A sail catches the power of the wind and uses it to propel a boat or ship across the water. The earliest boats were simple rafts made of logs or reeds lashed together, and it was to these that the first sails were added by the ancient Egyptians around 3,500 B.C. In its most essential form, a sailboat consists of the sail itself (made of papyrus or linen by the Egyptians), the mast (one or two poles attached vertically to the front half of the boat), and a long spar called a yard attached horizontally across the top of the mast (to which the sail is attached, and raised and lowered, by lines called halyards). These first Egyptian sails were square and harnessed the wind's power to travel upstream on the Nile River, since that was the way the winds usually blew. The boats were paddled rather than...