按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
so it must needs remain still。 This view is ingenious but not true。 The argument would prove that everything； whatever it be； which is put at the centre； must stay there。 Fire； then； will rest at the centre： for the proof turns on no peculiar property of earth。 But this does not follow。 The observed facts about earth are not only that it remains at the centre； but also that it moves to the centre。 The place to which any fragment of earth moves must necessarily be the place to which the whole moves； and in the place to which a thing naturally moves； it will naturally rest。 The reason then is not in the fact that the earth is indifferently related to every extreme point： for this would apply to any body； whereas movement to the centre is peculiar to
earth。 Again it is absurd to look for a reason why the earth remains at the centre and not for a reason why fire remains at the extremity。 If the extremity is the natural place of fire； clearly earth must also have a natural place。 But suppose that the centre is not its place； and that the reason of its remaining there is this necessity of indifference…on the analogy of the hair which； it is said； however great the tension； will not break under it； if it be evenly distributed； or of the men who； though exceedingly hungry and thirsty； and both equally； yet being equidistant from food and drink； is therefore bound to stay where he is…even so； it still remains to explain why fire stays at the extremities。 It is strange； too； to ask about things staying still but not about their motion；…why； I mean； one thing； if nothing stops it； moves up； and another thing to the centre。 Again； their statements are not true。 It happens； indeed； to be the case that a thing to which movement this way and that is equally inappropriate is obliged to remain at the centre。 But so far as their argument goes； instead of remaining there； it will move； only not as a mass but in fragments。 For the argument applies equally to fire。 Fire； if set at the centre； should stay there； like earth； since it will be indifferently related to every point on the extremity。 Nevertheless it will move； as in fact it always does move when nothing stops it； away from the centre to the extremity。 It will not； however； move in a mass to a single point on the circumference…the only possible result on the lines of the indifference theory…but rather each corresponding portion of fire to the corresponding part of the extremity； each fourth part； for instance； to a fourth part of the circumference。 For since no body is a point； it will have parts。 The expansion； when the body increased the place occupied； would be on the same principle as the contraction； in which the place was diminished。 Thus； for all the indifference theory shows to the contrary； earth also would have moved in this manner away from the centre； unless the centre had been its natural place。 We have now outlined the views held as to the shape； position； and rest or movement of the earth。
14
Let us first decide the question whether the earth moves or is at rest。 For； as we said； there are some who make it one of the stars； and others who； setting it at the centre； suppose it to be 'rolled' and in motion about the pole as axis。 That both views are untenable will be clear if we take as our starting…point the fact that the earth's motion； whether the earth be at the centre or away from it； must needs be a constrained motion。 It cannot be the movement of the earth itself。 If it were； any portion of it would have this movement； but in fact every part moves in a straight line to the centre。 Being； then； constrained and unnatural； the movement could not be eternal。 But the order of the universe is eternal。 Again； everything that moves with the circular movement； except the first sphere； is observed to be passed； and to move with more than one motion。 The earth； then； also； whether it move about the centre or as stationary at it； must necessarily move with two motions。 But if this were so； there would have to be passings and turnings of the fixed stars。 Yet no such thing is observed。 The same stars always rise and set in the same parts of the earth。 Further； the natural movement of the earth； part and whole alike； is the centre of the whole…whence the fact that it is now actually situated at the centre…but it might be questioned since both centres are the same； which centre it is that portions of earth and other heavy things move to。 Is this their goal because it is the centre of the earth or because it is the centre of the whole？ The goal； surely； must be the centre of the whole。 For fire and other light things move to the extremity of the area which contains the centre。 It happens； however； that the centre of the earth and of the whole is the same。 Thus they do move to the centre of the earth； but accidentally； in virtue of the fact that the earth's centre lies at the centre of the whole。 That the centre of the earth is the goal of their movement is indicated by the fact that heavy bodies moving towards the earth do not parallel but so as to make equal angles； and thus to a single centre； that of the earth。 It is clear； then； that the earth must be at the centre and immovable； not only for the reasons already given； but also because heavy bodies forcibly thrown quite straight upward return to the point from which they started； even if they are thrown to an infinite distance。 From these considerations then it is clear that the earth does not move and does not lie elsewhere than at the centre。 From what we have said the explanation of the earth's immobility is also apparent。 If it is the nature of earth； as observation shows； to move from any point to the centre； as of fire contrariwise to move from the centre to the extremity； it is impossible that any portion of earth should move away from the centre except by constraint。 For a single thing has a single movement； and a simple thing a simple： contrary movements cannot belong to the same thing； and movement away from the centre is the contrary of movement to it。 If then no portion of earth can move away from the centre； obviously still less can the earth as a whole so move。 For it is the nature of the whole to move to the point to which the part naturally moves。 Since； then； it would require a force greater than itself to move it； it must needs stay at the centre。 This view is further supported by the contributions of mathematicians to astronomy； since the observations made as the shapes change by which the order of the stars is determined； are fully accounted for on the hypothesis that the earth lies at the centre。 Of the position of the earth and of the manner of its rest or movement； our discussion may here end。 Its shape must necessarily be spherical。 For every portion of earth has weight until it reaches the centre； and the jostling of parts greater and smaller would bring about not a waved surface； but rather compression and convergence of part and part until the centre is reached。 The process should be conceived by supposing the earth to come into being in the way that some of the natural philosophers describe。 Only they attribute the downward movement to constraint； and it is better to keep to the truth and say that the reason of this motion is that a thing which possesses weight is naturally endowed with a centripetal movement。 When the mixture； then； was merely potential； the things that were separated off moved similarly from every side towards the centre。 Whether the parts which came together at the centre were distributed at the extremities evenly； or in some other way； makes no difference。 If； on the one hand； there were a similar movement from each quarter of the extremity to the single centre； it is obvious that the resulting mass would be similar on every side。 For if an equal amount is added on every side the extremity of the mass will be everywhere equidistant from its centre； i。e。 the figure will be spherical。 But neither will it in any way affect the argument if there is not a similar accession of concurrent fragments from every side。 For the greater quantity； finding a lesser in front of it； must necessarily drive it on； both having an impulse whose goal is the centre； and the greater weight driving the lesser forward till this goal is reached。 In this we have also the solution of a possible difficulty。 The earth； it might be argued； is at the centre and spherical in shape： if； then； a weight many times that of the earth were added to one hemisphere； the centre of the earth and of the whole will no longer be coincident。 So that either the earth will not stay still at the centre； or if it does； it will be at rest without having its centre at the place to which it is still its nature to move。 Such is the difficulty。 A short consideration will give us an easy answer； if we first give precision to our postulate that any body endowed with weight； of whatever size； moves towards the centre。 Clearly it will not stop when its edge touches the centre。 The greater quantity must prevail until the body's centre occupies the centre。 For that is the goal of its impulse。 Now it makes no difference whether we apply this to a clod or common fragment of earth or to the earth as a whole。