53,) of the lower circle being one-half the diameter, _a b_, of the upper circle.
This form has never been ascertained to be exactly correct, but the theoretical results deduced from a mathematical discussion of this figure have been fully verified in practice.

The radius, _p b_, of the upper circle is termed the _crater radius_; the line _o p_, drawn from the centre of the charge perpendicular to the surface where the explosion takes place, is termed the _line of least resistance_; the line _o b_, drawn from the centre of the powder to any point in the circumference of the upper circle, is termed the _radius of explosion_.
When the crater radius is equal to the line of least resistance, the mine is termed _common_; when this radius is greater than the line of least resistance, the mine is termed _overcharged_; and when the radius is less, _undercharged_.

A mine of small dimensions, formed by sinking a shaft in the ground, is termed a _fougasse_.

The term _camouflet_ is applied to a mine used to suffocate the enemy's miner, without producing an explosion.

Small mines made in rock or masonry, merely for the purpose of excavation, without any considerable external explosion, are called _blasts_.
From experiments made on common mines, whose line of least resistance did not exceed fifteen feet, it has been ascertained that the tenacity of the earth is completely destroyed around the crater to a distance equal to the crater radius, and that empty galleries would be broken in at once and a half that distance.