  Waves are important for the heating of the solar corona and the acceleration
of the solar wind. We have examined a long spectral time series of a southern
coronal hole observed on the 25th February 1997, with the SUMER spectrometer
on-board SoHO. The observations were obtained in a transition region N IV 765 A
line and in a low coronal Ne VIII 770 A line. Our observations indicate the
presence of compressional waves with periods of 18 min. We also find
significant power in shorter periods. Using Fourier techniques, we measured the
phase delays between intensity as well as velocity oscillations in the two
chosen lines over a frequency domain. From this we are able to measure the
travel time of the propagating oscillations and, hence, the propagation speeds
of the waves producing the oscillations. As the measured propagation speeds are
subsonic, we conclude that the detected waves are slow magneto-acoustic in
nature.
