would be developed by one engine in the case of a single fan producing at the same speed a similar ventilation. It appears to the writer a desirable arrangement in providing duplicate ventilators for a mine to erect them in such a manner with suitable connecting drifts so as to be able to use them either singly or as described in duplex or tandem fashion. In case of a breakdown, one is available for about 80 per cent. of the total quantity of air obtainable by the two ventilators, and would probably prevent the stoppage of the mine, while the capability of the two to give the larger range of ventilation without increasing the speed is of primary importance. The subject is one well worthy of further investigation, and the writer hopes that having called attention to it some members may take it up. The CHAIRMAN said he was sure the meeting would be glad to hear the observations of members on the subject. He had heard that the principle had been applied in a fan used for forcing purposes. Professor MERIVALE said there was one point that occurred to him in connexion with Mr. Cochrane's paper, and that was as to connecting the first fan at the bottom of the evasée chimney. He thought the general custom was to make it at the top; but he did not think it made much difference; if at the top, better results were obtained from the first fan, and if at the bottom, better results were obtained from the second. He also wondered what the result would be if the fans were run both duplex and tandem by removing the division. He would like to ask Mr. Cochrane if he could put him in the way of obtaining any results of experiments, or any formulæ, by means of which to calculate the orifice of passage for various fans? It was an important point to be taken into account in designing a fan. He concluded that one would not be inclined to ventilate a pit by means of two fans arranged tandem if starting with a new plant. It would be better to put up a fan sufficiently large in the first instance. Of course, if additional ventilation was required ten or twenty years after a fan had been erected, it would be a good plan, but he took it that the experiments were the result of an idea of twenty-five years ago, when it was not generally known that to get the best effect from two fans they must be placed tandem. Mr. T. H. M. STRATTON said that, according to the experiments, the fan nearest the upcast, and running at sixty revolutions, gave a water-gauge of 1·80 inches, and the leading fan 2:10 inches; but one had been taught to believe that a Guibal fan running at the same speed would always produce the same water-gauge, whatever the quanity of air. Had the fans separate orifices? The CHAIRMAN said this was one of the anomalous points mentioned in the paper, and it was due to the fact that the shutters were not adjustable. They had to take the fans as they found them-by a proper adjustment of the shutters they could vary the water-gauge. Mr. STRATTON said his point was that the efficiency of the fan was not at all dependent on the water-gauge. If a Guibal fan were revolved with its orifice directly communicating with the open air, would it not produce a water-gauge proportional to its peripheral velocity? Apart from its ability as a propeller of air would it not give a certain depression? The CHAIRMAN said the question as to water-gauge and equivalent orifice was one which had never been properly appreciated. There had only been fact, and not theory, as regards the Guibal. Guibal's formula was not V2÷2g, but very different from it. It was a complicated formula, the area of the chimney and the size of the inlet entering into it. Replying to Prof. Merivale, he did not know of any formula of the kind referred to. M. Guibal had investigated the subject, but he did not know whether his researches would be published. They had not tried the combined experiments suggested by Prof. Merivale. It might be noticed that they recorded a ventilation at 110 inches, which must absolutely require 1.80 inches. So that they were very anomalous. The Professor had also asked why they made the connexion as they had done instead of bringing the spiral inlet to the other side. To do this would have involved a very expensive structure, and although better results might have been obtained, the expense and difficulty prevented it. Professor MERIVALE-Then, you did practically do it in the ordinary way with the evasée chimney! The CHAIRMAN-Yes; we continued the evasée chimney up to the inlet. Upon the other question, that it would be preferable to put a sufficiently large ventilator in the first instance, they never knew what might ultimately be the requirements of the colliery, and in putting in a sufficiently large ventilator they provided no duplicate. As mentioned in the paper, he thought it desirable to put in duplicate fans. Mr. WALLAU said there were a great many arrangements possible