Although the Egyptians achieved extraordinary feats of engineering, they appear to have done so with relatively primitive technology. As far as is known they did not use wheels or pulleys. They transported massive stones over great distances using rollers, ropes and sledges hauled by large numbers of workers. The ancient Egyptians are credited with inventing the ramp, lever, lathe, oven, ship, paper, irrigation system, window, awning, door, glass, a form of plaster of Paris, the bath, lock, shadoof, weaving, a standardized measurement system, geometry, silo, a method of drilling stone, saw, steam power, proportional scale drawings, enameling, veneer, plywood, rope truss, and more. There are no surviving Egyptian manuals so there has been considerable speculation on how stones were lifted to great heights and obelisks erected. Most theories centre on the use of ramps.