As it seems the belief of many people that lightning cannot descend into the workings of a pit, the writer has thought that it might be desirable to add another well authenticated record of the presence of the electric fluid in a mine during a thunderstorm to the one at Tanfield Moor already recorded in page 31, Vol. XXX. of the Transactions of the Institute.
West Thornley Colliery (which has three times been the site of electrical discharges) is situated about a mile from Tow Law, which is 1,000 feet above the sea level; it is 25 fathoms deep, and is sheltered by a hill on the south side 50 feet high, which commences to rise about 50 yards from the pit, the ground on all other sides being fairly level.
The steam boiler chimney was 46 feet high, 12 feet south-west from the engine house, and 68 feet south-west from the centre of the pit.
The pulleys are 53 feet from the surface. The ropes are made of plough steel, the cages of steel, the guides or skeats are made of iron rails, secured by wrought iron buntons, and there are four rapper ropes in the pit, two of which only reach to the bottom seam. There are three ranges of pipes in the pit, viz., a sett of 5-inch steam pipes c, Plate III. covered with patent composition, which lead into a receiver x at the pit bottom, and from it to the underground hauling engine y, which is 40 yards on the south side of the pit bottom. A second range consists of an 8-inch rising main b, which rests upon a large balk at pit bottom, and a third range c of 10-inch pipes, which goes into the sump about 15 feet below the flat sheets, and is now used for conveying exhaust steam to bank.
About 10 P.m. on Tuesday, December 11th, 1883, there was a violent wind, with heavy rain, accompanied with much thunder and lightning. About 1015 the winding engine brakesman, Mark Adams, who was watching the storm out of the engine house east window, saw a flash of forked lightning about the pulleys, and heard a heavy peal of thunder, and about five minutes afterwards, at 10'20, when still looking out of the window, saw another brilliant flash of forked lightning strike the pulleys and light everything up in a blaze, almost blinding him, instantly accompanied by a terrific clap of thunder. Hearing a great noise on the other or west side of the engine house, he opened the door and found the boiler chimney had been struck at the top on the north-west side, and a large zigzag rent made on the west side, varying in width from a few inches to two feet and reaching to within about 15 feet from the bottom, and bricks and lime were falling all over the place.
Robert Emery, the master shifter, who was engaged about three yards from the shaft bottom, said he heard brattles of thunder about 10 o'clock, and between that and 10-20 he heard a very heavy one, when at the same time a flash of lightning came down the pit on the south-west side, and apparently down the rapper rope r', glancing from the rapper handle, which pointed at an angle of 45 degrees towards the 5-inch steam pipes a, which were about three feet off and uncovered at that point, and producing a brilliant light and a noise like that of the firing of a gun.
There are metal flat sheets on both sides of the pit.
There were no marks of damage about, but the flash seemed to have left behind a sort of vapour which appeared to pass along the steam pipes inbye, and it may be assumed that the lightning struck the pulleys and went down the winding rope to the cages, which were standing in the shaft, and then passed to the rapper rope.
John Craggs, shifter, who was also working at the pit bottom, confirmed the above statement.
This occurrence the writer has investigated with great care, and puts it forth as a statement entitled to every credence; the following statements refer to the two previous discharges that had been noticed:
About three years ago, in the summer, and during the middle of the day, W. Newton, ousetter, said a flash of lightning came down the pit and appeared to strike the flat sheets on the north-west side, making a very brilliant light, and a report louder than any gun. Those at the pit bottom also saw it and were very much frightened.
At the same time the underground hauling engineman had his hand on the throttle valve handle and felt a strong shock in his wrist and arm, and saw the lightning most distinctly. Two shifters who were working about 15 yards from him saw the lightning most distinctly through a stenton at right angles, but there were no rails or pipes to conduct it to them through the stenton.
About fifteen years ago, at mid-day, when W. Newton was then banking out, and had his hand on the cage sneck, there was a very vivid flash of lightning, and he experienced a very severe shock in his arm and feet, and considerable pain and numbness for the remainder of the day.
It would appear in this case, too, as if the lightning had struck the winding rope, passed through the cage, which was at bank, to the skeats, and gone down the pit, at the bottom of which it was most distinctly seen. It then struck one of the flat sheets, and broke it into several pieces, passing probably along the rails to the face of the south stone drift, which was then about 80 yards from the pit bottom, and was most distinctly seen there by two stonemen who were getting their baits, who said that it lighted up the whole place.
