478 SCIENCE PROGRESS 



which takes the sun as the centre of our solar system, consisting of sun, planets, 

 satellites, asteroids, comets belonging permanently to it, and, finally, its meteorites 

 and cosmic dust, is not a more correct representation than the Ptolemaic system, 

 which took the earth as centre, but is merely a simpler one, enabling the 

 phenomena to be correlated by a simpler system of mechanics than would be 

 required if the latter were adopted. 



Consider further such problems as are presented by the shape assumed by the 

 surface of a liquid in a bucket rotating about a vertical axis through the centre ; 

 the figure of the earth, approximately an oblate spheroid ; the resistance of a 

 spinning body, such as a top, and of the earth itself, to any displacement of its 

 axis of spin except in either of the two opposite directions in which that axis might 

 be prolonged ; the resistance of a swinging pendulum to the displacement of its 

 plane of swing. A freely suspended pendulum of considerable length and mass 

 was, indeed, employed by Foucault as a means of demonstrating the actual, or 

 absolute, rotation of the earth, its movements being such that they could be 

 explained most simply, and, on the basis of the accepted principles of mechanics, 

 could be explained only on the assumption that the pendulum tended to conserve 

 the absolute aspect in space of its plane of swing. Sir Isaac Newton regarded 

 phenomena of this kind as definitely indicating absolute rotations, and therefore 

 absolute directions in space, and has expressed that opinion in unequivocal terms. 

 One of the ablest of his recent critics, H. Poincare, in La Science et PHypothese, 

 objects to Newton's position, that he attempts to explain relations between 

 phenomena by means of the notions of absolute direction and absolute rotation, 

 involving the notion of absolute space, which Poincare, in common with all the 

 thoroughgoing advocates of the older relativity, ancient and modern, maintain to 

 be physically inconceivable to the relatively constituted human mind. I have 

 dealt elsewhere "' with this question in much greater detail than is possible within 

 the limits of space here available, and I must confine myself to very briefly 

 summarising the conclusions there aimed at, to serve as a basis for understanding 

 the nature of the new relativity theory, which, unlike the older one, is not built 

 upon a philosophical foundation, but must be regarded rather as a legitimate 

 hypothetical point of view which appears at present to offer considerable promise 

 of leading us, it may be directly or it may be indirectly, to the discovery of new 

 physical truths. 



Space, for the philosophic relativist, is the geometrical abstraction of pure 

 extension, and in this sense, absolute space is an expression which does not appear 

 capable of representation in the form of a definite mental concept ; but we must 

 guard ourselves against drawing the conclusion that there cannot be physical 

 phenomena indicative of absolute motion and direction in space, for this would be 

 to assume that our minds are to be accepted as measuring the possibilities of the 

 physical universe. Newton's writings show clearly that, although he seldom 

 referred explicitly to a medium filling all space, and never discussed its nature, 

 he firmly believed in its necessity, if only for transmitting the action of gravitation. 

 His attitude on the subject is expressed in the remark that: "Although prac- 

 tically, and at present, nothing is to be accomplished with this conception, we 

 might still hope to learn more in the future concerning this hypothetical medium ; 

 and from the point of view of science it would be in every respect a more valuable 

 acquisition than the forlorn idea of absolute space." I therefore consider that 

 we are justified in the conclusion that Newton regarded space as occupied by a 



1 A Treatise on Electrical Theory and the Problem of the Universe (1910), 

 Chapter V : The Ether as a Framework for Absolute Motions. 



