  We have detected HI 21cm line absorption by the Warm Neutral Medium (WNM)
using the Westerbork Synthesis Radio Telescope. The absorption was detected
toward Cygnus A at LSR velocities of -40 and -70 km s^{-1}. These two velocity
ranges were previously identified as being relatively free of cold absorbing
clouds. The measured optical depth for the WNM along the line of sight to
Cygnus A is 8.9+/-1.9 x10^{-4} at -70 km s^{-1}, and 8.5+/-2.0 x10^{-4} at -40
km s^{-1}, with corresponding spin temperatures of 6000+/-1700 K and
4800+/-1600 K, respectively. The volume filling factor for the WNM appears to
be fairly high (f = 0.4).