Having had some conversation with Mr. Massingham, Dean Street, Newcastle, who has had large experience in putting up lightning conductors, etc., the writer asked him if he knew of any local reasons why this pit should have been so often struck, and he replied that when once a place had been struck by lightning it was rendered more liable to be struck again for some time after, and that some parts of the earth, owing to the nature of the soil, etc. (which at West Thornley was a wet bluish clay probably with metallic veins running through), had a greater affinity for lightning than a dry soil of sand, chalk, or granite.
When asked as to the area or space a conductor would protect, he said it was now generally agreed that the area protected was in the form of a cone whose base was equal to its height, so that it would appear all chimneys or buildings a certain distance apart should have a separate conductor; and he was of opinion that the lightning, being forked, had struck the West Thornley chimney and pulleys simultaneously, and that a conductor on the chimney would not in all probability have prevented the lightning going down the pit.
The pit being an upcast, the warm current of air would offer a great inducement to the lightning. All lightning conductors should be tested about once a year, as, owing to alterations, repairs, and earth connections being disturbed, they are apt to get out of order.
Mr. S. T. WALKER said, he thought the thanks of every person interested in this question were due to the author of this paper for the very careful way in which he had given them all the details. The action of lightning was the most difficult branch of electrical study that he knew of; and its difficulty was principally due to the fact that lightning was always doing something they did not expect. Lightning apparently acted in a promiscuous sort of way, and went where it liked, and did as much damage as it could, and there was no guarding against it; but he thought if they carried the matter back to first principles they would find the whole thing was simple enough; in fact, the difficulties were due entirely to the enormous tension at which the charge-which was the force through which lightning acted-was generated. He thought the same kind of difficulty was experienced in regard to the smelting of. iron. So far as he knew, in regard to smelting, there were enormous heats, and up to very recently, he believed, there was no method of testing the actual degree of heat which was required to complete the process, and still less the heat that was necessary economically to do so; simply because nobody could get near enough to measure the heat in the furnace with any degree of certainty. So they were under the same difficulty with lightning, because they had no lightning test. If they could know the conditions which were present in the lightning discharge, they could possibly find out all it could do, and all it could not do; but they could not have that knowledge, because the tension, so far as they were able to judge by reasoning upon the actual facts under which the lightning acted, was enormously in excess of any tension they could possibly experiment with. Dr. Spottiswoode, the late President of the Royal Society, and Dr. De la Rue, had spent many years in experimenting on this subject, and built up a battery of some 10,000 cells; but they did not get anything near the tension, not the hundredth part of the tension perhaps, developed in the ordinary lightning flash. When they knew the difficulty of setting up 10,000 cells, and keeping up the insulation, they could imagine the immense difficulty of finding out what really took place in a case such as now reported. They could only judge by reasoning as closely as they could upon what took place from time to time, and from a careful comparison of the different results they knew. Lightning was merely a very powerful electric spark; it was only a discharge from a cloud very highly charged. It was little matter how the cloud gathered it, or generated it; the charge would be increased by its friction with the atmosphere as it blew along by the action of the wind. The air had an enormous resistance.
The current equalled the force divided by the resistance. If the resistance was one million Ohms (and it might easily be many millions) it was necessary that there should be a hundredth part of that force between the cloud and the object which it was to strike, for it to deliver the current necessary to pass the spark; and so long as it did not get that difference in the tension, and satisfy that condition, no current could pass. The action of a cloud probably was that it would acquire higher and higher tension as it passed along, and he thought be was right in stating that in the case of thundery weather, the cloud came lower and lower, and the atmosphere between the cloud and the earth became more impregnated with moisture than on an ordinary day, and, therefore, the conditions of discharge became more and more favourable. As the cloud passed along, any elevated object, such as a chimney stack or a head stock, or anything high would reduce the resistance very considerably. A chimney 200 feet high would reduce it enormously; because the resistance of 200 feet of air would be enormously in excess of the resistance of 200 feet of dry brickwork, or perhaps wet brickwork. Bricks were porous, and when exposed to the atmosphere they would probably fill to a certain extent with moisture; while deposited inside the chimney there was usually a very compact layer of carbon, with it might be some salts, so that the chimney itself might be by no means a bad conductor, and the more approach of a cloud might determine the conditions of the discharge, and it might be that when a cloud arrived within a certain distance of a certain chimney the resistance might be sufficiently low to enable it to pass the required current. But in determining the resistance they had to consider the whole of the parts of the chimney. The outside brickwork was one part, and the inside of the chimney leading