Well if i understood what you mean.. the only ways i know that would make you able to see previous lines is using a znc which keeps the messages when you're offline and sends you them when you log in. You can find some free bnc services if you search Google for Free bnc.
And the other way is probably not so suitable for you cause it's a channel mode and i only have seen it in inspircd .. +H lines:seconds saves X lines from Y seconds ago.. and when someone joins the channel it sends the lines to them.
Apart from these there's not a way (or at least that's what i know) to be able to see messages that were sent when you weren't on the channel.
