Help with sincronizing two directories on remote servers

[symbulos - ethical services (external mailing lists)]

Dear friends,

a newbie here.

we have two webservers, a and b, both of them with rsync 
installed.

We would like to rsync one directory on one server with 
another directpory on another server.

What I do is to go to one of the server (server a) as root 
using ssh, navigate to the directory, and write

rsync -avz username at serverb:directory/     ./

But it does not work.

Am I doing something wrong?

Thanks in advance
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