ther and son took their picks； their sticks； and their lamps。 They went about searching； sounding the rock with a sharp blow； listening if it would return a favor…able sound。 So long as the soundings had not been pushed to the granite of the primary formation； the Fords were agreed that the search； unsuccessful to…day； might succeed to…morrow； and that it ought to be resumed。 They spent their whole life in endeavoring to bring Aberfoyle back to its former prosperity。 If the father died before the hour of success； the son was to go on with the task alone。
It was during these excursions that Harry was more particularly struck by certain phenomena； which he vainly sought to explain。 Several times； while walking along some narrow cross…alley； he seemed to hear sounds similar to those which would be produced by violent blows of a pickax against the wall。
Harry hastened to seek the cause of this mysterious work。 The tunnel was empty。 The light from the young miner's
lamp； thrown on the wall； revealed no trace of any recent work with pick or crowbar。 Harry would then ask himself if it was not the effect of some acoustic illusion； or some strange and fantastic echo。 At other times； on suddenly throwing a bright light into a suspicious…looking cleft in the rock； he thought he saw a shadow。 He rushed forward。 Nothing； and there was no opening to permit a human being to evade his pursuit！
Twice in one month； Harry； whilst visiting the west end of the pit； distinctly heard distant reports； as if some miner had exploded a charge of dynamite。 The second time； after many careful researches； he found that a pillar had just been blown up。
By the light of his lamp； Harry carefully examined the place attacked by the explosion。 It had not been made in a simple embankment of stones； but in a mass of schist； which had perated to this depth in the coal stratum。 Had the object of the explosion been to discover a new vein？ Or had someone wished simply to destroy this portion of the mine？ Thus he questioned； and when he made known this occurrence to his father； neither could the old overman nor he himself answer the question in a satisfactory way。
〃It is very queer；〃 Harry often repeated。 〃The presence of an unknown being in the mine seems impossible； and yet there can be no doubt about it。 Does someone besides ourselves wish to find out if a seam yet exists？ Or； rather； has he attempted to destroy what remains of the Aberfoyle mines？ But for what reason？ I will find that out； if it should cost me my life！〃
A fortnight before the day on which Harry Ford guided the engineer through the labyrinth of the Dochart pit； he had been on the point of attaining the object of his search。 He was going over the southwest end of the mine； with a large lantern in his hand。 All at once； it seemed to him that a light was suddenly extinguished； some hundred feet before him； at the end of a narrow passage cut obliquely through the rock。 He darted forward。
His search was in vain。 As Harry would not admit a supernatural explanation for a physical occurrence； he concluded that certainly some strange being prowled about in the pit。 But whatever he could do； searching with the greatest care； scrutinizing every crevice in the gallery； he found nothing for his trouble。
If Jack Ryan and the other superstitious fellows in the mine had seen these lights； they would； without fail； have called them supernatural； but Harry did not dream of doing so； nor did his father。 And when they talked over these phenomena； evidently due to a physical cause； 〃My lad；〃 the old man would say； 〃we must wait。 It will all be explained some day。〃
However； it must be observed that； hitherto； neither Harry nor his father had ever been exposed to any act of violence。 If the stone which had fallen at the feet of James Starr had been thrown by the hand of some ill…disposed person； it was the first criminal act of that description。
James Starr was of opinion that the stone had bee detached from the roof of the gallery； but Harry would not admit of such a simple explanation。 According to him； the stone had not fallen； it had been thrown； for otherwise； without rebounding； it could never have described a trajectory as it did。
Harry saw in it a direct attempt against himself and his father； or even against the engineer。
CHAPTER VI SIMON FORD'S EXPERIMENT
THE old clock in the cottage struck one as James Starr and his two panions went out。 A dim light perated through the ventilating shaft into the glade。 Harry's lamp was not necessary here； but it would very soon be of use； for the old overman was about to conduct the engineer to the very end of the Dochart pit。
After following the principal gallery for a distance of two miles； the three explorers……for； as will be seen； this was a regular exploration…… arrived at the entrance of a narrow tunnel。 It was like a nave； the roof of which rested on woodwork； covered with white moss。 It followed very nearly the line traced by the course of the river Forth； fifteen hundred feet above。
〃So we are going to the end of the last vein？〃 said James Starr。
〃Ay！ You know the mine well still。〃
〃Well； Simon；〃 returned the engineer； 〃it will be difficult to go further than that； if I don't mistake。〃
〃Yes； indeed； Mr。 Starr。 That was where our picks tore out the last bit of coal in the seam。 I remember it as if it were yesterday。 I myself gave that last blow； and it re…echoed in my heart more dismally than on the rock。 Only sandstone and schist were round us after that； and when the truck rolled towards the shaft； I followed； with my heart as full as though it were a funeral。 It seemed to me that the soul of the mine was going with it。〃
