ventilation are closed as soon as the next cross-heading is made, for reasons already explained. This closing is usually done by building up in the aperture a wall of slate, rock, and coal, and filling the chinks with dirt from the floor of the mine. Sometimes wooden partitions are put in instead, and between principal air passages the cross-headings are closed by heavy walls of masonry. When it is necessary to turn the air from any traveling way, or to prevent it from further following such traveling way, a partition is built across the passage, and in the opening left in the partition a door is swung. If this is across a way through which mine cars pass, a boy will be stationed at the door to open it when the cars come and close it as soon as they have gone through. He is called a “door boy.” All doors are so hung as to swing open against the current of air, and are therefore self-closing. The law directs that this shall be done. There are several patented devices for giving an automatic movement to mine doors; but few if any of them are in practical operation in the anthracite mines. The conditions here are not favorable for the use of self-acting doors, and besides this the act of 1885 provides that all main doors shall have an attendant. The law is very explicit on this subject of ventilation; it is a matter of the utmost importance in operating a mine. A failure of the air current for even an hour might, in some mines, result in the death of all those who chanced to be inside. For this current not only supplies air for breathing purposes, but it takes up the smoke, the dust, the dangerous and the poisonous gases, and carries them to the surface. In the same way pure air is drawn into the lungs, loaded with the refuse matter brought there by the blood, and then expelled. So life is preserved in both cases.