New cat my dog hates!?

I just got a new cat from the humane society. He is so nice and really wants to get to know my dog, who is very weary of him. everything was fine for about an hour of them meeting, then the cat got to smelling him and my dog attacted him and now he is hiding under my bed and won't come out. What do i do?? both animals are soooo sweet, they just arent fond of each other yet.