a history of science-1-第52章
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cean is divided into two seas by narrow isthmuses so placed as to prevent circumnavigation。 〃How much more probable that it is confluent and uninterrupted。 This theory；〃 he adds； 〃goes better with the ebb and flow of the ocean。 Moreover （and here his reasoning becomes more fanciful）； the greater the amount of moisture surrounding the earth； the easier would the heavenly bodies be supplied with vapor from thence。〃 Yet he is disposed to believe； following Plato； that the tradition 〃concerning the island of Atlantos might be received as something more than idle fiction； it having been related by Solon； on the authority of the Egyptian priests； that this island； almost as large as a continent； was formerly in existence although now it had disappeared。〃＇2＇ In a word； then； Strabo entertains no doubt whatever that it would be possible to sail around the globe from Spain to India。 Indeed； so matter…of…fact an inference was this that the feat of Columbus would have seemed less surprising in the first century of our era than it did when actually performed in the fifteenth century。 The terrors of the great ocean held the mariner back； rather than any doubt as to where he would arrive at the end of the voyage。 Coupled with the idea that the habitable portion of the earth is an island； there was linked a tolerably definite notion as to the shape of this island。 This shape Strabo likens to a military cloak。 The comparison does not seem peculiarly apt when we are told presently that the length of the habitable earth is more than twice its breadth。 This idea； Strabo assures us； accords with the most accurate observations 〃both ancient and modern。〃 These observations seemed to show that it is not possible to live in the region close to the equator； and that； on the other hand； the cold temperature sharply limits the habitability of the globe towards the north。 All the civilization of antiquity clustered about the Mediterranean； or extended off towards the east at about the same latitude。 Hence geographers came to think of the habitable globe as having the somewhat lenticular shape which a crude map of these regions suggests。 We have already had occasion to see that at an earlier day Anaxagoras was perhaps influenced in his conception of the shape of the earth by this idea； and the constant references of Strabo impress upon us the thought that this long； relatively narrow area of the earth's surface is the only one which can be conceived of as habitable。 Strabo had much to tell us concerning zones； which； following Posidonius； he believes to have been first described by Parmenides。 We may note； however； that other traditions assert that both Thales and Pythagoras had divided the earth into zones。 The number of zones accepted by Strabo is five； and he criticises Polybius for making the number six。 The five zones accepted by Strabo are as follows： the uninhabitable torrid zone lying in the region of the equator； a zone on either side of this extending to the tropic； and then the temperate zones extending in either direction from the tropic to the arctic regions。 There seems to have been a good deal of dispute among the scholars of the time as to the exact arrangement of these zones； but the general idea that the north…temperate zone is the part of the earth with which the geographer deals seemed clearly established。 That the south…temperate zone would also present a habitable area is an idea that is sometimes suggested； though seldom or never distinctly expressed。 It is probable that different opinions were held as to this； and no direct evidence being available； a cautiously scientific geographer like Strabo would naturally avoid the expression of an opinion regarding it。 Indeed； his own words leave us somewhat in doubt as to the precise character of his notion regarding the zones。 Perhaps we shall do best to quote them： 〃Let the earth be supposed to consist of five zones。 （1） The equatorial circle described around it。 （2） Another parallel to this； and defining the frigid zone of the northern hemisphere。 （3） A circle passing through the poles and cutting the two preceding circles at right… angles。 The northern hemisphere contains two quarters of the earth； which are bounded by the equator and circle passing through the poles。 Each of these quarters should be supposed to contain a four…sided district； its northern side being of one…half of the parallel next the pole； its southern by the half of the equator； and its remaining sides by two segments of the circle drawn through the poles； opposite to each other； and equal in length。 In one of these （which of them is of no consequence） the earth which we inhabit is situated； surrounded by a sea and similar to an island。 This； as we said before； is evident both to our senses and to our reason。 But let any one doubt this； it makes no difference so far as geography is concerned whether you believe the portion of the earth which we inhabit to be an island or only admit what we know from experience namely； that whether you start from the east or the west you may sail all around it。 Certain intermediate spaces may have been left （unexplored）； but these are as likely to be occupied by sea as uninhabited land。 The object of the geographer is to describe known countries。 Those which are unknown he passes over equally with those beyond the limits of the inhabited earth。 It will； therefore； be sufficient for describing the contour of the island we have been speaking of； if we join by a right line the outmost points which； up to this time； have been explored by voyagers along the coast on either side。〃＇3＇ We may pass over the specific criticisms of Strabo upon various explorations that seem to have been of great interest to his contemporaries； including an alleged trip of one Eudoxus out into the Atlantic； and the journeyings of Pytheas in the far north。 It is Pytheas； we may add； who was cited by Hipparchus as having made the mistaken observation that the length of the shadow of the gnomon is the same at Marseilles and Byzantium； hence that these two places are on the same parallel。 Modern commentators have defended Pytheas as regards this observation； claiming that it was Hipparchus and not Pytheas who made the second observation from which the faulty induction was drawn。 The point is of no great significance； however； except as showing that a correct method of determining the problems of latitude had thus early been suggested。 That faulty observations and faulty application of the correct principle should have been made is not surprising。 Neither need we concern ourselves with the details as to the geographical distances； which Strabo found so worthy of criticism and controversy。 But in leaving the great geographer we may emphasize his point of view and that of his contemporaries by quoting three fundamental principles which he reiterates as being among the 〃facts established by natural philosophers。〃 He tells us that 〃（1） The earth and heavens are spheroidal。 （2） The tendency of all bodies having weight is towards a centre。 （3） Further； the earth being spheroidal and having the same centre as the heavens； is motionless； as well as the axis that passes through both it and the heavens。 The heavens turn round both the earth and its axis； from east to west。 The fixed stars turn round with it at the same rate as the whole。 These fixed stars follow in their course parallel circles； the principal of which are the equator； two tropics； and the arctic circles； while the planets； the sun； and the moon describe certain circles comprehended within the zodiac。〃＇4＇ Here； then； is a curious mingling of truth and error。 The Pythagorean doctrine that the earth is round had become a commonplace； but it would appear that the theory of Aristarchus； according to which the earth is in motion； has been almost absolutely forgotten。 Strabo does not so much as refer to it； neither； as we shall see； is it treated with greater respect by the other writers of the period。
TWO FAMOUS EXPOSITORSPLINY AND PTOLEMY While Strabo was pursuing his geographical studies at Alexandria； a young man came to Rome who was destined to make his name more widely known in scientific annals than that of any other Latin writer of antiquity。 This man was Plinius S
