How to partial dump the log in SQL?

I am use Postgres 7.1 going to upgrade to 7.2 run in Redhat Linux 7.2.

Each time user do update, insert and delete will save to the logfile,
right?

1) Where is that log file?
2 ) How do I do partial dump the log file in SQL statements?
For example, I want to dump all log
from 6:00 to 9:00 ,
from 9:00 to 12:00 ,
from 12:00 to 15:00,
from 15:00 to 18:00,
etc.

I can use these partial dump as incremental backup instead full dump.
I can also use these partial dump as replicate data to other Postgres
server.
Please tell me how do these.

P.S. our NNTP USENET server doesn't have all *.pgsql-* news groups.
Therefore, please CC a copy to me if you response my message.
Thank Q!