Chapter 6

Mr. T.R. Cramton, who at the Southampton meeting of the British Association suggested a method of tunneling which, under certain conditions, seems of excellent promise, brought forward a suggestion at Southport for the construction of three-way tunnels. Now, the undoubted aim of all engineers is economy of construction and the securing of permanent advantages. Mr. Crampton maintains that the suggested system will give these, that three tunnels of, say, 17 ft. diameter, can be constructed cheaper than one of 30 ft. diameter. After describing Sir J. C. Hawkshaw's scheme for the ventilation of long tunnels, the three-way scheme was discussed. Three separate tunnels of 17 ft. diameter each, or 227 ft. area, are to be connected by large passages about midway of their length. These passages are without valves; in fact, free air passages. Between these midway connections and the ends, say again midway between, is formed a branch at right angles either above or below with separate openings from the branch into the other tunnels, such openings being provided with doors or valves quite clear of the main tunnel, any two of which may be closed, thus separating at this point the corresponding tunnels from the third. The branch is to be led to any convenient position where the exhustion apparatus can be placed. If two of the tunnels are left open to this branch, and the third one shut off from it by closing the doors, the vitiated air will be drawn from the two working tunnels, through the connecting branch, while fresh air will be partly sucked down the vertical shafts through their open ends and partly at the center tunnel, which is supplied by forcing air down the vertical shaft in communication with it, a stop or door being placed just outside of the bottom of the shaft so as to compel the air to flow to the center of the tunnel. It will be observed that no trains are running in this air tunnel so long as it is so used; there are similar doors for the working tunnel, but they are kept open, unless either of them is required to be made into an air tunnel, so that the passing trains run no risk of running into the doors. By means of the doors above mentioned, any one of the three tunnels can be used as a fresh-air tunnel, in which the men doing the repairs to the road would be clear of the traffic, while the other two are used for the traffic, as well as outlets for the mixed impure gas and air. If a breakdown of a train occurs in any one tunnel, that tunnel can at once be converted into a fresh-air one, while its traffic is transferred to the one previously used for air, thereby avoiding delay. The system described for splitting the air and drawing off the noxious gases is very similar to that described by Mr. Hawkshaw at Southampton. The valves and other details being added, to make the system applicable to three tunnels, it will be obvious that other modes of ventilation may be adopted. In order to reduce the number of men working in the tunnel it is proposed, if found practicable, not to adopt the ordinary ballast and cross sleepers, but to substitute the longitudinal timber system, the timbers to be secured to brickwork or concrete, forming a part of the tunnel lining, placing efficient elastic material between the foundation and longitudinals for their whole area, also between the rails and sleepers. An open drain is formed between the rails; by this plan any water accumulating flows over smooth surfaces through small channels into a drain, the tunnel on each side being dry. The saving of labor in repairs, if this system can be employed, is so evident that a large amount of money might be expended in endeavoring to discover a suitable elastic material for the purpose. There are data on many long viaducts sufficient to justify experiments being made on the subject, and it is not unreasonable to expect that suitable material may be met with. In very long tunnels nothing should be omitted tending to reduce the number of men working in them. The opinion was expressed that in tunnels passing through solid materials, and proper foundations being made for the longitudinals to rest upon, with good elastic material placed between the rails and sleepers and foundations, one-half of the men employed on the ordinary cross sleeper road resting on ballast would be saved, more particularly as the repairs are effected in pure air free from the traffic as explained. The estimate as to the cost of this system was upon the dimensions given by Sir J. Hawkshaw, and the following gives the comparison:

