This simple drive mechanism is complicated by the fact that the runner stone must be held suspended above the bed stone. Ideally, the faces of the two stones—however close together—should never touch. To accomplish this, the runner stone is balanced to turn freely on the point of a spindle that comes up from below through the eye of the bed stone. The spindle, in turn, rests at its lower end in a pivot bearing that can be raised or lowered very slightly by a series of levers. By this means the miller can adjust the distance between stones according to the kind of grain he is grinding and also according to the speed at which his mill is operating. In a variable wind, for instance, he may have to make the adjustment continuously as the feel of the meal coming from the stone indicates. The faces of both stones must be “dressed” periodically—perhaps once every ten days when the mill is in constant operation. For this the runner stone must be removed and turned over so that the stone dresser, with his pickaxe-like “millbill,” can operate on both faces. The dresser deepens the furrows, if necessary, and roughens the “lands” between the furrows toward the outer edges of the stones. When the stones are in good condition, the grain kernels are opened out near the eye of the stones, gradually reduced in the middle area, and the bran scraped and cleaned in the outer one-third of the face. The condition of the bran is the best index of the stone dressing, while the feel of the meal tells the miller whether his mill is running at the best speed and if the stones are set at the proper distance apart. French burr stones produced the best quality flour in colonial mills, and most Virginia mills that ground for export seem to have had a pair of them. Cologne, or “cullin,” stones from the Rhine were somewhat less choice, and the same mills often had a pair of these for country work, especially for grinding corn. Stones quarried in Virginia, Pennsylvania, New York, and elsewhere were also used. The pair in the Williamsburg mill are of quartz-bearing granite quarried in Rowan County, North Carolina. They are four and one-half feet in diameter. The bed stone is seven inches thick and the runner stone ten inches. Together they weigh more than two tons. Although colonial millers ground all varieties of grain between the same set of stones—making only the necessary adjustment in distance between the stones—Robertson’s windmill today processes only corn. But whatever the grain, the difference of a newspaper’s thickness makes the difference between a good grind and a poor one. So for all its clumsiness in appearance and lumbering clatter in operation, the windmill is an extremely precise mechanism at the point where precision counts.