Sec. 924. [=Ventilation of mines while persons working therein.=] At each mine where the ventilation is not continuous, it shall be started a sufficient length of time prior to the appointed time for any person, or persons, working therein to enter, to clear the mine of explosive, poisonous and noxious gases, and shall be kept in operation a sufficient length of time after the appointed time for such employes to leave their working places, for all persons to be out of the mine. (Sec. 922, 923, 952; Penalty, Sec. 976.) [=Pressure gauges.=] At each mine generating fire-damp so as to be detected by a safety lamp, and wherein twenty or more persons are employed, a recording pressure gauge for the purpose of recording the pressure or vacuum of the main air current, shall be provided and maintained, which shall be kept in constant use, and records preserved for ninety days, subject to the inspection of the chief inspector of mines and the district inspector of mines. (Penalty, Sec. 976.) Sec. 925. [=Competent person or persons shall be designated as fire-boss.=] The owner, lessee or agent of a mine generating fire-damp so as to be detected by a safety lamp, shall designate a competent person or persons as fire boss or fire bosses, who shall make a thorough examination of each working place in the mine every morning with a standard safety lamp, not more than three hours prior to the appointed time for the employes to enter the mine. As evidence of such examination, the fire boss shall mark with chalk upon the face of the coal, or in some other conspicuous place, his initials and date of the month upon which the examination is made. If there is any standing gas discovered, he must leave a danger signal across every entrance to such place. [=Examination of other than working places.=] Each mine generating fire-damp so as to be detected by a safety lamp, shall be kept free from standing gas. All traveling ways, entrances to old workings, and places not in the actual course of working, shall be carefully examined with a safety lamp by the fire boss not more than three hours before the appointed time for persons employed therein to enter. Parts of the mine not in the actual course of working and available, shall be examined not less than once each three days, and shall be so fenced as to prevent persons from inadvertently entering therein. (Sec. 955, 959; Penalty, Sec. 976.)