Hi.
If you are using Apache, you can define a VirtualHost, with ServerName www.yourhost.com and DocumentRoot /path/public_html. See the Apache documentation. It is easy to do. I think the same can be do with IIS.
So, any time you access http://www.yourhost.com the request will go to the public_html.
Sorry, my english is bad, but I hope you undestand me.