The quantity of excavation and brickwork or concrete in each case will be as follows: Single tunnel: 30 ft. diameter lining, 3 ft. thick, with the brickwork forming the air passage = to 36.5 cubic yards per yard forward. Excavation to outside of brickwork 36 ft. diameter = to 113 cubic yards per yard forward. Three tunnels 17 ft. diameter and 18 in. brickwork. Brickwork lining for three tunnels = 24.5 cubic yards per yard forward. Excavation outside brickwork for the same 105 cubic yards per yard forward. It is assumed that three 17 ft. tunnels are stronger, more conveniently formed, and involve less risks in construction than one of 30 ft. diameter; at the same time there is no difficulty in making the latter. The above shows the saving in the three tunnels of 23 per cent. in brickwork, and about 7 per cent. of earthwork, compared with one of 30 ft. With regard to ventilation, it is well known that the power required to force air along passages is practically as the cube of the velocity; and as the area of the air passages in the single tunnel is 106 ft. with speed ten miles per hour, and that of one of the 17 ft. diameter is 227 ft., or rather more than double, giving only five miles per hour velocity, it follows that the power for this portion would be eight times less. That for the working tunnels would be practically the same, the velocities being nearly alike in both cases, which would be about 2½ miles per hour--the 30 ft. having an area of 470 ft., the two single ones together about 450 ft. Upon the face of it the system deserves a trial. A full consideration of the scheme by engineers preparing plans for new tunnels would no doubt throw further light upon the subject and be of interest wherever such work is contemplated.--_Contract Journal_.

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MONT ST. MICHEL.

Every one who has the slightest regard for historical monuments, who values mediæval architecture, or cares in the least degree for the beautiful and the picturesque, must heartily sympathize with M. Victor Hugo in his protest against the proposed scheme for uniting the wonderful island of Mont St. Michel with the mainland by means of a _causeway_, and possibly a _railway_!

Those who know Mont St. Michel well, and, like the writer, have spent several days upon the island, cannot but feel that such a scheme would not only be a frightful disfigurement, but would entirely destroy all the associations and the poetry of the place. Practical people will say, "Modern improvement cannot stop in its march forward to consider poetical associations and mere artistic whims and fancies." Now, this would be a possible argument if Mont St. Michel were a busy, thriving town, a commercial port, or the seat of great industries; but in a case where the only trade is that of touting, the only visitors sightseers, the only "stock-in-trade" mediæval remains, surely, from a practical point of view, anything which will injure these antiquities will really destroy the importance of the island, as its _only_ value consists in its wonderful historic and artistic associations.

The first glimpse of Mont St. Michel is strange and weird in the extreme. A vast ghostlike object of a very pale pinkish hue suddenly rises out of the bay, and one's first impression is that one has been reading the "Arabian Nights," and that here is one of those fairy palaces which will fly off, or gradually fade away, or sink bodily through the water. Its solemn isolation, its unearthly color, and its flamelike outline fill the mind with astonishment.

Mont St. Michel is by far the most perfect example of a mediæval fortified abbey in existence, with its surrounding town and dependencies, all quite perfect; just, in fact, as if time had stood still with them since the fifteenth century. The great granite rock rises to the height of two hundred and thirty feet out of the bay; it is twice an island and twice a peninsula in the course of twenty-four hours. The only approach is at low water, by driving or walking across the sands. When, however, one arrives within a few yards of the solitary gate to the "town," walking or driving has to be abandoned, and here the commercial industries of the inhabitants commence. A number of individuals, half sailors and half fishermen, are standing ready to carry you on their shoulders over the small gully, which is very rarely quite dry. Entering through the old gate one sees two ancient pieces of cannon taken from the English, who unsuccessfully laid siege to the place in 1422. Close to the gate are the two rival inns, which are very primitive in their arrangement, the entrance hall forming the kitchen, as in many old Breton houses. A second frowning old gateway leads to the single street, which, passing between two rows of antique gabled houses, and under the chancel of the little parish church, conducts one to the almost interminable flight of stone steps leading to the gateway of the monastery. Upon ringing the bell a polite lay brother opens the iron-studded door, and we are admitted into a solemn, vaulted hall, with another stone staircase opposite. Here we go up and up, to a second vaulted hall, where, in olden times, we should have had to give up any arms which we were carrying. Then another stone staircase, which lands us in a small court with a well in it, at the opposite end of which is a heavy and solid arched doorway. We pass through this, expecting to find ourselves on the top of the central tower of the church at least, and are surprised to find ourselves in the solemn and almost dark crypt of the church. Here we have climbed up some 230 feet above the world and the sea to find ourselves in an underground vault; up in the air and down under the rock at the same time. Wonderfully beautiful is this strange crypt, when one's eye gets accustomed to the gloom, with its exquisite ribbed and vaulted roof, supported upon huge circular columns. Returning to the court, another doorway conducts us into a most superb Gothic hall, with a row of slender columns down the center. This was the monks' refectory in ancient times; adjoining this is another grand hall, divided into four aisles by rows of granite columns, all of the most perfect thirteenth century work. Above these are two other halls, still more magnificent than those below. One of these, called the "Salle des Chevaliers," is probably the most beautiful Gothic hall in existence. Again a flight of stone stairs, and we find ourselves, where we should certainly not have expected, in the cloisters of the monastery, the exquisite architecture of which, with its countless marble columns and delicate double arcades, cannot be described.

