RSS

Dec/09

the outline of science Page 6

0 Comments | Posted by admin in 英文

matter, and been (at least in part) vaporised by the friction.

But the difficulties are considerable, and some astronomers prefer tothink that the blazing star may merely have lit up a dark nebula whichalready existed. It is one of those problems on which speculation ismost tempting but positive knowledge is still very incomplete. We may becontent, even proud, that already we can take a conflagration that hasoccurred more than a thousand trillion miles away and analyse itpositively into an outflame of glowing hydrogen gas at so many miles asecond.

THE SHAPE OF OUR UNIVERSE

§ 4

Our Universe a Spiral Nebula

What is the shape of our universe, and what are its dimensions? This isa tremendous question to ask. It is like asking an intelligent insect,living on a single leaf in the midst of a great Brazilian forest, to saywhat is the shape and size of the forest. Yet man’s ingenuity has provedequal to giving an answer even to this question, and by a method exactlysimilar to that which would be adopted by the insect. Suppose, forinstance, that the forest was shaped as an elongated oval, and theinsect lived on a tree near the centre of the oval. If the trees wereapproximately equally spaced from one another they would appear muchdenser along the length of the oval than across its width. This is thesimple consideration that has guided astronomers in determining theshape of our stellar universe. There is one direction in the heavensalong which the stars appear denser than in the directions at rightangles to it. That direction is the direction in which we look towardsthe Milky Way. If we count the number of stars visible all over theheavens, we find they become more and more numerous as we approach theMilky Way. As we go farther and farther from the Milky Way the starsthin out until they reach a maximum sparseness in directions at rightangles to the plane of the Milky Way. We may consider the Milky Way toform, as it were, the equator of our system, and the line at rightangles to point to the north and south poles.

Our system, in fact, is shaped something like a lens, and our sun issituated near the centre of this lens. In the remoter part of this lens,near its edge, or possibly outside it altogether, lies the great seriesof star clouds which make up the Milky Way. All the stars are in motionwithin this system, but the very remarkable discovery has been made thatthese motions are not entirely random. The great majority of the starswhose motions can be measured fall into two groups drifting past oneanother in opposite directions. The velocity of one stream relative tothe other is about twenty-five miles per second. The stars forming thesetwo groups are thoroughly well mixed; it is not a case of an innerstream going one way and an outer stream the other. But there are notquite as many stars going one way as the other. For every two stars inone stream there are three in the other. Now, as we have said, someeminent astronomers hold that the spiral nebulæ are universes like ourown, and if we look at the two photographs (Figs. 25 and 26) we see thatthese spirals present features which, in the light of what we have justsaid about our system, are very remarkable. The nebula in Coma Berenicesis a spiral edge-on to us, and we see that it has precisely thelens-shaped middle and the general flattened shape that we have found inour own system. The nebula in Canes Venatici is a spiral facing towardsus, and its shape irresistibly suggests motions along the spiral arms.This motion, whether it is towards or away from the central, lens-shapedportion, would cause a double streaming motion in that central portionof the kind we have found in our own system. Again, and altogether apartfrom these considerations, there are good reasons for supposing ourMilky Way to possess a double-armed spiral structure. And the greatpatches of dark absorbing matter which are known to exist in the MilkyWay (see Fig. 22) would give very much the mottled appearance we noticein the arms (which we see edge-on) of the nebula in Coma Berenices. Thehypothesis, therefore, that our universe is a spiral nebula has much tobe said for it. If it be accepted it greatly increases our estimate ofthe size of the material universe. For our central, lens-shaped systemis calculated to extend towards the Milky Way for more than twentythousand times a million million miles, and about a third of thisdistance towards what we have called the poles. If, as we suppose, eachspiral nebula is an independent stellar universe comparable in size withour own, then, since there are hundreds of thousands of spiral nebulæ,we see that the size of the whole material universe is indeed beyond ourcomprehension.

[Illustration: _Photo: Mount Wilson Observatory._

FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON

Notice the lens-shaped formation of the nucleus and the arm stretchingas a band across it. See reference in the text to the resemblancebetween this and our stellar universe.]

