I downloaded Emacs source files, and installed it using make. After installing Emacs successfully, I manually removed the downloaded source files to save disk space. Now I want to remove Emacs and I tried to use sudo apt-get purge emacs. But it says Emacs has not been installed and will not be removed. But I can run Emacs by typing emacs in the terminal. Also the command which emacs shows the result /usr/local/bin/emacs.
Why can't apt-get detect it? How can I remove Emacs completely in this situation? My OS is Ubuntu 12.04, and my Emacs version is 24.3.1.