The four arms are fixed into the hub of the massive “windshaft.” This is the main horizontal axle that brings the power of the turning sails into the mill. Actually, it is not exactly horizontal but slants at a ten-degree angle. Since the wind has always been thought to descend from heaven, this ancient arrangement may at one time have been intended to let the sail wheel “look up” a bit into the wind. It has, in any case, both structural and aerodynamic advantages. Just inside the front wall of the mill the windshaft rests in a metal bearing, and next to it is the large gear wheel, known as the “brake wheel,” that can be seen when one looks up through the trap door from the mill’s little back platform. The platform, incidentally, was not unusual in post mills, although it was often a convenience added after the mill was built and on which the miller could enjoy a pipe and a moment of repose while the mill ground merrily away. The brake wheel, a little more than seven feet in diameter, is firmly fixed to the windshaft and turns at the same speed as the sails. Its fifty-one hickory gear teeth and eighteen other major pieces, plus at least as many wooden pegs to hold the pieces together, were all carefully shaped and fitted together by hand. Around its outer edge is the brake band—of bent hickory—that can stop the sails and hold them at any position when the long, heavy brake lever is lowered. As an emergency aid in slowing down the sails in a strong wind, the stones can be choked with grain, thus making them harder to turn. The mill can also be operated with partially furled sails. The stones themselves consist of the lower or “bed stone,” which does not move, and the upper or “runner stone,” which turns at a little more than five times the rate of the sails. Its normal speed for best results is 108 revolutions per minute, at which speed it produces five to ten pounds of meal per minute. [Illustration: _Another engraving from Diderot’s encyclopedia showing the passage of grain from hopper to millstones, and its reappearance as meal. Notice the arrangement to set the bell ringing when the hopper is about empty and the weighted lever at the left by which the miller can control the distance between the millstones._] The cogs of the brake wheel stay in constant mesh with the staves of a sort of wooden-bird-cage gear called a “wallower.” This is fixed to the upper end of the wrought-iron vertical drive shaft, whose lower end stands in the center or “eye” of the runner stone and turns it.