[Illustration: _Photo: H. J. Shepstone._

100-INCH TELESCOPE, MOUNT WILSON

A reflecting telescope: the largest in the world. The mirror is situatedat the base of the telescope.]

[Illustration:

________________________________________________________________ | | | THE SOLAR SYSTEM | |________________________________________________________________| | | | | | | | | MEAN DISTANCE | PERIOD OF | | | | NAME | FROM SUN (IN | REVOLUTION | DIAMETER | NUMBER OF | | | MILLIONS OF | AROUND SUN | (IN MILES) | SATELLITES | | | MILES) | (IN YEARS) | | | |_________|_______________|____________|____________|____________| | | | | | | | MERCURY | 36.0 | 0.24 | 3030 | 0 | | VENUS | 67.2 | 0.62 | 7700 | 0 | | EARTH | 92.9 | 1.00 | 7918 | 1 | | MARS | 141.5 | 1.88 | 4230 | 2 | | JUPITER | 483.3 | 11.86 | 86500 | 9 | | SATURN | 886.0 | 29.46 | 73000 | 10 | | URANUS | 1781.9 | 84.02 | 31900 | 4 | | NEPTUNE | 2971.6 | 164.78 | 34800 | 1 | | SUN | ------ | ------ | 866400 | -- | | MOON | ------ | ------ | 2163 | -- | |_________|_______________|____________|____________|____________|

FIG. 27]

[Illustration:

[A] ESTIMATED

FIG. 28

The above distances are merely approximate and are subject to furtherrevision. A “light-year” is the distance that light, travelling at therate of 186,000 miles per second, would cover in one year.]

In this simple outline we have not touched on some of the more debatablequestions that engage the attention of modern astronomers. Many of thesequestions have not yet passed the controversial stage; out of these willemerge the astronomy of the future. But we have seen enough to convinceus that, whatever advances the future holds in store, the science of theheavens constitutes one of the most important stones in the wonderfulfabric of human knowledge.

ASTRONOMICAL INSTRUMENTS

§ 1

The Telescope

The instruments used in modern astronomy are amongst the finest triumphsof mechanical skill in the world. In a great modern observatory thedifferent instruments are to be counted by the score, but there are twowhich stand out pre-eminent as the fundamental instruments of modernastronomy. These instruments are the telescope and the spectroscope, andwithout them astronomy, as we know it, could not exist.

There is still some dispute as to where and when the first telescope wasconstructed; as an astronomical instrument, however, it dates from thetime of the great Italian scientist Galileo, who, with a very small andimperfect telescope of his own invention, first observed the spots onthe sun, the mountains of the moon, and the chief four satellites ofJupiter. A good pair of modern binoculars is superior to this earlyinstrument of Galileo’s, and the history of telescope construction, fromthat primitive instrument to the modern giant recently erected on MountWilson, California, is an exciting chapter in human progress. But theearly instruments have only an historic interest: the era of moderntelescopes begins in the nineteenth century.

During the last century telescope construction underwent anunprecedented development. An immense amount of interest was taken inthe construction of large telescopes, and the different countries of theworld entered on an exciting race to produce the most powerful possibleinstruments. Besides this rivalry of different countries there was arivalry of methods. The telescope developed along two different lines,and each of these two types has its partisans at the present day. Thesetypes are known as _refractors_ and _reflectors_, and it is necessary tomention, briefly, the principles employed in each. The _refractor_ isthe ordinary, familiar type of telescope. It consists, essentially, of alarge lens at one end of a tube, and a small lens, called the eye-piece,at the other. The function of the large lens is to act as a sort ofgigantic eye. It collects a large amount of light, an amountproportional to its size, and brings this light to a focus within thetube of the telescope. It thus produces a small but bright image, andthe eye-piece magnifies this image. In the _reflector_, instead of alarge lens at the top of the tube, a large mirror is placed at thebottom. This mirror is so shaped as to reflect the light that falls onit to a focus, whence the light is again led to an eye-piece. Thus therefractor and the reflector differ chiefly in their manner of gatheringlight. The powerfulness of the telescope depends on the size of thelight-gatherer. A telescope with a lens four inches in diameter is fourtimes as powerful as the one with a lens two inches in diameter, for theamount of light gathered obviously depends on the _area_ of the lens,and the area varies as the _square_ of the diameter.

