces exist behind the changes we see. There is nothing of what is ordinarily called picti'.rexnue science in his essays though he makes us realise all ho tells, it is tor the sake of more clearly understanding tUo operative powers, and not for the sake of dazzling the imagination, that he describes. A purely intellectual kind of vividness marks the stylo ot ait these lectures. The first, and one of the most interesting, is on earthquakes and volcanoes, as u restorative and conferral ire force in naturo. Sir John Herschel shows that the sen, by constant inotion, wears away the land, and carries oil 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 ot pressure, this added pressure in one part and diminished pressure in another neighbouring part, produces a tendency to crack somewhere near the sea-coast. Whenever such a crack takes place, " down goes the laud on tho 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 ot 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 lall into the chasm elsewhere. Here is tho destructive proccss:— What tho sea is doing tho rivers are helping; it to do. Look at the sand-banks at tho mouth of the Thames. What nre they but the materials of our islands c rrie:.l oul to sea by tho stream ? Ihe Ganges carries away the soil of India, and delivers into tho sea, twice as much solid substance weekly as is.contained in tho Great Pyramid of l"gypt. r -fbo Irruwaddy sweeps otT from i3urni.li S2 cubic foot of earth every second of time on an average, and there avo BG, 100 seonds in every day, and 3G5 doyo in every year ; and bo on for tho othor rivers. Wh&t has rocomo of all th-\t great bed of chalk which onca covered all that weald of Kent, and formed a continuous mass from Dovor ti ISecehy Head, running inland to Madamsconrt Hill and Hoven Oaks? All clean gone, and r wept out into the bosom of the Atlantic, and there forming otli-:r chalk beds. JS'ow geology assures us, on tho most conclusive and undeniablo evidenco, that ill our present land, and our continents and islands, have been formed in this way out of the ruins of former ones. The oM onos which existed at tho beginning of things liavo all porished, and what we now stand upon has most assuredly been, at ono time or othor, perhaps many timos, the bottom of tho soa. And then Sir John Herschel describes the restorative process, of how the whole coast lino of Chili for 100 miles, with the Andes that border it, were hoisted at.one effort from two to seven feet above its former level on the 12th. November, 1522; how in 1822 in India the territory of Outeli, for fifty miles long and sixteen broad, was hoisted 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 tho floor of the .Baltic Sen, which is rising up out of the sea at the rate of two feet per hundred years. Active volcanoes, which arc 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 seacoast —just because the sea is the power which thins away the crust of the earth and thickens another, so as to tend to produce a crack: — Well, now, it is a remarkable fact in tho history of volcanoes, that there is hardly an instanco of an active volcano at any considerable distance from tho soa-coast. All tho great volcanic chain of tlio Andes is closs to tho western coas'J line of America. Etna is close to the sea ; so is "Vesuvius ; Tencriffo is very near tho African coast; Mount Krobus is on tho edge of the great Antarctic continent. Out of 225 volcanoes which are known to have been in actual eruption over tho whole earth within tho last lot) years, I remember only a single instance of one more than 320 miles from tho soa, and even that is on tho edge of tho Caspian, tho largest of all tho inland soas —I mean Mound Demawcnd, in Porsia. — 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 tho land into line sand and dust oil its own bottom, and does much to thrust out of the sea at one placc what has been washed into it at another. The lectures on the sun, and the comets, and celestial weighings and measurings are full of still more striking and graphic description. Pfot, 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 beforo been rather abstract figures and general statements, than conceptions reprcsentable to the mind's eye. Take this, for instance, as realizing the actual brightness of the sun Let me say something now of tho tight of the sun. The means wo have of mcsuring tho intensity of light arc not nearly so exact as in tho caßo of heat; but this at least wo know, that tho most intense lights we can produce artificially, are as nothing compared surf act for surface with tho sun. Tho most brilliant and boiutiful light which can be artificially produced is that of a ball of quicklime kept violently hot by a flame of mixed ignited oxygon and hydrogen gases playing on its surface. Such a ball, if brought near 1 enough to appear as the same size as tlie Bun does,

can no more be looked at without hurt than the sun; but if i obu held between the cyo and the Ban, and both bo enfeobled by a dark glass as to allow of their beiun- looked at altogether, it appears aa a black spot on the sun, or aa the black outlines of the moon in an eclipso, seen thrown upon it. It has teen ascertained by experiments which I cannot now describe, that the brightness, the intrinsic splendour of the j surface of such a liine-ball is only 14Gth part of that I lof tlio sun's surface. That is to say, that the sun ; gire3 out as much light as 146 balls of quicklime, | each the size of the sun, and each heated all over surface in the way I liavo describad, which is the most intense heat we can raiso, and in which platina molts liko lead. I —and tlieu in a further section Sir Jolm Her- I sciiel tells us that the nucleus or kernel of tlio sun itself, at an immeasurable depth beneath its intensely luminous photosphere, emits so little light as to appear, iu the comparison, quite black, 1 " though that docs not prevent its being in as vivid a state of fiery glare as a white-liot iron ; when wc remember what has been said of tlio lime light appearing black against the light of tlie 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 jiarts of the spots so that even the kernel of the sun is probably a luminous body, though so much Jess luminous than its outer envelopes as to seem quite dark in the comparison. The chapter on comcts is perhaps the most interesting and romantic in tlio book. The vivid description which Sir John Ilcrschel gives of the adventures of the diffcrct comets, of tlie sad way thej' got misled and thrown out of their own individual carcer by tlie 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— ! actcd upou without apparent reaction, changed from comets of one period of revolution to comets ' of a very different period—is much more inter- ' esting than most novels ; and the speculation as L " to the probable constitution of comets with " which he concludes, is a dream of quite new ' philosophical possibilities. But take, first, this ' exquisite little bit of eometary biography : —

