ploration trip by rescue crews on March 13 proved that the fire had been extinguished. A full account of this occurrence is given under "Procedure used in unsealing Horning mine fire.”

TABLE 1.-Analyses of air samples taken at the Horning mine fire and explosion (fire and explosion occurred February 3, 1926)

TABLE 2.-Analyses of air samples taken in Oakmont mine sealed area1 (fire discovered January 14, 1925)

On January 14, 1925, a section (see fig. 18) of the Oakmont mine was sealed, following the discovery of fire. It will be observed from Table 2 that the first sample collected on January 15 contained 15.45 per cent of oxygen, and another sample collected on the 24th contained 4.2 per cent. This was the lowest oxygen content shown over a long period, as a sample collected on March 6 contained 5.2 per cent. When the oxygen content showed no reduction the stoppings surrounding the fire area were reinforced, and every effort was made to stop leakage of air. Water-gauge readings taken at the stoppings on the intake and return side of the area showed about 3 inches inward pressure on the intake side and about 3 inches outward pressure on the return side. On April 4, in order to ascertain definitely conditions within the sealed area, explorations were made by rescue crews behind the seals adjacent to the fire. The explorations were made through air locks A and B, thus admitting a minimum amount of oxygen. When the rescue crew found unmistakable indications of fire within air lock B, the area was resealed and flooded. The flooding was afterwards found to have been unsuccessful owing to the difference in elevation within the sealed area. Table 2 shows that after resealing the subsequent samples contained a slight increase in oxygen, and virtually no reduction was effected until after July 10.

The increase in oxygen was probably due largely to water being pumped in the sealed area. The carbon monoxide did not decrease materially until after June 11. On June 12 the ventilation was changed by building a stopping in the return from No. 10 butt and by opening trapdoor E on the intake airway outby No. 10 butt. The change in ventilation placed an equal pressure on all stoppings that inclosed the fire area, and the water-gauge readings at all stoppings became zero. The carbon monoxide also began to decrease rapidly. A sample taken on August 22 showed 2.1 per cent of oxygen, and samples taken on October 8 showed 0.8 per cent at No. 20 butt and 0.4 per cent at No. 10 butt, respectively. On October 15 the area was explored, and as no indications of fire were found it was reventilated.

EFFECT OF BOREHOLES, REGION SEALED, AND TEMPERATURE WITHIN SEALED AREA

In addition to the factors just mentioned, which generally govern the time for unsealing, there may be local factors that will necessitate careful scrutiny, such as the proximity of gas wells to fire areas, the position of boreholes, the extent of the region under seal, and temperature within the sealed area. Ordinarily a large acreage under seal will require more time before unsealing than a smaller area; especially is this true if the seals in the former are poorly placed. Temperatures at the site of seals are not representative of the temperature within the sealed area. However, when explorations are made with oxygen breathing apparatus within the sealed area temperatures should be taken, preferably with maximum and minimum thermometers, by inserting the thermometer in the caved material. Especial care should be taken to do this at or near the supposed origin of the fire.

CONCLUSIONS REGARDING TIME TO UNSEAL FIRES

Very careful consideration obviously must be given to all of the foregoing factors in the proper determination of the time for unsealing, as an error in any of them may necessitate resealing and may be accompanied by disaster.

METHODS OF UNSEALING

The method to be used for the recovery of a fire area must be planned and outlined in detail before the seals are broken. In general, two systems may be employed-(1) recovery of an area in successive blocks by means of air locks and (2) reventilation of an area after there is conclusive evidence that the fire has been extinguished. When the sealed area is extensive, the fire inaccessible, or the exigen

