For the firefighters: bell questions?

I live directly behind a firehouse in Philadelphia. Im constantly hearing fire bells go off, but I hear different bells sometimes. Theres a long bell that lasts maybe a second and a half, then theres another that gives off several short rings. I know I've heard more than that but I just can't think of the patterns right now. My question is: what do each of these bells mean? They are rarely done one after another, so I know its not all for the same run. Is one bell set for each of a shoe run, a wires/fumes run, a RIT run, a real fire run, and a multiple alarm run?