This model of Saverys 1698 pumping engine was built from odds and sods around the workshop. It is purely an experimental model to get an understanding of the problems involved in the production of such an engine. I plan at some stage in the future to build a more realistic looking model of the Savery pumping engine, set in a mine or pumping water to a fountain as one rich Duke in the late 17th or early 18th century did. I'll keep you posted!
By explaining the working principle of the Savery engine, I will be doing little more than repeating what I have read in books over the years - it's probably better for you to google it, there are some fantastic animations showing it's operation, but to start you off, here is a link to a Wikipedia entry on Thomas Savery.