  The extraction problem of information about the location and shape of the
cavity from a single set of the temperature and heat flux on the boundary of
the conductor and finite time interval is a typical and important inverse
problem. Its one space dimensional version is considered. It is shown that the
enclosure method developed by the author for elliptic equations yields the
extraction formula of a quantity which can be interpreted as the {\it travel
time} of a {\it virtual} signal with an arbitrary fixed propagation speed that
starts at the known boundary and the initial time, reflects at another unknown
boundary and returns to the original boundary.
