Moving on path

[Blom]

I want an actionscript to make a movieclip go along a path like in the attachted image. I know how to change the x position after what xposition the mouse has got. But how should i make the movieclip go along the path when mouse is moved.
It should have the same x position when mouse is in that x area. Like from 0-250px.
But the y position of the movieclip should be on that circled path all the time? Anyone understadning?

How should i do that?