with two fans, and it was a question as to which arrangement recommended itself most under certain circumstances. There were cases where two small fans upon one axle would give better results and be more economical to begin with than one large fan. The cases he meant were those where the water-gauge required could be readily obtained by a smaller fan, and two fans having each an inlet and being connected with the mine, of sufficiently large area, by running at a high speed would give the proper water-gauge, and the two small inlets would do as much work as the larger one. The smaller fans would be the more economical arrangement, for in arranging the ventilation in that way it was possible to have a much more simple drift than with the ordinary fan. For, if they put the fans on one axle, they could have bearings clear from the drift, and therefore free from the soot and dirt naturally present in a mine. This was a matter that had been considered of so much importance that Messrs. Black, Hawthorn & Co. had taken provisional protection for a certain arrangement of two fans. They were now making a model to thoroughly test it, and perhaps before long it would be brought under the notice of the Institute. The CHAIRMAN-To get the same air from the mine, notwithstanding that smaller fans have to run at the same velocity of periphery as the larger one? Mr. WALLAU-That is so. Mr. THOS. BELL said before passing to the next paper he wished to propose a vote of thanks to the President for the very able paper he had laid before them. In doing this he would suggest that the discussion be adjourned until the paper was in the hands of the members. The vote of thanks was unanimously adopted. DISCUSSION ON MR. M. WALTON BROWN'S PAPER ON "EXPERIMENTS WITH EXPLOSIVES USED IN MINES," AND ON MESSRS. M. WALTON BROWN AND W. FOGGIN'S PAPER ON "EXPERIMENTS WITH CARBONITE." Mr. M. WALTON BROWN said it appeared from the Report of the French Commission that an explosive whose detonation was due to an explosive wave may be harmless in the presence of fire-damp, if its temperature of detonation be below the temperature of the ignition of fire-damp. The normal temperature of ignition of fire-damp is about 1,200 degs. Fahr., but it was necessary that the effect of such a temperature should be prolonged for some time in order that the mixture might be ignited. The experiments of the French Commission showed that the retarda. tion of ignition characteristic of fire-damp mixtures. the almost instantaneous mixture of the gases resulting from the explosive with the surrounding air, and the quick cooling consequent thereon, combine to make explosives whose temperature of detonation is less than 4,000 degs. Fahr. incapable of igniting explosive fire-damp mixtures under normal conditions of use; that is to say, in holes properly stemmed. The requirement was therefore, although not entirely sufficient, the discovery of an explosive whose temperature of detonation was less than 4,000 degs. Fahr. It naturally followed that the degree of safety became greater as the temperature of detonation fell below this value, and it might be assumed that the degree of safety was sufficient when the temperature of detonation of the explosive varied from 2,700 to 3,000 degs. Fahr. In the present stage of knowledge, which was specially derived from the conclusions of the French Commission, it appeared that dual mixtures were the most suitable. One of the substances was intended to diminish the temperature of detonation, and as regards the useful effect it was essential that this substance should be itself an explosive. Up to the present time the most suitable substance for this purpose was nitrate of ammonia, whose temperature of detonation was about 1,850 degs. Fahr. All high explosives, such as dynamite, gun-cotton, etc., possessed a very high temperature of detonation. This temperature was largely reduced by the mixture therewith of nitrate of ammonia, or other similar substances having a lower temperature of detonation, and the mixture formed a dual explosive which had an intermediate temperature. In the use of an explosive the guarantees of safety diminished as the stemming diminished in length, or became otherwise imperfect. Without stemming, or when the explosive was detonated unconfined, the conditions of safety could not be formulated with exactness; in fact, one cartridge might be fired without ignition and another with ignition, haphazard, so to speak. The following table summarized the results of a series of experiments made at North Biddick Colliery, and I Denotes ignition of gaseous mixture, and N non-ignition of gaseous mixture. showed the effects produced by variations in the length of stemming. Four ounces of explosive were used in each experiment, and were fired into a mixture of 3 parts of coal-gas and 22 parts of atmospheric air, laden with coal-dust. No doubt a