To build the Car-glass timer, you need lots of cars. These become the particles of sand in your appropriately gigantic timer.
Instead of slipping through the gap between the top and bottom container, where they might jam up, there is a double action rotating gate mechanism which releases one car at a time. Once the bottom gate opens, a single car drops down the hundreds of feet into the lower chamber, where it smashes against the bottom of the toughened glass. Cars which don't totally make it through the two gates are simply severed into segments by the guillotine action of the closing gate.
That's all there is to it, apart from equipping it with an engine powerful enough to crank the mechanism and invert it, once all the cars have dropped through the gated orifice separating the top from the bottom holding vessels.
In time all of the cars will be reduced to smaller and smaller particles, eventually becoming no bigger than the grains of sand they initially represent.-- xenzag, May 30 2008 Hour Glass http://www.pwcphoto...tudio/hourglass.jpgeach grain is a car [xenzag, May 30 2008] Make it airtight, fill with oxygen, a bit of water vapor and set in the sun. Over time, the metal oxidizes and the plastic degrades and falls to the bottom. Slower, but more fun IMHO.-- Klaatu, May 30 2008 random, halfbakery