The sailcloths themselves are handmade, of imported Scottish linen. To meet changes of weather they are furled by hand, each arm in turn being stopped at the lowest position while the miller unties the outward corners and twists the long strip into a more or less tight roll. For this purpose he can set and release the brake from the ground, using the rope that hangs out the side of the mill. Whereas each sailcloth of Robertson’s windmill must be wholly furled or not trimmed at all, the sails of many early mills could be partially reefed. The four degrees of reefing were known as full sail, first reef, dagger point, and sword point. Trimming the sails was a difficult and sometimes dangerous task, for a sudden storm with sleet and shifting gusts of wind could make the job almost impossible at the very moment that it had to be accomplished quickly. A miller caught with his sails up in such a storm might suffer what was known as “tail winding” if the wind veered faster than he could work. In this event he might be lucky to get off with nothing worse than having the sailcloth stripped from the frame. A hurricane could do more serious damage—and might overturn the whole mill—no matter which way it faced. For the wind was the miller’s master as well as his servant, an evil genius that he feared as well as a heavenly blessing for which he prayed. Without wind the mill stood idle and the miller earned nothing. When the wind arose the miller must heed its call to work, whether in the middle of a meal or in the middle of the night. And always, he must keep a weather eye on the horizon for signs of too much wind. Fire and lightning were other great perils to every windmill. If the hopper ran empty of grain, the friction of the stones rubbing against one another could generate enough heat for combustion. So could the friction of the brake if it were used continuously in a strong wind. In either event, a building made entirely of wood and open to every breeze burned readily, and more windmills probably fell victim to fire and storm than to old age. Similarly, its height and exposed position made the windmill an attractive target for lightning. The sail wheel of the Williamsburg mill has a diameter of fifty-two feet—about average for a small post mill of this type. In a wind of about twenty miles an hour, which seems to be generally best for windmill operations, it will turn at about twenty revolutions per minute. This apparently slow and majestic rate is deceptive; for at twenty rpm the tips of the sails travel at a linear speed of 3,266 feet a minute or about thirty-seven miles an hour! An operating windmill is something to stay well clear of, as Don Quixote and uncounted innocently grazing cows and sheep have discovered to their sorrow.