IMPROVED METHOD OF MINE VENTI

LATION.

SIR, AS you have always shown an anxious desire to promote the safety of the working miner, by disseminating, through the medium of your useful periodical, any information bearing on that object, I am induced to trouble you with the particulars of a plan for assisting ventilation, which I have introduced successfully at a colliery under my management in Derbyshire, and which has been tried at two neighbouring collieries with equal effect. It is, perhaps, necessary I should first state, that the principle of ventilation is simply that of a pneumatic balance, where one scale is made to preponderate, either by receiving an increased supply of air, or by a reduction of weight in the opposite scale by rarefaction. To promote this object of rarefaction in the north, powerful furnaces are placed at the bottom of the upcast shaft, the heat from which expands, and lightens the long column of air above, and thus promotes a constant circulation. This method, though efficacious, is open to several objections, amongst which are the danger and expense of feeding the fire, and the necessity of confining the upcast shaft entirely to the purposes of ventilation, as suffocation must inevitably ensue should any one attempt to descend through the smoke and vitiated air voided therefrom. The plan I have adopted, though, perhaps, not so powerful as that used in the north, is certainly much more simple and less expensive; and the result of three years' experience has proved it to be quite sufficient in all ordinary cases for the removal of those pests to the miner, carbonic acid gas, and the more dangerous hydrogen, both of which it has had to contend with in formidable quan ities. My plan (see sketch) is as follows:

At a distance of 4 feet from the side of the upcast shaft A, (which is here used for drawing coals,) I sink a small shaft B, 4 feet diameter, and 14 yards deep, the top of which is closely covered with a pair of folding iron doors C. At 2 feet from the bottom of this shaft an opening D is made into the upcast shaft 2 feet 6 inches high, and 2 feet 6 inches wide, the sides of which are secured with brick walls, and the top supported with broken cast rails. At 2 yards from

the top of this hole another opening E of the same dimensions cominunicates with the main shaft, and is secured in a similar manner. A fire-lamp, F, 2 feet 6 inches high, and 18 inches wide is then filled with fire and lowered into the small pit to the space between the holes D and E; the doors are now closed, when the heat and smoke, with a portion of rarefied air, rushes through the top hole E, and mixing with the air in the upcast shaft, so lightens it as to produce a very strong and continuous current through the mine to the surface. A few weeks ago I effected a considerable improvement by suspending to the chain in the small shaft, at a little distance above the uppermost hole E, an iron cover G, which nearly fills the shaft, and thus preventing the escape of any portion of the heated air or smoke through the folding doors above, renders the whole available for the purpose of rarefying the air in the upcast shaft A. In practice I find the depth I have chosen for the small shaft B (14 yards), to be the most convenient, as one man can with ease wind up the fire and repair it, and there is no inconvenience to the workmen in descending for this distance through the smoke.

The strength of a cylindrical tube to resist an external pressure exerted on its outward surface, is a very different thing from the strength of the same tube to resist an internal pressure.

In the latter case, that is, when the force is exerted on the inside of the tube, and tending to burst or rend it asunder, the relative strength or power of resistance of the tube is very easily estimated, it is well known to be, under like circumstances, in the simple ratio of the thickness of the metal of which the tube is formed, and inversely as the diameter of the tube.

But in the other case, when the pressure is external, the strength of the tube to resist such pressure will depend upon very different principles :-it is generally supposed that the strength of a cylindrical tube under such circumstances must be immeasurably great; and there is no doubt that such is really the fact, provided the pressure is uniform all round the tube, and that the true cylindrical figure is strictly preserved; because in such case the tube is like a well-formed arch; it cannot be destroyed except by the absolute crushing of the particles of metal one into the other, which is altogether improbable. But if the true cylindrical or circular figure is not preserved, and indeed if the deviation from the true figure of greatest resistance is ever so trifling, the principle of the arch is gone at once, it is then like an arch without abutments; and the tube under such circumstances, instead of being able to resist almost infinite pressure, will in fact be unable to resist a comparatively moderate pressure.

