With the single brattice screen across the Haigh Yard level still in position next morning, the intake to the district, apart from the air which escaped past the screen, was No. 2 drift. Gas was still present in a cavity 2 feet 6 inches high above some of the arch girders supporting the short length of level road at the top of the drift, air samples showing the presence of up to 5.92 per cent. of methane. The amount of air flowing up No. 2 was very small; a measurement at the lifted screens in the connection showed a quantity of 1,120 cubic feet per minute to be passing through them; it was impossible to detect a flow on the face.

A quantity of 4,860 cubic feet per minute, however, was flowing in the return airway, showing that in spite of the screen on the Haigh Yard level, 3,740 cubic feet per minute were passing up No. 1 intake. A very significant discovery was that gas had accumulated at the roof of a short level length of road at the apex of No. 1 intake where the Plodder seam was exposed and where the gradient changed from that of the rising one of the stone drift to the dipping one of the road driven in the seam, the counterpart of the spot in No. 2 intake where the Pikrose was fixed. The gas extended over a length of 16 yards and the layer was three feet deep at the thickest part.

After the screen on the Haigh Yard level had been taken down, the gas at the high points in both intakes was removed by setting hurdle sheets; it was considered advisable to leave a hurdle sheet set in No. 2 for some days.

A close examination of the control switch at the Pikrose haulage at the top of No. 2 drift in the condition in which it was found on the previous evening was made. The switch had two compartments, a main enclosure containing the contactors, overload protective devices and control circuits, and a busbar compartment above containing an isolator. As already mentioned, the front cover of the main chamber had been removed and the cover of the busbar chamber had been twisted round. The switch was so constructed that the isolator was mechanically interlocked with the front cover of the main enclosure; thus it was not possible to remove the front cover until the isolator was in the "off " position. As found, the apparatus inside the open main enclosure was therefore "dead ". In the busbar chamber the busbars were constructed so as to be slightly proud of the top of the chamber, the top cover being domed to give the necessary clearance. Thus, when the cover was swung into the position in which it was found after the explosion, the inside edge of the flange of the top cover would foul one of the busbars (see Plate II (b) and inset) and as already mentioned previously, at the corresponding points of contact of the cover flange and the busbar there was evidence of electrical fusion. This was a plain indication that the busbars had been "live" when the cover was twisted around and that arcing had occurred when contact was made.

This conclusion was reinforced by the fact that the section switch controlling the supply to the haulage had been found, after the explosion, to have tripped out on an earth leakage fault. A detailed examination and test of the control switch revealed no other defect in the tripping or operating mechanism. The difficulty experienced by driver Woodward was found to have been caused by burnt contacts on the isolating switch on the haulage, but this had no connection with the arcing which occurred at the time of the explosion.

The cables and other apparatus in the district were examined and tested and found to be in good condition, with no indication of any fault having occurred on them at the time of the explosion.

Two D.C. intrinsically safe bells which were near the haulage unit were tested at the Safety in Mines Research Establishment at Sheffield; the tests indicated that in the condition in which they were found they were incapable of igniting firedamp.

A 500-volts insulation tester which was found lying at the side of the control switch was not of an intrinsically safe type. Owing to the position in which it was found, however, and the fact that the handle was not in the operating position, it is unlikely that this instrument was being used immediately before the explosion.

From the foot of No. 2 drift the flames were found to have extended some 40 yards along the Haigh Yard level towards the pit bottom and also, in the other direction, about ten yards. In the direction of the face, i.e. on the return side of the Pikrose haulage, there were slight indications of burning for about 30 yards. The total length of road traversed by flame was about 150 yards. The only burning material which required to be quenched were a few articles of clothing, some small pieces of brattice cloth and a wooden chock; the other indications of heat observed were the charring of wooden supports, of paper, notices and cable coverings (see Plates III and IV).

To assess the extent of the flame it is necessary, therefore, to consider its effect on the unfortunate victims of the explosion, distasteful though this course may be. Of the five men who were within a few yards of the Pikrose haulage, four sustained superficial burns affecting about a third of their bodies, while W. Pearson, who lived for a few days, had more extensive burns over about half of his body surface. The five men working near the foot of the drift were more severely affected both as regards the extent of their burns — some had burns over three-quarters of their total skin surfaces — and their depth. Some of the burns were of the third degree.

One of the survivors, T. Dzundza, who had been working at the foot of the drift, said that flame came down the drift twice; another survivor, J. Howcroft, said he was blown over by the blast and was on the floor when the flame struck him. It appears that in parts of the roadway, at least, the whole area became full of flame. This likelihood, together with the severity of the burns sustained by the men, suggests that a considerable quantity of gas exploded. It can only be conjectured, however, how far the gas extended before the explosion and how thick was the layer of gas at the explosion point.

No physical damage was caused. A stone dust barrier extended laterally across No. 2 intake ten yards inbye of the electric switch whence the arcing emanated, but neither the boards nor, as far as could be seen, the dust, were disturbed. It is likely of course that the explosion had not developed much force so close to the igniting point and, as has been said already, there was little indication of flame having extended in an inbye direction. Furthermore, the construction of the barrier, cross boards 3 feet long placed edge to edge parallel with the roadway and supported by wooden spars set on the sides, was such that a force of some magnitude would have been necessary to overturn it, as the spars gave solid support at the ends of the cross boards instead of at the middle. Central support would have assisted in causing the easy collapse of the barrier.

Coal dust played no part in the explosion; apart from the fact that little violence was produced, there was an almost complete absence of coal dust on the roads, along none of which had coal been transported to any considerable extent.

