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are not invariant at infinity. Then, of course, inertia is not explained: 
we must then prefer to leave it unexplained rather than explain it 
by the undetermined and undeterminable constant A. It cannot be 
denied that the introduction of this constant detracts from the 
symmetry and elegance of Einstein's original theory, one of whose 
chief attractions was that it explained so much without introducing 
any new hypothesis or empirical constant. 
Postscript. 
Prof. Einstein, to whom I had communicated the principal contents 
of this paper, writes (March 24, 1917): “Es wäre nach meiner 
Meinung unbefriedigend, wenn es eine denkbare Welt ohne Materie 
gabe. Das g,,.-Feld soll vielmehr durch die Materie bedingt sein, 
ohne dieselbe nicht bestehen können. Das ist der Kern dessen, was 
ich unter der Forderung von der Relativität der Trägheit verstehe”. 
He therefore postulates what I called above the logical impossibility 
of supposing matter not to exist. We can call this the “material 
postulate” of the relativity of inertia. This can only be satisfied by 
choosing the system A, with its world-maiter, i.e. by introducing 
the constant 2, and assigning to the time a separate position amongst 
the four coordinates. 
On the other hand we have the ‘‘mathematical postulate’ of the 
relativity of inertia, i.e. the postulate that the y,, shall be invariant 
at infinity. This postulate, which, as has already been pointed out 
above, has no real physical meaning, makes no mention of matter. 
It can be satisfied by choosing the system 4, without-a world- 
matter, and with complete relativity of the time. But here also we 
need the constant 2. The introduction of this constant can only be 
avoided by abandoning the postulate of the relativity of inertia 
altogether. 
Astronomy. — “On the Theory of Hyperion, one of Saturn’s Satel- 
lites.”. By J. Woutser Jr. Communicated by Prof. W. pe Sirrrr). 
(Communicated in the meeting of April 27, 1917). 
1. Among the-peculiar disturbances, which the satellites of Saturn 
undergo by their mutual attraction, those, produced by Titan in the 
motion of Hyperion, are of much importance. In this paper I intend 
to give a short development of the theory of the latter satellite; 
my dissertation will contain more extensive calculations on this 
subject. 
