With that he picked up a try-square and pencil and began laying out some work for Paul to cut on the circular saw, while John busied himself at the boring-machine in putting a hole through the center of the big twelve-foot balsa-wood propeller which a little later would be reinforced with a thin jacket of a new metal called "salinamum," which was made chiefly from salt but whose fused components made it as light as aluminum and stronger than tool steel.