The largest telescopes at present in existence are _reflectors_. It ismuch easier to construct a very large mirror than to construct a verylarge lens; it is also cheaper. A mirror is more likely to get out oforder than is a lens, however, and any irregularity in the shape of amirror produces a greater distorting effect than in a lens. A refractoris also more convenient to handle than is a reflector. For these reasonsgreat refractors are still made, but the largest of them, the greatYerkes’ refractor, is much smaller than the greatest reflector, the oneon Mount Wilson, California. The lens of the Yerkes’ refractor measuresthree feet four inches in diameter, whereas the Mount Wilson reflectorhas a diameter of no less than eight feet four inches.

[Illustration: THE YERKES 40-INCH REFRACTOR

(The largest _refracting_ telescope in the world. Its big lens weighs1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about12,000 pounds. The parts to be moved weigh approximately 22 tons.

The great _100-inch reflector_ of the Mount Wilson reflectingtelescope--the largest _reflecting_ instrument in the world--weighsnearly 9,000 pounds and the moving parts of the telescope weigh about100 tons.

The new _72-inch reflector_ at the Dominion Astrophysical Observatory,near Victoria, B. C., weighs nearly 4,500 pounds, and the moving partsabout 35 tons.)]

[Illustration: _Photo: H. J. Shepstone._

THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE

The smaller telescope at the top of the picture acts as a "finder"; thefield of view of the large telescope is so restricted that it isdifficult to recognise, as it were, the part of the heavens beingsurveyed. The smaller telescope takes in a larger area and enables theprecise object to be examined to be easily selected.]

[Illustration: MODERN DIRECT-READING SPECTROSCOPE

(_By A. Hilger, Ltd._)

The light is brought through one telescope, is split up by the prism,and the resulting spectrum is observed through the other telescope.]

But there is a device whereby the power of these giant instruments,great as it is, can be still further heightened. That device is thesimple one of allowing the photographic plate to take the place of thehuman eye. Nowadays an astronomer seldom spends the night with his eyeglued to the great telescope. He puts a photographic plate there. Thephotographic plate has this advantage over the eye, that it builds upimpressions. However long we stare at an object too faint to be seen, weshall never see it. With the photographic plate, however, faintimpressions go on accumulating. As hour after hour passes, the starwhich was too faint to make a perceptible impression on the plate goeson affecting it until finally it makes an impression which can be madevisible. In this way the photographic plate reveals to us phenomena inthe heavens which cannot be seen even through the most powerfultelescopes.

Telescopes of the kind we have been discussing, telescopes for exploringthe heavens, are mounted _equatorially_; that is to say, they aremounted on an inclined pillar parallel to the axis of the earth so that,by rotating round this pillar, the telescope is enabled to follow theapparent motion of a star due to the rotation of the earth. This motionis effected by clock-work, so that, once adjusted on a star, and theclock-work started, the telescope remains adjusted on that star for anylength of time that is desired. But a great official observatory, suchas Greenwich Observatory or the Observatory at Paris, also has _transit_instruments, or telescopes smaller than the equatorials and without thesame facility of movement, but which, by a number of exquisiterefinements, are more adapted to accurate measurements. It is theseinstruments which are chiefly used in the compilation of the _NauticalAlmanac_. They do not follow the apparent motions of the stars. Starsare allowed to drift across the field of vision, and as each starcrosses a small group of parallel wires in the eye-piece its precisetime of passage is recorded. Owing to their relative fixity of positionthese instruments can be constructed to record the _positions_ of starswith much greater accuracy than is possible to the more general andflexible mounting of equatorials. The recording of transit iscomparatively dry work; the spectacular element is entirely absent;stars are treated merely as mathematical points. But these observationsfurnish the very basis of modern mathematical astronomy, and withoutthem such publications as the _Nautical Almanac_ and the _Connaissancedu Temps_ would be robbed of the greater part of their importance.

