a history of science-1-第30章
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d to the science of astronomy。 Looking back from the latter…day stand…point； it would seem as if the mind of the philosopher must have taken one other step： the mind that had conceived sun； moon； stars； and earth to be of one substance might naturally； we should think； have reached out to the further induction that； since the moon is a sphere； the other cosmic bodies； including the earth； must be spheres also。 But generalizer as he was； Anaxagoras was too rigidly scientific a thinker to make this assumption。 The data at his command did not； as he analyzed them； seem to point to this conclusion。 We have seen that Pythagoras probably； and Parmenides surely； out there in Italy had conceived the idea of the earth's rotundity； but the Pythagorean doctrines were not rapidly taken up in the mother… country； and Parmenides； it must be recalled； was a strict contemporary of Anaxagoras himself。 It is no reproach； therefore； to the Clazomenaean philosopher that he should have held to the old idea that the earth is flat； or at most a convex diskthe latter being the Babylonian conception which probably dominated that Milesian school to which Anaxagoras harked back。 Anaxagoras may never have seen an eclipse of the moon； and even if he had he might have reflected that； from certain directions； a disk may throw precisely the same shadow as a sphere。 Moreover； in reference to the shadow cast by the earth； there was； so Anaxagoras believed； an observation open to him nightly which； we may well suppose； was not without influence in suggesting to his mind the probable shape of the earth。 The Milky Way； which doubtless had puzzled astronomers from the beginnings of history and which was to continue to puzzle them for many centuries after the day of Anaxagoras； was explained by the Clazomenaean philosopher on a theory obviously suggested by the theory of the moon's phases。 Since the earth… like moon shines by reflected light at night； and since the stars seem obviously brighter on dark nights； Anaxagoras was but following up a perfectly logical induction when he propounded the theory that the stars in the Milky Way seem more numerous and brighter than those of any other part of the heavens； merely because the Milky Way marks the shadow of the earth。 Of course the inference was wrong； so far as the shadow of the earth is concerned； yet it contained a part truth； the force of which was never fully recognized until the time of Galileo。 This consists in the assertion that the brightness of the Milky Way is merely due to the glow of many stars。 The shadow… theory of Anaxagoras would naturally cease to have validity so soon as the sphericity of the earth was proved； and with it； seemingly； fell for the time the companion theory that the Milky Way is made up of a multitude of stars。 It has been said by a modern critic＇1＇ that the shadow…theory was childish in that it failed to note that the Milky Way does not follow the course of the ecliptic。 But this criticism only holds good so long as we reflect on the true character of the earth as a symmetrical body poised in space。 It is quite possible to conceive a body occupying the position of the earth with reference to the sun which would cast a shadow having such a tenuous form as the Milky Way presents。 Such a body obviously would not be a globe； but a long…drawn…out； attenuated figure。 There is； to be sure； no direct evidence preserved to show that Anaxagoras conceived the world to present such a figure as this； but what we know of that philosopher's close…reasoning； logical mind gives some warrant to the assumptiongratuitous though in a sense it be that the author of the theory of the moon's phases had not failed to ask himself what must be the form of that terrestrial body which could cast the tenuous shadow of the Milky Way。 Moreover； we must recall that the habitable earth； as known to the Greeks of that day； was a relatively narrow band of territory； stretching far to the east and to the west。
Anaxagoras as Meteorologist The man who had studied the meteorite of aegospotami； and been put by it on the track of such remarkable inductions； was； naturally； not oblivious to the other phenomena of the atmosphere。 Indeed； such a mind as that of Anaxagoras was sure to investigate all manner of natural phenomena； and almost equally sure to throw new light on any subject that it investigated。 Hence it is not surprising to find Anaxagoras credited with explaining the winds as due to the rarefactions of the atmosphere produced by the sun。 This explanation gives Anaxagoras full right to be called 〃the father of meteorology；〃 a title which； it may be； no one has thought of applying to him； chiefly because the science of meteorology did not make its real beginnings until some twenty…four hundred years after the death of its first great votary。 Not content with explaining the winds； this prototype of Franklin turned his attention even to the tipper atmosphere。 〃Thunder；〃 he is reputed to have said； 〃was produced by the collision of the clouds； and lightning by the rubbing together of the clouds。〃 We dare not go so far as to suggest that this implies an association in the mind of Anaxagoras between the friction of the clouds and the observed electrical effects generated by the friction of such a substance as amber。 To make such a suggestion doubtless would be to fall victim to the old familiar propensity to read into Homer things that Homer never knew。 Yet the significant fact remains that Anaxagoras ascribed to thunder and to lightning their true position as strictly natural phenomena。 For him it was no god that menaced humanity with thundering voice and the flash of his divine fires from the clouds。 Little wonder that the thinker whose science carried him to such scepticism as this should have felt the wrath of the superstitious Athenians。
Biological Speculations Passing from the phenomena of the air to those of the earth itself； we learn that Anaxagoras explained an earthquake as being produced by the returning of air into the earth。 We cannot be sure as to the exact meaning here； though the idea that gases are imprisoned in the substance of the earth seems not far afield。 But a far more remarkable insight than this would imply was shown by Anaxagoras when he asserted that a certain amount of air is contained in water； and that fishes breathe this air。 The passage of Aristotle in which this opinion is ascribed to Anaxagoras is of sufficient interest to be quoted at length： 〃Democritus； of Abdera；〃 says Aristotle； 〃and some others； that have spoken concerning respiration； have determined nothing concerning other animals； but seem to have supposed that all animals respire。 But Anaxagoras and Diogenes （Apolloniates）； who say that all animals respire； have also endeavored to explain how fishes； and all those animals that have a hard； rough shell； such as oysters； mussels； etc。； respire。 And Anaxagoras； indeed； says that fishes； when they emit water through their gills； attract air from the mouth to the vacuum in the viscera from the water which surrounds the mouth； as if air was inherent in the water。〃＇2＇ It should be recalled that of the three philosophers thus mentioned as contending that all animals respire； Anaxagoras was the elder； he； therefore； was presumably the originator of the idea。 It will be observed； too； that Anaxagoras alone is held responsible for the idea that fishes respire air through their gills； 〃attracting〃 it from the water。 This certainly was one of the shrewdest physiological guesses of any age； if it be regarded as a mere guess。 With greater justice we might refer to it as a profound deduction from the principle of the uniformity of nature。 In making such a deduction； Anaxagoras was far in advance of his time as illustrated by the fact that Aristotle makes the citation we have just quoted merely to add that 〃such things are impossible；〃 and to refute these 〃impossible〃 ideas by means of metaphysical reasonings that seemed demonstrative not merely to himself； but to many generations of his followers。 We are told that Anaxagoras alleged that all animals were originally generated out of moisture； heat； and earth particles。 Just what opinion he held concerning man's development we are not informed。 Y