to the furnace, and more or less in metallic connection with the boiler, was another. The heated air and smoke from the chimney would also lessen the resistance. If there was a lightning conductor on the chimney it would deliver the whole of the charge, no matter how great it might be, silently into the earth, provided it was carried sufficiently above the chimney, and was in such a position that the resistance offered by the lightning conductor was very much less than any other resistance. It would be useless for a lightning conductor to be at such a distance from the object it was intended to protect, that the resistance offered by the distance between it and the cloud left a fair path in some other direction not protected. There was another law in electricity which said that when a current had two or more paths open to it, it divided in the inverse proportion to the resistance, the largest portion went to where the smaller resistance was, and the smaller portion to where there was the largest resistance. In this case the lightning appeared to have divided, so far as he could see, into five or six different paths. There were two parts in the chimney itself-the brickwork of the chimney, and the inside of the chimney leading to the furnace-where the discharge would take place naturally. Whatever the cause, the share of the charge which the chimney itself took was sufficiently great to do considerable damage. There was another law, and that was, that where a current encountered resistance it did damage in exact proportion to the resistance it encountered. If the current was large it developed heat according to the square of the current. If the current was small and the force large, the damage would be as the square of the force. In this case the current going down the chimney might be very small, but the force would be so large as to be sufficient to do the damage that was done. Then another portion would strike the pulleys. The head stocks were of wood but the pulleys would be iron, and would be in connection with the rope, but not in perfect electrical connection, because there was always a good deal of grease, and grease was not a good conductor; consequently the current would again split, and part would go down the rope. Another law was, that when there was a charge of electricity, involving a very small quantity, but at very high tension, if any other object were brought in connection with it, or near it, as a conductor, it would take a portion of that charge. In this case the rope and engine would relieve the charge of a large portion of its intensity and another part would pass on to the boiler, and to the feed place or pond where the water was drawn from. It was his opinion 'that if the ropes were in perfect electrical connection with the pulleys and the pond made good earth no charge would have found its way down the pit. It was owing to the imperfect connection between the ropes and the pulleys, and again, probably, the drums and the ropes, owing to the layer of grease which offered a certain resistance to the passage of the charge, that a portion went down the rope leading into the pit and to both cages. The cages, he presumed, would be in connection with the guides, and the guides would take a portion of the charge, and the cages a portion. The guides would carry the charge into the sump, where still more of it would be dissipated. If the sump were making perfect earth, and able to dissipate the whole of the charge, there would be no flash going to the rapper handle, but the sump took only a certain portion of the charge, the sides of the shaft another portion, the pipes another portion, and still there was enough left to get to the rapper wire. He thought most probably the original flash struck at the same time the pulley and something in connection with the rapper wire. Then the charge going down this wire apparently could find only one outlet, and flashed across to the pipes which led away to the hauling engine, in connection with which there was always a mass of metals, rails, and rope in addition to the whole mass of coal. A gentleman made experiments in South Wales, about two years ago, on this subject-which, however, he had not had time to verify -to prove that coal, although an imperfect conductor, would, if the tension was high, accept a charge. If that were so, and if the charge did find its way into the mine, the enormous surface of the mine would lead the charge away, and it would be dissipated. So far as he could learn, in every case he knew of, that seemed to be the course the lightning adopted. In one of the other cases mentioned, the man at the underground hauling engine saw the lightning and received a shock in his hand or arm, and men some distance off, round a corner, saw the lightning, as well. There were two explanations of this. One was that it was an optical effect, and the other that it was electrical. He was inclined to think that what the men saw was the remainder of the charge distributing itself harmlessly over the coal, and he was borne out in this opinion by the fact that no harm seemed to have been done. In no case where lightning penetrated into a pit had any man been seriously injured by what was left. It had been said that explosions had occurred, but he had seen no conclusive proof of that. The mere fact of a charge with all these paths open to it still finding its way down, and then only hurting a man's arm and doing no serious injury, was conclusive proof that the charge must have been considerably dissipated. He believed the cases in which lightning descended a pit were very few indeed, and he did not know of any properly authenticated instance in a working mine of an explosion occurring through lightning. He did not say it could not take place; but to be safe, a properly fixed lightning conductor, as large and with as many points in as many different directions as possible, should be carried above the head stock as high as possible, and the end carried into good damp ground or a river. If such a lightning conductor were placed over every colliery