Petard, a 16th-century explosive instrument for destroying doors and similar obstacles to hostile entry. It was generally formed of gun-metal, and took the form of a conical hat and contained from 7 to 20 lbs. of powder. It was fastened by means of a plank and fired by a slow-match. An excellent account of the petard and its action occurs in Scott's Woodstock, where it is employed by Cromwell against Sir Henry Lee's dwelling. The petard is now obsolete, as bags of powder and discs of gun-cotton are more effectual.