††††The wheel is regarded as one of the most important human inventions. Its invention made it much easier to move or turn things. Besides the wheels we see on the roads or railways, almost every machine or instrument with moving parts has wheels in it. Before the wheel was invented, heavy loads were moved using tree trunks as rollers. But someone had to keep picking up the rollers at the back and move them around to the front. The oldest evidence of the use of a wheel is a rough sketch of a cart on a clay tablet found in Mesopotamia and dating from about 3500 BC. The oldest surviving wheels, found in Mesopotamian tombs of 3000 to 2000 BC, were made of three planks clamped together with cross struts and cut into a circular shape. A knot hole in the central plank was used as a pivot.
The rim of a Mesopotamian wheel of 2500 BC is studded with copper nails. These may have been put there to resist wear or to hold on a leather tyre. Metal tyres for wheels first appeared in about 2000 BC.
