So engineers turned to steam engines to return the carts back up the line to the slate quarries before they were returned down the line under their own gravity.

BBC: The Princess in 1871

This exploits one of the predictions of Einstein's general theory of relativity: that the path of a beam of light (which is a straight line in empty space) is bent inwards by the gravity of a massive object.

ECONOMIST: Cosmology: I spy with my little gravitational lens | The

If an object floating through space passes near the line of sight between the earth and a distant star, its gravity should, according to the theory of relativity, bend and focus the light from that star.

ECONOMIST: The dark side of cosmology

Along with a current line-up of more than 90 compatible games, including the five star No Gravity, Outfoxed, Silverfish, Temple Run, Super Crossfire HD, Air Attack HD and Terra Noctis, Gametel for iOS devices also supports iCADE's suite of classic retro games with over 100 titles available to download from the Apple iTunes App Store.

ENGADGET: Gametel Bluetooth mobile controller lands stateside for $49.99