  We investigate some classical aspects of fundamental strings via numerical
experiments. In particular, we study the thermodynamics of a string network
within a toroidal universe, as a function of string energy density and space
dimensionality. We find that when the energy density of the system is low, the
dominant part of the string is in the form of closed loops of the shortest
allowed size, which correspond to the momentum string modes. At a certain
critical energy density corresponding to the Hagedorn temperature, the system
undergoes a phase transition characterized by the formation of very long loops,
winding a number of times around the torus. These loops correspond to the
winding string modes. As the energy density is increased, all the extra energy
goes into these long strings. We then study the lifetime of winding modes as a
function of the space densionality. We find that in the low--energy density
regime, long winding strings decay only if the space dimensionality of the
toroidal universe is equal to 3. This finding supports the proposed
cosmological scenario by Brandenberger and Vafa, which attempts to explain the
space dimensionality and to avoid the initial singularity by means of string
theory.