Now, practically speaking, it is almost impossible to form a tube that shall be

*As a strong corroboration of this fact it may be stated that about fifteen years back some interesting

strictly cylindrical, or of any other figure of greatest resistance; the very weight of the material is sufficient of itself to destroy the true figure; the circumstance of the tubes of steam boilers being formed of metal plates with lap joints rivetted to gether precludes the possibility of obtain→ ing the true figure; moreover, in the case of horizontal tubes, as they are employed in steam boilers, the pressure is not uniform; for while the pressure on the upper part of a tube six feet diameter may be only 13 lbs. upon the square inch, the pressure on the lower part of the tube will be nearly 16 lbs. to the square inch, because the weight of a colunin of water six feet high has to be added to the pressure on the lower part of the tube; therefore the cylindrical or circular form is not in that case the true figure of greatest resistance; and it is not very likely that in the ordinary way of business, of boiler making, much care or correctness can or will be bestowed to the calculating, or afterwards in the making of the tube, agreeable to the true figure of greatest resistance.

Moreover, if all the above difficulties be overcome, and the tube is formed according to the true figure of greatest resistance, there is little chance that it will long remain so, in the practical working of a boiler; the unequal contraction and expansion of the plates by being partially overheated and then suddenly cooled, accidents of constant occurrence, will cause the plates to be drawn and buckled, and thereby soon destroy the true figure. And it should be borne in mind that however trifling the alteration of form may be at first, yet the moment a slight alteration has taken place, the destructive change then goes on in an accelerated ratio. And here a very important distinction should be observed, which is, that when the force is exerted within a tube tending to burst it outwards, the force exerted will not induce any change of form such as to render the tube weaker; because if the tube was originally made tolerably near to the true figure of resistance, any change of figure which afterwards takes place, must be such as

experiments were made to ascertain the relative force which copper tubes were able to support internally and externally: the tubes were beautifully made, and as perfectly cylindrical as hands could form them, but it was found in every case that a much less force was sufficient to crush or collapse the tube than what was required to burst it asunder.

will render it in fact stronger-that is supposing the metal plates to have some de gree of elasticity-it will cause the tube to assume the figure of greatest resistance. But it is not the same with a tube that supports an external pressure, because in this case, any change of figure must demonstrably produce a greater departure from the figure of greatest resistance, and thereby render the tube weaker and weaker; and this is a very important reason why a tube that has been proved to a pressure of 30 or 40lbs to the square inch, may afterwards fail under a pressure of less than half that

amount.

For the above reasons it is therefore clear, that a practical estimate of either the absolute or relative strength of tubes, supporting an external pressure, cannot be based upon the idea of these tubes being correctly formed agreeably to the figure of greatest resistance; the safe mode is to estimate the strength of the tubes by the capacity of the metal plates (of which the tubes are formed) to resist a transverse strain, in the same manner as we should estimate the strength of a flat plate, a bar, or a beam, the strength of which is known to be in the ratio of the square of the thickness or depth, in the direction of the strain.

Under this view of the matter, therefore, it would be correct to consider, that the strength of tubes under an external pressure would vary as the square of the thickness of the metal, but it is also clear that the strength will also vary inversely as the square of the diameter of the tube; because an increase of diameter not only increases the leverage, but also the absolute quantity of force in a like ratio.

But it would not be right to suppose that the absolute strength of curved plates in a tube is no greater than that of perfectly flat plates; this is not the case: there can be no question that the curved form enables the plates to sustain a much greater force than flat plates are able to support; and it is clear that the nearer the curvature of the plates approximates to the figure of greatest resistance, the greater force will they be able to bear; for although, as is stated above, the slightest deviation from the figure of greatest resistance destroys the principle of the arch, and thereby reduces the comparative strength of such tubes from al

most infinity to a strength of very moderate limits, nevertheless curved plates will support a greater or less strain the nearer or the more remotely they approach to the true figure of greatest re sistance.

It is therefore evident, that besides the capacity of resisting a transverse strain, there is also another element of strength in tubes subject to external pressure; that is, the strength derived from the curvature of the plates; but, as this latter strength depends entirely upon the greater or less approximation of the curvature to the figure of greatest resistance, and as the degree of approximation will vary in every individual case, and will also be liable to rapid alteration in the same tube, it is clear that no general rule can be given for determining the strength thereby gained. And, indeed, this is not requisite in the present instance, because it is not intended to offer a rule for estimating the positive strength of tubes, but simply the relative strength of different tubes; which, as above stated, is, in like circumstances, as the square of the thickness of metal, and inversely as the square of the diameter of the tube. The positive or absolute strength of such a tube can only be known by actual proof, but this once known the strength of other tubes may be estimated by the foregoing rule:thus, if a tube 3 feet diameter and made of inch plate is capable of sustaining a given external pressure, what will be the relative strength of a tube 6 feet diameter made of inch plate? Answer-the former is 16 times stronger than the latter.