On tho 27th February, IS2C, Professor Biela, an Austrian, astronomer of Jose phi t.idt, d.seovercd a small eomot. When its motions were carofully studic;! it was found bv M. Uatuoa, another nf tlioso

indefatigable Gcrm?.n compulists, that it revolved in an ecliptic orbit i i a perixl of six yeirs and oight months, on looking back into tho list of comets that had been observed in 1772, 1805, and perhaps iu 1817- Its return was according!r predicted, an ! tlio prediction verified with tho mo3t striking exactness- And this went on regularly till its appearance

(also predicted) in ISIG. In that j'Oar it w,is observed us usual, anu all ECjined to be going on quietly and comfortably, -when, behold! suddenly on tho l3tli of January, it split i to two distinct comets! ouch with a head and a co rn mid a little nucleus o' its own. There is .some little contradiction about tlio exact dato; Lieutenant Muury, of tlio United States Observatory of Washington, reported officially on the having seen it double ou thelSl/i, but Professor Wichman who saw it double on the 1 <th avers that lio hod a good view of it on the l-l-//i, and remarked nothing particular in its appearance. Bs that, as it may, the comet from a single became a doulle one. What domeslic troubles caused the secession it is impossible ti conjecture, but the tvo receded farther ard farther from each other up to n cortiin mod-rate distance, with some degree of mutual communication and a very odd intercliango of light—ono day ono head being brighter, and another the other—till they seem to have agreed finally to part company. The odi'.est part of the story, however, is yet to comj. Tho year 1852 brought round Ihi time for their reappearance, and behold! there they both were, at about the same distance from cich other, and both visible in one tolescopo. Tho orbit of this comet very nearly indeed intorsects that of the earth on tho place which tho ea'tli occupies on tho 30th of November. If ever the earili is to be swallowed up by a comet, or to swallow np one, it will bo On or ab.<ut that day ofthn year In tlio year 1832 we missed it by a month. The head of tho comet enveloped that point of our orbit, but this happened on the 29th of October, so that we escaped that timo. Had a meeting taken place, from what wo know of comets, it is most probable that no harm would have happened, and that nobody would have l.niwn nnything about it. It would appear that we are happily relieved from the dread of such a collision. It is now (February 18GG) overdue ! Its orbit lias been recomputed and an ephotneris calculated. Astronomers have beon looking eagerly out for its reappearance for tho last two months, when, according to all former experience, it ought to have boon conspicuously visible, but without success, giving rise to the strangest '.heorics. At all events, it se:ms to have fairly disappeared, and that wi'hout any such excuse as in tho case of Ltxell's, the preponderant attraction of some great planet. Can it have come in contact or exceedingly close approach to sonio asteroid as yot undiscovered ; or peradventuro plunged into and got bewildered among the ring of meteorolitca, which astronomers more than suspect ? Here is a comet dividing, as, it is said, worms cut iu two Trill do into two quite independent comets, whicli sail as consorts for a few years in the sky, return at the right moment still in company and then, when tliey are expected back once again, plunge into invisibility as if they had both Rono down together in a squall of the celestial firmament. Singular and most fascinating in its suggestion of philosophic vistas is Sir John Herschcl's final speculation as to the sun's probable analysis of the matter of which comets are composed into two components—one, matter on which the sun exorcises an attractive force as it does on tho material of all our planets —tlieother,matter compOFingagreat part of what is called the tail of tho comets, on which this influence is not attractive, but repulsive, and which recedes further and further into space as the head of tho comet approaches the sun. Sir John Ilerscliel suggests that, just as St. Clair Deville has shown that the chemical afliuity between tho oxygen and hydrogen of which water consists is so much weakened by a very high tcmporature, that " the mere difficulty in traversing an earthenware tub, suffices to set them frco of one another," so the action of the sun's lieat might weaken sufficiently tho atomic bond of union between that portion of tho comctary matter which the sun attracts, aud that portion which it repels; that at every return to tho neighbourhood of the sun, a good deal of tho matter liable to repulsion by the sun should be cast off in space, and the rest more and more contracted till it settles down into the com paratively hard nucleus of a planet. The plausibility of this theory is that those comets which have very little or lio tail;?, like Eneke's comet, always contract after passing round the sun (passing perihelion) ..'"id «- panel again as tliey get to a distance. Sir John Herscliel suggests that at each visit this temporary evaporation, 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 cut off wholly into space. What a train of speculation the mere existence of elements in cemctary matter liable to repulsion from the sun, instead of attraction to it, suggests ! If any such elements of matter linger, bound up m 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 now are, to theearthandsolavsystem bythe law of gravitation, be, by the very force of repulsion, projected into universes beyond the solar univcrso to which wo belong. At present, the highest idea we have of physical impossibility 'is of corporeal frames getting beyond tne attraction of the earth, and still more of the solar central force ; but if there be a sort of matter imprisonable in gravitating matter, and get also separable from it, which, when separate, is simply repelled by tlic sun tlien tliere is a Lmd of material frame which might (conceivably) be made in this system and yet made by the law of its nature to travel out of it. We have noted but one or two points in a book of the most profound and romantic scientific charm. We leave our readers to find out inuumerabie others for themselves.