1341 
perhaps hope that the error introduced by this assumption will not 
be considerable *). 
We mentioned already the analogy between the problem treated 
in $$ 11—18 and that of the thermal expansion. In the one case 
the torsion plays the same part as the heat motion in the other 
and the quantities that have been indicated by q in the two 
problems are comparable with each other; the similarity of the mathe- 
matical treatment in the two cases is likewise evident. PoyNriNG 
remarks that a dilatation of the wire will also take place when it 
executes torsional vibrations or when vibrations of this kind are 
propagated in it. With similar phenomena we are generally concerned, 
when an elastic body is traversed by waves, and when we consider 
the very short waves especially, this leads us directly to an insight 
into the nature of thermal dilatation. 
Finally it deserves our attention that, though the phenomena 
discussed in this paper are chiefly determined by the change of the 
elastic constants caused by a previous deformation, yet there are 
as well in equation (17) as in (29) and (30) terms that are independent 
of this change. . 
Physics. — “On Einsteins Theory of gravitation.” I. By Prot. 
H. A. Lorenz. 
(Communicated in the meeting of February 26, 1916). 
§ 1. In pursuance of his important researches on gravitation 
Einstein has recently attained the aim which he had constantly kept 
in view; he has succeeded in establishing equations whose form is not 
changed by an arbitrarily chosen change of the system of coordinates *). 
Shortly afterwards, working out an idea that had been expressed 
already in one of Erystern’s papers, HiBert*) has shown the use 
that may be made of a variation law that may be regarded as 
Hamitton’s principle in a suitably generalized form. By these results 
the “general theory of relativity” may be said to have taken a 
definitive form, though much remains still to be done in further 
1) This paper had already gone to press, when an article of FöRSTERLING 
came under my notice (Ann. d. Phys. 47 (1915) p 1127) in which considerations 
similar to those here developed are put forward. 
2) A. Einstein, Zur allgemeinen Relativitätstheorie, Berliner Sitzungsberichte 
1915, pp. 778 799; Die Feldgleichungen der Gravitation, ibid. 1915, p. 844. 
3) D. Hizpert, Die Grundlagen der Physik |, Göttinger Nachrichten, Math.-phys. 
Klasse, Nov. 1915, 