But, in the case of the force acting inside the tube with a tendency to burst or rend it asunder, the strength of the tube will be as the thickness of the metal directly and inversely as the diameter; therefore if a tube is 3 feet diameter and made of inch plate, it will be 4 times as strong as a tube 6 feet diameter made of inch plate.

The foregoing is a safe, easy, practical rule, and if employed by boiler makers in the planning of high-pressure boilers, I believe it will be the means of preventing many serious errors and fatal accidents.

JOHN SEAWARD.

Limehouse, August 20, 1838.

ON THE CAUSE OF STEAM BOILER EXPLOSIONS.

Sir,-An opinion has lately prevailed with many persons, that the frequent explosions of high-pressure steam boilers may be attributed to the igniting of explosive gases generated within the boilers themselves. The subject is highly important, and has engaged my attention for some time past; and the result of my observations and reflections is, that the above opinion is not based upon any satisfactory or valid foundation. I therefore offer a few remarks, in the hope of engaging more competent persons than myself, to investigate this interesting subject.

That under certain circumstances hydrogen gas may be formed inside a boiler in consequence of the overheated iron plates decomposing the water of the steam by abstracting and uniting with its oxygen, is a fact generally admitted; but the circumstances under which this process may go on, I conceive must be exceedingly rare, and the effect of very trifling amount. If it were possible for the gas to be formed in any considerable quantity, the circumstance must be immediately known by the very perceptible effect it would have upon the working of the engines, but which effect I have never been able to discover myself, nor have I ever heard of any well-authenticated account of such fact having been noticed by others: although I, as well as others, have had the opportunity of noticing many boilers in which a portion of the interior flue or fire-place plates, have become red hot.

It is quite certain, that even if a considerable quantity of the gas were formed, it must be carried off with the steam so rapidly, either through the cylinders or through the waste steam-pipe, that no great accumulation could ever remain in the boiler; in high-pressure boilers which alone are exposed to the accidents of explosion, the steam space is so limited that the whole quantity of steam contained in the boiler is carried off, and again renewed, at least every eight seconds, or about seven or eight times per minute; and as the gas is carried off with the steam, it is clear that the former must be generated at a most abundant rate, or otherwise the quantity at any one time in the boiler, must be very small indeed.

But admitting the fact, that hydrogen gas may be present inside a boiler, how are we to account for the presence of oxygen gas also, which is so essential to form the explosive inixture? It is true, some persons suppose that the latter is supplied from outside the boiler, from the atmospheric air; but this is an idea wholly untenable: others suppose that the oxygen is also formed inside the boiler by the de-oxidizing of a part of the metal plates that had been previously oxidized; but this idea is as difficult to conceive as the former; for either the two operations of oxidizing and de-oxidizing must be going on conjointly, or the one must cease before the other commences; if the latter, then it is certain that the whole of the hydrogen gas will have escaped from the boiler before the oxygen begins to be formed; and if we suppose the other mode, then we must admit that two opposite antagonist operations are promoted at the same time, under the same circumstances, and by the same means; a fact somewhat marvellous, though perhaps quite within the range of chemical affinities.

But, granting that the two gases may both be produced simultaneously in the boiler in sufficient quantity and proportion to form the explosive mixture, what will then take place? Why, the gas will be so saturated or diluted by the steam or vapour of the water, that it is difficult to conceive any other than that the compound will be inexplosive and wholly innoxious.

The above are some of the difficulties which militate against the opinion that steam boiler accidents are occasioned by an explosion of gases, difficulties that cannot readily be explained away; there is, however, one fact of so strong and irrefragable a character, that in my opinion it most conclusively decides the fate of this ingenious hypothesis; and clearly proves that steam boiler accidents cannot be attributed to an explosion of gases. The fact alluded to is this, that an explosion or a collapsing, in the numerous class of low-pressure boilers are things never heard of, while they are but too numerous among the class of high-pressure boilers; although it is most certain that the former boilers are quite as likely to form the gases as the latter; the plates of the fire-places and flues of the former boilers are probably quite as much