§ 2

The Spectroscope

We have already learnt something of the principles of the spectroscope,the instrument which, by making it possible to learn the actualconstitution of the stars, has added a vast new domain to astronomy. Inthe simplest form of this instrument the analysing portion consists of asingle prism. Unless the prism is very large, however, only a smalldegree of dispersion is obtained. It is obviously desirable, foraccurate analytical work, that the dispersion–that is, the separationof the different parts of the spectrum–should be as great as possible.The dispersion can be increased by using a large number of prisms, thelight emerging from the first prism, entering the second, and so on. Inthis way each prism produces its own dispersive effect and, when anumber of prisms are employed, the final dispersion is considerable. Aconsiderable amount of light is absorbed in this way, however, so thatunless our primary source of light is very strong, the final spectrumwill be very feeble and hard to decipher.

Another way of obtaining considerable dispersion is by using a_diffraction grating_ instead of a prism. This consists essentially of apiece of glass on which lines are ruled by a diamond point. When thelines are sufficiently close together they split up light falling onthem into its constituents and produce a spectrum. The moderndiffraction grating is a truly wonderful piece of work. It containsseveral thousands of lines to the inch, and these lines have to bespaced with the greatest accuracy. But in this instrument, again, thereis a considerable loss of light.

We have said that every substance has its own distinctive spectrum, andit might be thought that, when a list of the spectra of differentsubstances has been prepared, spectrum analysis would become perfectlystraightforward. In practice, however, things are not quite so simple.The spectrum emitted by a substance is influenced by a variety ofconditions. The pressure, the temperature, the state of motion of theobject we are observing, all make a difference, and one of the mostlaborious tasks of the modern spectroscopist is to disentangle theseeffects from one another. Simple as it is in its broad outlines,spectroscopy is, in reality, one of the most intricate branches ofmodern science.

BIBLIOGRAPHY

(The following list of books may be useful to readers wishing to pursuefurther the study of Astronomy.)

BALL, _The Story of the Heavens_. BALL, _The Story of the Sun_. FORBES, _History of Astronomy_. HINCKS, _Astronomy_. KIPPAX, _Call of the Stars_. LOWELL, _Mars and Its Canals_. LOWELL, _Evolution of Worlds_. MCKREADY, _A Beginner’s Star-Book_. NEWCOMB, _Popular Astronomy_. NEWCOMB, _The Stars: A Study of the Universe_. OLCOTT, _Field Book of the Stars_. PRICE, _Essence of Astronomy_. SERVISS, _Curiosities of the Skies_. WEBB, _Celestial Objects for Common Telescopes_. YOUNG, _Text-Book of General Astronomy_.

II

THE STORY OF EVOLUTION

INTRODUCTORY

THE BEGINNING OF THE EARTH–MAKING A HOME FOR LIFE–THE FIRST LIVINGCREATURES

§ 1

The Evolution-idea is a master-key that opens many doors. It is aluminous interpretation of the world, throwing the light of the pastupon the present. Everything is seen to be an antiquity, with a historybehind it–a _natural history_, which enables us to understand in somemeasure how it has come to be as it is. We cannot say more than”understand in some measure,” for while the _fact_ of evolution iscertain, we are only beginning to discern the _factors_ that have beenat work.

The evolution-idea is very old, going back to some of the Greekphilosophers, but it is only in modern times that it has become anessential part of our mental equipment. It is now an everydayintellectual tool. It was applied to the origin of the solar system andto the making of the earth before it was applied to plants and animals;it was extended from these to man himself; it spread to language, tofolk-ways, to institutions. Within recent years the evolution-idea hasbeen applied to the chemical elements, for it appears that uranium maychange into radium, that radium may produce helium, and that lead is thefinal stable result when the changes of uranium are complete. Perhapsall the elements may be the outcome of an inorganic evolution. Not lessimportant is the extension of the evolution-idea to the world within aswell as to the world without. For alongside of the evolution of bodiesand brains is the evolution of feelings and emotions, ideas andimagination.