theoretical explosive should consist of one substance only. whose detonation should develop a sufficiently low temperature and only give uncombustible products. It was somewhat unfortunate that nitrate of ammonia, the only substance which was particularly valuable for use in dual mixtures, did not perfectly realize these conditions, and its facility of detonation was also somewhat unfortunate. In the formation of a dual explosive there was a certainty that the mixture, however carefully made, was never absolutely intimate. Consequently, if one of the mixed substances was combustible, or when detonated alone was susceptible of developing a temperature higher than 4,000 degs. Fahr., or if on detonation it yielded combustible gases, there was always a chance that a particle of that substance might exist on the surface of the cartridge, and might at the time of detonation ignite the surrounding explosive mixture. The irregularities in the ignition of unconfined explosives was due probably to this cause. These accidental ignitions naturally increased in proportion as the substance which might cause them was used in high proportions to form the dual mixture, as the mixture was less intimate and as the weight of the explosive increased. Practically, therefore, the essential for a safe explosive was the application of machinery, so as to ensure the utmost perfection in its mixture and to diminish to the lowest possible point, without reducing the aptitude of detona+ Used without the protection of the Trench flame-extinguishing compound. VOL. II.-1890-91. 3 M tion and power of the explosion, the proportion of the substance capable of igniting the gases. A further point in the use of explosives appeared to be the fact that the safety of their ignition in an explosive atmosphere depended upon the almost instantaneous mixture of the gases of the detonation with a sufficient volume of surrounding air. It was highly probable that it might be dangerous to fire a shot in a too limited space and with a weight of explosive too great for the volume of the surrounding air, as compared with the volume of the gases produced by the detonation. Continuing, he said that, in the experiments at Cwmaman and also at Washington Station, the volume of the tube in which the experiments were made as compared with the quantity of material used was about two to one. Possibly the certainty with which the ignition took place in each set of experiments was 'due to this. Since the issue of the report of the French Commission on Explosives the French Government had enacted that makers of explosives should mark upon the cartridges intended for sale the composition of their products in such a way as to allow of their temperature of detonation being calculated; and the government had given to the colliery owners the means of compelling the manufacturers to deliver to them (under penalty) only such materials as afforded the greatest guarantees of safety when used. The French Government had also felt it necessary to pass from the period of recommendation to that of enactment, and had now ordered the use of the new explosives, not only in fiery mines, but also in dusty mines, whose dusts were inflammable. For this purpose the Inspectors of Mines had forwarded to the government lists of fiery mines and dusty mines, whose dusts were inflammable, and had formally notified the owners of the class into which their mine had been distributed. Time would be allowed to colliery owners for choice of explosives suitable under the new law; but at all times they would be allowed to have, at their own risk, every latitude in the choice of explosives, the duty of the government being fulfilled when they had indicated, with all desirable precision, the technical conditions which the explosives must satisfy. The explosives recommended by the French Government were as follows: -1. Mixtures of dynamite, No. 1 (75 per cent. of nitro-glycerine and 25 per cent. of silica) and nitrate of ammonia, in which the proportion of dynamite did not exceed 40 per cent. for stone-work, and 20 per cent. for coal-getting. 2. Mixtures of blasting-gelatine (917 per cent. of nitro-glycerine and 8.3 per cent. of nononitric cotton) and nitrate of ammonia, in which the proportion of blasting-gelatine did not exceed 30 per cent. for stone-work, and 12 per cent. for coal-getting. 3. Mixtures of octonitric cotton with nitrate of ammonia, in which the percentage of gun-cotton did not exceed 20 per cent. for stone-work, and 95 per cent. for coal-getting. 4. Mixtures of dinitro-benzol and nitrate of ammonia, in which the proportion of dinitro-benzol did not exceed 10 per cent. for stone-work. The mixtures of octonitric cotton with nitrate of ammonia were manufactured by the State at the National Powder Mills, whilst the other mixtures were obtained from private manufacturers. The decree applicable to fiery mines and dusty |