Although a probable igniting agent was evident at an early stage in the investigation, the possibility that another method of ignition had been involved was not lost sight of. The remaining electrical apparatus, as already mentioned, was found to be in order. The electric cap lamps of the four men who were killed outright and the two flame lamps which had been taken in by the men working at the ripping near the drift foot as firedamp detectors were examined at the Safety in Mines Research Establishment at Sheffield. All the lamps were found to have no defects which could have caused the explosion. The clothing of the men who had worked near the Pikrose haulage was examined but no contraband material was found.

A tentative suggestion that there had been a blower of gas was put forward half-heartedly at the inquest. I am unable to accept this suggestion; no evidence, such as a recognised liability of the seam or the simultaneous occurrence of any related physical phenomena, was brought forward to support it.

I conclude, therefore, that gas which had accumulated at the top of No. 2 drift through a progressive reduction in the amount of air flowing up the drift, following the fall on the east side of the dip development face nearly five days previously, was exploded when an arc was struck as an electrician twisted the cover plate of the busbar chamber of the switch for the Pikrose haulage, and the earthed flange of the cover plate came into contact with a live busbar. The road had a height of 9 feet 6 inches and the igniting arc was struck at a point 1 foot 9 inches from the floor. The source of the gas was the Plodder seam where it was exposed by the driving of No. 2 drift.

A reduction in the amount of air flowing along the east dip face and thus up No. 2 drift had been noted the day the fall occurred. The possibility that the position at the face might get worse was realised by the under-manager, who rightly had the screens in the crosscut raised. This measure was only a partial relief, however, since the resistance of the crosscut and its two frames was greater than that of the face, as shown by measurements carefully made some time after the explosion. It is probable that there was always a small "make" of gas at the top of the drift and the reduction in the quantity of air flowing would be sufficient to cause an accumulation to develop (or even an existing one, in the cavity above the girders, to increase) giving a reduction in the effective area available for air to travel; this restriction in area would cause a further reduction in the quantity of ventilation flowing. The process was thus cumulative until, as was stated by one of the officials concerning the position on the morning of the explosion, "only a slight current of air was passing in the cut through and only a small current was passing through the fall on the face ". The air passing up No. 2 drift was the sum of these two small quantities. This view of the position was confirmed by F. Holmes, night deputy, who said that after firing shots in the ripping in No. 2 drift during the night shift of 4th — 5th March "the fumes cleared up the tunnel slowly ".

It must be remembered that all this time an excellent road, No. 1 intake, with an area of at least 100 square feet along all its length, was available for air to enter the district at the middle point of the face. When comparing this, area with that of No. 2 drift circuit it should be noted also that, in addition to the restriction provided by the two screen frames, both open but only of an area of 18 square feet, parts of No. 2 drift itself had an area of no more than 45 square feet. Strong confirmation of the suggestion that gas gathered at the top of No. 2 drift through a fall in the quantity of air passing was provided by the finding of the gas at the high point of No. 1 drift the day after the explosion. It will be remembered that a screen had been put across the Haigh Yard level on the night of the explosion to drive air up No. 2. The quantity flowing up No. 1 fell from 6,418 cubic feet of air per minute to 3,740 yet, in spite of this flow, within 15 hours of the screen being set, a quantity of 600 cubic feet of gas capable of extinguishing a flame safety lamp gathered at the exact counterpart in No. 1 drift to the place in No. 2 drift where gas was found after the explosion. A common effect suggests a common cause.

When it was considered that the heat effects of the explosion had been dissipated, measurements of the quantity of air passing were made at a number of points under conditions as far as possible comparable with those just before the explosion. It was found that the quantity in Gibfield Stret was 5,688 cubic feet of air per minute compared with one of 10,000 on 1st March, five days before the explosion. Every effort was made by practical investigation and enquiry to find out whether any changes other than the fall on the face had occurred in the ventilation circuit (and it will be remembered that inasmuch as the air for the Plodder district came from the downcasts of two other collieries and returned to the upcast of a third, there was scope for changes, involuntary or otherwise, to have been made), yet no proof of any such changes could be established. Whilst looking upon the fall on the face as the originating cause in the accumulation of the gas I still feel that there may have been hidden in the complexities of the ventilating circuit some factor which did not come to light.

Survivors stated that the party were at the top of the drift for some 20 minutes before the explosion occurred; the atmosphere was thus capable of being breathed though W. Pearson, one of those present at the top of the drift, stated just before he died "The atmosphere was a bit sticky. it seemed too warm and the ventilation wasn't so good. There was a slight smell of gas, and when we got to the top of the brow we were panting." The fact remains however that the arc, which was of limited extent as shown by tests made after the explosion, was struck at a level as low as 1 foot 9 inches from the floor, showing that gas in the inflammable range was present at that point. There was doubtless an extensive rich layer near the roof (some of it possibly even spilling down the drift) and the movement of the men would cause it to be displaced. The evidence of T. Dzundza — that flame went down the drift twice — was an indication that the flame of the burning gas, as is often experienced on these occasions, was of a pulsating nature.

The two deputies, Houghton and Robinson, had each visited the Pikrose hauler not very long before the explosion occurred. It is not impossible — as indeed has happened on many occasions in similar circumstances previously — that the fact that gas had not been found since the reopening of the Plodder seam had led to the search for gas being not so thorough as good inspection demanded, particularly at this vulnerable position at the apex of No. 2 intake where electricity was in use. Houghton's visit was towards the end of his first shift, during which no persons had been at work there. Robinson stated that he visited the place some time after Houghton did, in fact less than an hour before the explosion, that he stood on the top of the very switch which was later to figure so dramatically in the explosion, made tests for gas at arm's length and found nothing. I find it impossible to accept that any tests, if indeed any were made at all, were more than of a casual, perfunctory and superficial nature.