in the kingdom they would not hear of lightning going underground into workings. The first office of a lightning conductor was to discharge the cloud long before it arrived at the object which the conductor was to protect. What was known as the first discharge took place at the points of a lightning conductor, and the area over which it was discharged would be very great. They had in Germany, or they had some years ago, a very large machine, and the brush discharge from that machine was, he thought, 30 feet. If that were so with the comparatively small tension they were able to get with the largest and most perfect machine they could make, then the brush discharge from a point with a charge of far higher tension must be very much greater than that. The second office of the conductor was this-It might happen that, notwithstanding the brush discharge, the charged cloud would be driven along by a strong wind, and arrive at the object before the conductor had time to discharge it, in that case the conductor would take it harmlessly to earth. If the conductor was properly fixed, and bad the capability of conducting the highest current and the greatest tension, then it ought to be possible to take that conductor through a powder magazine without doing any harm. The human body-the object most sensitive to electricity he knew of-would probably not feel it, always provided the conductor were perfect. Conductors required to be looked at occasionally, for it became a serious matter if the conductor were tampered with, and the defect not found out. He understood there had been a controversy in the North as to whether lightning went from the earth to the cloud or from the cloud to the earth, and that there were no such thing as discharges, but only thunderbolts. So far as he could understand he could see no difference, whether it went from the earth to the cloud or from the cloud to the earth, in the effects produced. As to thunderbolts, he remembered many years ago seeing an object which he was told was a thunderbolt. It was a small ball which had been broken open, and had a beautiful crystalline structure inside. With the knowledge which he had acquired since, he imagined that if that was a thunderbolt, it would be formed simply by the sudden condensation of metallic vapour held in suspension in the atmosphere-supposing that it was metallic vapour. If, from disturbance in the atmosphere, such a bolt fell it would kill any man it struck, and injure a building, but that was very different from the damage done by lightning.
Professor LEBOUR said, the thunderbolts alluded to by Mr. Walker were undoubtedly lumps of iron pyrites, and were common in the South of England. They certainly had not come from above, but from the chalk and the green sand, and other formations.
Professor HERSCHEL said, he did not think a discussion had ever been raised in the North as to whether lightning went up or down. The question of its going up or down would not affect the danger of its action; but the question of what road it took was a far .more important point. This subject had already been placed before the Institute at considerable length in papers contained in Vol. XXX., where there was a carefully described instance of the lightning's course in a pit; and therefore they could not doubt that it did enter the pit, and ramify about the wagonways for the long distance of 800 yards, and always continuing to have great intensity and tension. So far from diffusing itself in the coal, as they hoped and wished it would without risk, it had been attended with reports and flashes which could not but be attended with risk in a fiery mine. The course which the lightning pursued in this case might certainly be too difficult for them to enter upon without much more full details as to the metallic parts of the connection between the point of the stroke of the lightning above ground, and where it was received and perceived below ground. Such probabilities as that the grease would affect the direction the lightning would take, could not possibly be entertained. Mr. Walker spoke about the tension of lightning, and instanced that of 10,000 cells as below the mark. If 50,000 cells were necessary to produce a spark a quarter of an inch long, what would be necessary when they had lightning flashes of a mile or more long? They could not describe the tension by thousands of thousands of cells; and grease would present no obstacle to such an electric force. He believed the evidence brought before the Institute on former occasions of lightning strokes in mines, and those which had now been freshly introduced, all pointed to this: that the stroke of the lightning must be warded off, and not guided into the mine in any manner. Mr. Walker was right in saying that a good lightning conductor should be provided about all the prominent parts of a colliery, and have a good earth connection. In page 43, Vol. XXX. of the Proceedings of the Institute, Mr. Heaviside wrote a letter in which he gave three instances of earth being unsuitable for electrical purposes-at the Ballast Hill, North Shields, at Throckley, and at West Stanley, when they were unable to find earth for the electric telegraph wires. At the North Shields Ballast Hill the return wire had to be taken down to the River Tyne. There was no doubt they would not get a good earth for a lightning conductor at all collieries; but at most collieries there was a stream of running water, to which the conductor must be taken, no matter how distant. This was really the point in the matter of protecting mines. It was unnecessary to enter into elaborate details what route the lightning took on one occasion or another. They could not foretell or foresee what direction it would take. The supposed probability that it would repeat its stroke a second time at the same place was perhaps ideal. The lightning did not strike the chimney on the previous occasion. A long iron rope leading into the sump was a suitable channel for the lightning to take, but whether it would take the same channel another time one could not say.
