The oldest kind of windmill is called a _post_ mill, because the whole structure is centred on a post, or pivot, and, when the wind shifts, the mill has to be turned bodily to meet it, by means of a long lever. The _smock_, or _frock_, windmill is an improvement upon the post mill; the building itself is stationary and permanent, but the head or cap, where is the wind shaft, rotates, and this is more easily managed. For hundreds of years people were contented with the four and six arms to their windmills, and it was only in modern times that Messrs. J. Warner and Sons, of Cripplegate, London, patented their annular sails, which, as is plain to the meanest capacity, are vastly superior. The shutters, or ‘vanes,’ are connected with spiral springs, which keep them up to the best angle of ‘weather, for light winds. If the strength of the wind increases, the vanes give to the wind, forcing back the springs, and thus the area on which the wind acts diminishes. In addition, there are a striking lever and tackle for setting the vanes edgeways to the wind, when the mill is stopped, or a storm expected. We have seen how from the very first man used stones wherewith to triturate his corn, and to this day stones are still used for grinding, although their days are in all probability numbered, and in a very little time they, with the windmill, will be relegated to limbo. The _Encyclopædia Britannica_ gives such an excellent description of these mill-stones, that I quote it in its entirety. [Illustration: THE GRINDING SURFACE OF A MILLSTONE.]