The gravity with which the old man uttered these words impressed the engineer； who was not far from sharing his sentiments。 They were those of the sailor who leaves his disabled vessel…… of the proprietor who sees the house of his ancestors pulled down。 He pressed Ford's hand； but now the latter seized that of the engineer； and； wringing it：
〃That day we were all of us mistaken；〃 he exclaimed。 〃No！ The old mine was not dead。 It was not a corpse that the miners abandoned； and I dare to assert； Mr。 Starr； that its heart beats still。〃
〃Speak； Ford！ Have you discovered a new vein？〃 cried the engineer； unable to contain himself。 〃I know you have！ Your letter could mean nothing else。〃
〃Mr。 Starr；〃 said Simon Ford； 〃I did not wish to tell any man but yourself。〃
〃And you did quite right； Ford。 But tell me how； by what signs； are you sure？〃
〃Listen； sir！〃 resumed Simon。 〃It is not a seam that I have found。〃
〃What is it； then？〃
〃Only positive proof that such a seam exists。〃
〃And the proof？〃
〃Could fire…damp issue from the bowels of the earth if coal was not there to produce it？〃
〃No； certainly not！〃 replied the engineer。 〃No coal； no fire…damp。 No effects without a cause。〃
〃Just as no smoke without fire。〃
〃And have you recognized the presence of light carburetted hydrogen？〃
〃An old miner could not be deceived；〃 answered Ford。 〃I have met with our old enemy； the fire…damp！〃
〃But suppose it was another gas；〃 said Starr。 〃Firedamp is almost without smell； and colorless。 It only really betrays its presence by an explosion。〃
〃Mr。 Starr；〃 said Simon Ford； 〃will you let me tell you what I have done？ Harry had once or twice observed something remarkable in his excursions to the west end of the mine。 Fire； which suddenly went out； sometimes appeared along the face of the rock or on the embankment of the further galleries。 How those flames were lighted； I could not and cannot say。 But they were evidently owing to the presence of fire…damp； and to me fire…damp means a vein of coal。〃
〃Did not these fires cause any explosion？〃 asked the engineer quickly。
〃Yes； little partial explosions；〃 replied Ford； 〃such as I used to cause myself when I wished to ascertain the presence of fire…damp。 Do you remember how formerly it was the custom to try to prevent explosions before our good genius； Humphry Davy； invented his safety…lamp？〃
〃Yes；〃 replied James Starr。 〃You mean what the 'monk；' as the men called him； used to do。 But I have never seen him in the exercise of his duty。〃
〃Indeed； Mr。 Starr； you are too young； in spite of your five…and…fifty years； to have seen that。 But I； ten years older； often saw the last 'monk' working in the mine。 He was called so because he wore a long robe like a monk。 His proper name was the 'fireman。' At that time there was no other means of destroying the bad gas but by dispersing it in little explosions； before its buoyancy had collected it in too great quantities in the heights of the galleries。 The monk； as we called him； with his face masked； his head muffled up； all his body tightly wrapped in a thick felt cloak； crawled along the ground。 He could breathe down there； when the air was pure； and with his right hand he waved above his head a blazing torch。 When the firedamp had accumulated in the air； so as to form a detonating mixture； the explosion occurred without being fatal； and； by often renewing this operation； catastrophes were prevented。 Sometimes the 'monk' was injured or killed in his work； then another took his place。 This was done in all mines until the Davy lamp was universally adopted。 But I knew the plan； and by its means I discovered the presence of firedamp and consequently that of a new seam of coal in the Dochart pit。〃
All that the old overman had related of the so…called 〃monk〃 or 〃fireman〃 was perfectly true。 The air in the galleries of mines was formerly always purified in the way described。
Fire…damp； marsh…gas； or carburetted hydrogen； is colorless； almost scentless； it burns with a blue flame； and makes respiration impossible。 The miner could not live in a place filled with this injurious gas； any more than one could live in a gasometer full of mon gas。 Moreover； fire…damp； as well as the latter； a mixture of inflammable gases； forms a detonating mixture as soon as the air unites with it in a proportion of eight； and perhaps even five to the hundred。 When this mixture is lighted by any cause； there is an explosion； almost always followed by a frightful catastrophe。
As they walked on； Simon Ford told the engineer all that he had done to attain his object； how he was sure that the escape of fire…damp took place at the very end of the farthest gallery in its western part； because he had provoked small and partial explosions； or rather little flames； enough to show the nature of the gas； which escaped in a small jet； but with a continuous flow。
An hour after leaving the cottage； James Starr and his two panions had gone a distance of four miles。 The engineer； urged by anxiety and hope； walked on without noticing the length of the way。 He pondered over all that the old miner had told him； and mentally weighed all the arguments which the latter had given in support of his belief。 He agreed with him in thinking that the continued emission of carburetted hydrogen certainly showed the existence of a new coal…seam。 If it had been merely a sort of pocket； full of gas； as it is sometimes found amongst the rock； it would soon have been empty； and the phenomenon have ceased。 But