Organic evolution means that the present is the child of the past andthe parent of the future. It is not a power or a principle; it is aprocess–a process of becoming. It means that the present-day animalsand plants and all the subtle inter-relations between them have arisenin a natural knowable way from a preceding state of affairs on the wholesomewhat simpler, and that again from forms and inter-relations simplerstill, and so on backwards and backwards for millions of years till welose all clues in the thick mist that hangs over life’s beginnings.

Our solar system was once represented by a nebula of some sort, and wemay speak of the evolution of the sun and the planets. But since it hasbeen _the same material throughout_ that has changed in its distributionand forms, it might be clearer to use some word like genesis. Similarly,our human institutions were once very different from what they are now,and we may speak of the evolution of government or of cities. But Manworks with a purpose, with ideas and ideals in some measure controllinghis actions and guiding his achievements, so that it is probably clearerto keep the good old word history for all processes of social becomingin which man has been a conscious agent. Now between the genesis of thesolar system and the history of civilisation there comes the vastprocess of organic evolution. The word development should be kept forthe becoming of the individual, the chick out of the egg, for instance.

Organic evolution is a continuous natural process of racial change, bysuccessive steps in a definite direction, whereby distinctively newindividualities arise, take root, and flourish, sometimes alongside of,and sometimes, sooner or later, in place of, the originative stock. Ourdomesticated breeds of pigeons and poultry are the results ofevolutionary change whose origins are still with us in the Rock Dove andthe Jungle Fowl; but in most cases in Wild Nature the ancestral stocksof present-day forms are long since extinct, and in many cases they areunknown. Evolution is a long process of coming and going, appearing anddisappearing, a long-drawn-out sublime process like a great piece ofmusic.

[Illustration: _Photo: Rischgitz Collection._

CHARLES DARWIN

Greatest of naturalists, who made the idea of evolution currentintellectual coin, and in his _Origin of Species_ (1859) made the wholeworld new.]

[Illustration: _Photo: Rischgitz Collection._

LORD KELVIN

One of the greatest physicists of the nineteenth century. He estimatedthe age of the earth at 20,000,000 years. He had not at his disposal,however, the knowledge of recent discoveries, which have resulted inthis estimate being very greatly increased.]

[Illustration: _Photo: Lick Observatory._

A GIANT SPIRAL NEBULA

Laplace's famous theory was that the planets and the earth were formedfrom great whirling nebulæ.]

[Illustration: _Photo: Natural History Museum._

METEORITE WHICH FELL NEAR SCARBOROUGH, AND IS NOW TO BE SEEN IN THENATURAL HISTORY MUSEUM

It weighs about 56 lb., and is a "stony" meteorite, i.e., an aerolite.]

§ 2

The Beginning of the Earth

When we speak the language of science we cannot say “In the beginning,”for we do not know of and cannot think of any condition of things thatdid not arise from something that went before. But we may qualify thephrase, and legitimately inquire into the beginning of the earth withinthe solar system. If the result of this inquiry is to trace the sun andthe planets back to a nebula we reach only a relative beginning. Thenebula has to be accounted for. And even before matter there may havebeen a pre-material world. If we say, as was said long ago, “In thebeginning was Mind,” we may be expressing or trying to express a greattruth, but we have gone BEYOND SCIENCE.

The Nebular Hypothesis

One of the grandest pictures that the scientific mind has ever thrownupon the screen is that of the Nebular Hypothesis. According toLaplace’s famous form of this theory (1796), the solar system was once agigantic glowing mass, spinning slowly and uniformly around its centre.As the incandescent world-cloud of gas cooled and its speed of rotationincreased the shrinking mass gave off a separate whirling ring, whichbroke up and gathered together again as the first and most distant

No tags