Common grindstone-spindles, with a crank at one end, are open to the objection that the stone will never keep round, because every person is inclined, more or less, to follow the motion of his foot with his hand, which causes the pressure on the stone to be unequal. The harder pressure is always applied to the very same part of the stone, and will soon make it uneven, so that it is impossible to grind a tool true. To avoid this, put in place of the crank a small cog-wheel to the spindle, say with twelve cogs; have another short spindle, with a crank and a cog-wheel of thirteen cogs, to work into the former. The stone will make about 0.07 of a revolution more than the crank, and the harder pressure of the tool on the stone will change to another place at every turn; and the stone will keep perfectly round if it is a good one.