To the Abbé d’Arnal
Paris July 9. 1787.
I had the honour of informing you when at Nismes that we had adopted in America a method of hanging the upper stone of a grist mill which had been found so much more convenient than the antient as to have brought it into general use. Whether we derive the invention from Europe, or have made it ourselves, I am unable to say. The difference consists only in the Spindle and horns. On the former plan, the horns were of a single peice of iron in the form of a cross, with a square hole in the middle, thus which square hole fitted on the upper end of the spindle. The horns were then fixed in cross grooves in the bottom of the upper stone, which was to be laid on the spindle so that the plane of it’s grinding surface should be perfectly perpendicular to the spindle. This was a difficult and tedious operation, and was to be repeated every time the stones were dressed. According to the present method, two distinct peices of iron are substituted for the horns: the one in this form of such breadth and thickness as to support the whole weight of the stone. It’s streight ends are to be firmly fixed in one of the cross grooves of the stone, the circular part should rise through the hole in the center of the stone so as to be near it’s upper surface. In the middle of this semicircular part, and on it’s under surface (at a) should be a dimple to which the upper end of the spindle should be adjusted, by giving it a convexity fitted to the concavity of the dimple. The other peice of iron is only a streight bar, to be firmly fixed in the other of the cross grooves of the stone, and to have a square hole in it’s center thus . The corresponding part of the spindle must be squared to fit this hole. The office of the first peice of iron is to suspend the stone, that of the last is to give and continue it’s motion. The stones [bein]g dressed, and these peices firmly fixed in it, it is turned over on the spindle so that the point of the spindle may enter the dimple of the semicircular iron, and the stone be suspended on it freely. It will probably not take at first it’s true position which is that of the plane of it’s grinding surface being truly perpendicular to the spindle. The workman must therefore chip it at top, with a chissel, till it hangs in that just position. This being once done, is done for ever: for whenever they dress the stones afterwards they have only to return the upper one to it’s pivot and it will resume it’s equilibrium. It sometimes happens that one side of the stone, being softer than the other, wears faster, and so the equilibrium is lost in time. Experience has shewn that a small departure from the equilibrium will be rectified by the bedstone, which serves as a guide to the running stone till it assumes it’s motion in a true plane, which it will afterwards keep. But should a defect of the stone render this [depar]ture from the equilibrium too considerable, it may be necessary to set it to rights, at certain periods, by chipping it again on the top. I had promised, when I had the honour of seeing you at Nismes, to send you a model of this manner of fixing the mill-stone: but the expence of sending a model by post, the danger of it’s being lost or destroyed by the Messagerie, and the hope that I could render it intelligible by a description and figures, have induced me to prefer the latter method. I shall with great pleasure give any further explanations which may be necessary for your perfect comprehension of it, and the more so as it will furnish me with new occasions of assuring you of those sentiments of respect and esteem with which I have the honor to be Sir your most obedient & most humble servant,
PrC (DLC); at foot of first page: “M. l’Abbé D’Arnal, ancien chanoine d’Alais à Nimes.”