cies of the case necessitate the removal of bodies from a previous explosion the area should be recovered in sections by using air locks. and advancing to a point where observations can be made or the bodies recovered. Air locks consist of two stoppings 10 or 15 feet apart equipped with doors through which oxygen breathing-apparatus crews can enter or take material into the sealed area. One door of the air lock is kept closed at all times. After an air lock is completed the crews work through it and erect another air lock about 200 to 500 feet inby and perform such other work as may be required for reventilation of the region between air locks. The distance between locks depends largely on conditions encountered, such as ease of travel, height of passageway, work to be done, etc. As an area between air locks is recovered the locks outby should be removed. After the air locks have been advanced close enough to the seat of the fire to permit observations to be made it may be possible to load out heated material through the air locks, as was done at the fire in the Sunnyside mine, Sunnyside, Utah." When the area under seal is relatively small, so that it can be explored by rescue crews under reasonably safe conditions, and there is every indication that the fire has been extinguished, the area should be reventilated and the fire gases removed as quickly as possible, preferably with all persons out of the mine.

VENTILATION AND ROCK-DUSTING

The successful opening of any fire area will necessitate a certain amount of preparatory work. Necessary adjustments in ventilation should be made so that the return from the fire area can be directed into main return. Return airways should be inspected and necessary arrangements made to confine the fire gases to the main return to prevent the gases from circulating or accumulating in other portions of the mine. In coal mines all entries leading to and from the fire area should be heavily coated with rock dust at and outby the seals that are to be broken.

ELECTRIC POWER CUT-OUT

In coal mines it is essential that the electric current be cut out from the mine even before the actual work of unsealing is undertaken. Moreover, when the seals are broken it is absolutely necessary that the electric current be cut off from the whole mine to eliminate possible ignition of gas through arcs or sparks.

* Allen, C. A., and Watts, A. C., Apparatus Men Load Out Heated Material from a Mine Fire in a Cooling Current of Oxygen-Depleted Air: Coal Age, vol. 21, Mar. 23, 1922, pp. 479-483.

RECOVERING SEALED AREA BY USE OF AIR LOCKS

Before the seal is broken on the intake side of the area a tightly constructed air lock, preferably of matched lumber, should be erected a convenient distance from the seal. When barometric conditions are favorable and all other preparations have been made, such as efficient organization, necessary equipment, and materials, a crew of at least five men wearing oxygen breathing apparatus, fully equipped for the work at hand and supported by a reserve rescue crew of at least five men at the air lock, should enter and break the seal. The advance crew, using a life line, should explore to the point where the next air lock is to be erected, a distance generally not to exceed 500 feet. General conditions should be observed, temperature readings taken, measurements made for materials required to construct the air lock, and an air sample collected to check previous analyses. The crew should then return to the fresh-air base. Crews wearing breathing apparatus, with a reserve crew at the fresh-air base, should next construct an air lock at the place previously selected, erect necessary stoppings in parallel entries to insure resealing of the inby area, examine the unexplored parts of the isolated section for possible fire, and make the changes necessary so that the fire gases from this localized area may be directed to the return airway. Next, a seal on the return side should be broken by a rescue crew; this area should then be reventilated by so regulating the quantity of air that the return from the mine will be kept below the lowest explosive limit.

In like manner advances should be made by successive blocks until the entire area is recovered. As the work progresses frequent analyses should be made to determine the composition of fire gases within the air locks. The oxygen should be kept under control and within safe limits at all times. It is imperative that the oxygen be kept below 6 per cent at all times to allow for a limited amount of infiltration of air as the work progresses, as when 12 per cent of oxygen is reached explosive mixtures of methane and air are possible.

20 men.

PROCEDURE USED IN UNSEALING HORNING MINE

A section (see fig. 19) of the Horning mine of the Pittsburgh Terminal Coal Co., Pennsylvania, was sealed after an explosion that occurred during the sealing of a fire on February 3, 1926, and killed Three bodies had been recovered by rescue crews when a second explosion occurred, making it extremely dangerous to attempt to recover the other 17 victims. After careful deliberation by those in charge of the situation it was decided to seal the affected region and attempt to recover the bodies by erecting a system of air locks when the oxygen content of the sealed area had been reduced to 4 per cent.