takes the place of
in our former equations. Einstein then shows, as before, that for the universe to be cylindrical, hence stable, a connection must exist between this mysterious pressure
and the average density of matter throughout the universe, hence also between the pressure and the universal curvature. In this respect, at least, a slight advance in our understanding of the binding pressure of matter appears to have been obtained (see [Chapter XXXVI]). Furthermore, the connection between the density of matter
and the internal pressure
is easier to understand than that between