why the position next to the keeper is called a SLIP?

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hi friends.yes ofcourse we all watch matches and most of us know whats happening every ball by reading or hearing commentary because we know what the positions mid-on,cover,mid-wicket,third-man means but did you ever thought how they were given these names? i did but it didnt make any sense. [b]the position next to the wicket keeper is called a slip because it is to catch the balls slipping away from the keeper? why call one as cover and the exact same position on the opposite as mid-wicket? is it named gully because most of the runs flow from that region? why there is a off spinner but no onspinner?[/b] if anybody know why and how the positions are named,let me and all others know.