Effect on Fire of the Blast.

The nature of the blast has a very great effect on the fire because, on it, depends the amount of air which is drawn into the box through the grate and fire-hole door. If the blast is too sharp it will disturb the fire excessively; if too gentle, it will not draw in sufficient air. The correct size of the exhaust pipe cap is always determined (by experiment) by the designer of the engine.