The author would like to thank Neal Jackson for giving useful comments on sections of this paper and Roeland Rengelink for all discussions about the work I have been doing concerning the Westerbork Northern Sky Survey.
During my stay at the observatory I have visited the IOA and MRAO in Cambridge (UK) and presented a poster at the ADASS'93 conference in Victoria B.C. (Canada). Financial support was given by the Leids Kerkhoven Bosscha Fonds.
The Westerbork Synthesis Radio Telescope is operated by the Netherlands Foundation foe Research in Astronomy (NFRA) with financial support from the Netherlands Organization for scientific research (NWO).