If you have a computer of your own, there are numerous free versions
of Prolog available on the web.
SWI Prolog is available for download over the Web. More information is available
at :
http://www.swi.psy.uva.nl/projects/SWI-Prolog/
There is also a free version of the commercial Prolog produced
by Amzi! Corporation available at:
http://www.amzi.com/download/amzi_logic_explorer.htm
For Macintoshes, the best free Prolog is Open Prolog, available
from Trinity College Dublin at:
ftp://ftp.cs.tcd.ie/pub/languages/open-prolog/OpenProlog.sea.hqx