There are grindstones with frames too light. The competition to manufacture and sell a grindstone for farm use at the cheapest possible price has resulted in turning out thousands of grindstone frames that possess very little stability. [Illustration: Figure 35.--Grindstone. The speed of a grindstone varies with the diameter of the stone. It should turn just fast enough to keep a flow of water on the upper face surface. If the stone turns too slow the water will run down; if too fast, it will fly off.] Grindstones should be kept under cover; the best stone will be injured by leaving it in the hot sun. The sun draws the moisture out of the upper side and leaves the lower side damp and soft so that in use the stone soon becomes flat sided. The wet side freezes in winter, which is a disintegrating process. The best stones, with good care, will become uneven in time. The remedy is to true them with a quarter-inch soft iron round rod used like a lathe tool over an iron rest placed close to the stone on a level with the center of the stone. The rod is held against the stone in such a way as to cut away the high bumps and make the stone truly round. The stone cuts away best when it is dry. A small rod is better than a large rod. It digs into the stone better and takes out a deeper bite. Large power stones in machine shops are trued up in this way frequently. Farm stones often are neglected until they wabble so badly that it is difficult to grind any tool to an edge. If the grindstone is turned by a belt from an engine the work of truing may be done in a few minutes. If the stone is turned by hand the work of making it round takes longer and requires some muscle, but it pays. The face of a grindstone should be rounded slightly, and it should be kept so by grinding the tools first on one side of edge of the stone, then on the other, with the cutting edge of the tool crosswise to the face of the stone. For safety and to prevent a sloppy waste of water the stone should turn away from the operator. The best way to keep a stone moist is by a trickle of water from an overhead supply. Troughs of water suspended under the stone are unsatisfactory, because the water soon gets thick and unfit for use. Such troughs are forgotten when the job is done, so that one side of the stone hangs in the water. An overhead supply of water leaks away and no damage is done. Grindstone frames are best made of wood 3" x 4" thoroughly mortised together and well braced with wooden braces and tied across with plenty of iron rods. A good grindstone frame could be made of angle iron, but manufacturers generally fail in the attempt. There are good ball-bearing grindstone hangers on the market, both for hand crank stones and for belt use.