Colonel PARNELL said that the matter required careful study. There had been a good deal of controversial matter mentioned which had no connection with this incident at all, and he hardly thought this the time or place to enter into these matters. He thought Mr. White's paper very interesting, and he hoped they would soon be able to obtain some light as to the best means of protecting collieries in regard to lightning strokes.
Mr. MASSINGRAM said, he had been courteously invited to attend the meeting, not as a scientist, but as a practical man. He had advanced the theory, that after a chimney or other building had once been struck by lightning, that it had a greater affinity for the electric fluid than it had before, and was thereby rendered more liable to be struck again. He had no scientific authority for this assertion, although he had carefully searched all the books that he could find bearing upon the subject, but could not find that this particular fact had been noticed at all. The suggestion was based solely on his own personal observation and knowledge of the fact, that numerous cases of spires, chimneys, etc., had been struck by lightning several times over (although previous to the first stroke they had stood for years unharmed), whilst others in the immediate vicinity had escaped uninjured, and he thought that there must be some particular reason for this. He knew that this had been the case in a great many local instances, and no doubt there were hundreds of similar cases which he knew nothing of. He was inclined to think that when a building or chimney had been struck by lightning it remained charged with electricity, the same as a piece of magnetised iron, for an indefinite time, and being in this charged state, the chimney or building would have a greater affinity for lightning than it had before, and be more liable to be struck again.
Professor MERIVALE said, that as to a place once struck being struck again by lightning, he supposed it would not be a matter of wonder; because the conditions which would make the place liable to be struck once, would make it liable to be struck again.
Mr. WALKER said, he could fall in with the view of the last speaker, and not with those of Mr. Massingham, that the chimney remained in a charged condition. An object was struck because the meterological and electrical conditions were favourable to discharge by that path; and if these conditions remained the same, the object might be again struck.
Mr. MASSINGHAX said, he could not agree to the remarks of Professor Merivale, for he had known a chimney built on a certain spot of earth fifty years ago, on a certain spot which would presumably remain the same during the whole fifty years. For the first forty years the chimney was not visited by lightning, but in its fortieth year it received a stroke, and in the ensuing ten years several other strokes. What is to be inferred from this? Did the conditions remain the same as they always were, or were they altered ? If altered, in what way ? May not the building and its surroundings have become charged as suggested? He did not pretend to be able to solve this question, but certainly considered that such cases as this, of which he had numerous instances, will go a long way to prove, at least, the consistency of the theory advanced. Mr. Walker said that he could fall in with the views of the last speaker, but would not admit "that the chimney remained in a charged condition," but as he did not give satisfactory reasons for saying so, he (Mr. Massingham) could not accept his opinion as final or conclusive. Professor John Murray, F.S.A., F.L.S., F.H.S., F.G.S., &c., in his work entitled, "A Treatise on Atmospherical Electricity, Lightning Rods, and Paragreles," said :-"Once struck, we should presume that the same spot is always liable to a revisit, as there are several instances of buildings in this country having been similarly visited." This exactly coincided with his own observations and experience, and, coming from so eminent a scientist, he submitted that it is a theory that should receive a full and free investigation, as it must be a point of great importance, more particularly in the protection of collieries and their surroundings. Mr. Walker also said, that in the case of the West Thornley pit, if the rope and the pulley had been electrically connected, the lightning would not have gone down the pit! Where would it have gone? He was of opinion that whilst the rope was running over the pulley, both were electrically connected, the bright parts of the rope coming in contact with the bright parts of the inside of the pulley, forming a perfect electrical connection, and yet the stroke went down the pit. There is but one way of preventing lightning strokes from going down pit shafts, and that is, to have properly constructed lightning conductors erected considerably higher, and independent of the head gearing of the shaft, and carried to a perfect and good earth contact; indeed this, the earth contact, is the principal part of a lightning conductor, and the neglect or ignorance of this fact, is the most frequent cause of their failure. It still remains to solve the question or theory, as to whether a place having been once struck is thereby rendered more liable to be struck again; and as this very important point in the subject of lightning strokes, appears hitherto to have received very little attention at the hands of our great scientists, it could not fail to be both useful and instructive, if it were more specially considered by experts.
The PRESIDENT proposed a vote of thanks to Mr. White for his paper, and said that it showed one of the many dangers to which coal-mining was liable; and the use of such an Institute as this was to bring these matters before the members for discussion.
The motion was agreed to.