The method has been described and also the apparatus for air blowing into the ground in order to determine the cracking condition of mine root and particularly to fix the interstratal fissures. Examples are given of precise location of fissures and of their distance from the roof as well as of other uses of the method. The usefulness of the method has been stated, particularly for checking the early stages, from origin of the cracking to the formation of fissure of width less than 0,001 of inch.

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