G is shown in position in Fig.

221.
[Illustration: Fig.224.Agacite Grinder.] All such devices necessitate a perfectly true stone.

The essential features are, to have a rigid support against which the tool may be pushed by the revolving stone, to hold the tool at a fixed angle which may be adjusted, and to press the tool against the stone with considerable pressure.

The wheel should revolve toward the edge which is being ground, for two reasons.

It is easier to hold the tool steadily thus, and the danger of producing a wire edge is lessened.
The edge as it becomes thin, tends to spring away from the stone and this tendency is aggravated if the stone revolves away from the edge.
If the stone does not run true and there is a consequent danger of digging into the stone with the tool which is being sharpened, the stone would better revolve away from the edge.