The church deserves a few words, as it is a veritable cathedral as to size and grandeur. The choir is immensely lofty, and constructed of granite most elaborately wrought in the later Gothic or flamboyant style. The nave and transepts are in the old Romanesque style, with solid pillars and low round arches. The church is beautifully kept, and contains some very interesting old reredoses and altars with carving in alabaster. The one modern altar in the Lady Chapel is composed entirely of silver! Our space will not permit us to describe the numerous interesting old Abbey buildings--the library, the prior's lodging, the vast kitchen, the prisons, the dungeons, and the means of supplying the place in times of siege. The proposed causeway would join the island to the left of our view, and our readers can imagine the abominable effect of a high embankment disfiguring this point, and breaking through the interesting old walls and towers, with, perhaps, a Brummagem Gothic station against the old time-worn gateway.--_H. W. Brewer, in London Graphic_.

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ADORNMENTS OF THE NEW POST OFFICE AT LEIPZIG.

The cuts given herewith, taken from the _Illustrirte Zeitung_, represent two statues for the new Post Office at Leipzig. The sculptor, Kaffsack, has represented the post and the telegraph as winged female figures. The figure representing Mail holds a horn or trumpet in her left hand, and a letter in her right hand. The figure representing Telegraphy holds a bunch of thunderbolts in her left hand, and unrolls a band for receiving dispatches with her right hand. It will be observed that the figure representing Telegraphy is made much lighter and more graceful than the figure representing Mail, and has also a more energetic expression of countenance, thus indicating the greater speed of Telegraphy.

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COAL GAS AS A LABOR-SAVING AGENT IN MECHANICAL TRADES.

By THOMAS FLETCHER, F.C.S.

Gas, as a fuel, is an absolute necessity to the economical carrying out of many commercial processes. It is often used in the crudest and most costly way; a burner may be perfect for one purpose, yet exceedingly wasteful for another, and however good it may be, an error of judgment in its application may lead to its total condemnation. An excess of chimney draught, in cases where a flue is necessary, may pull in sufficient excess of cold air to almost neutralize the whole power of the burner, unless a damper is used with judgment. With solid fuel, an excess of draught causes more fuel to be burnt, but with gas the fuel is adjusted and limited; there is no margin or store of fuel ready to combine with the excess of air, which, therefore, lowers the amount of work done by its cooling power. The power of any burner, for any specified purpose, depends not only on its perfection, but to a far greater extent on the difference in the temperature of the flame and of the object to be heated. For instance, if a bright red heat is required, it is not possible to obtain this temperature economically with any burner working without an artificial blast of air; the difference between the temperature of the flame and that of the object heated is too little to enable the heat to be taken up freely or quickly, and the result is a large loss of costly fuel. If we want to obtain high temperatures economically, an artificial blast of air is necessary, and the heavier the pressure of air, the greater the economy. On the contrary, low temperatures and diffused heat are obtained best by flames without any artificial air supply.

For such purposes as ovens, disinfecting chambers, japanners' stoves, founders' core drying, and similar requirements the best results are obtained by a number of separate jets of flame at the lowest part of the inclosed space, and the use of either illuminating or blue flames is a matter of no importance, as the total amount of heated air from either character of flame is the same. If there is any preference, it may be given to illuminating flames, as the proportion of radiant heat is greater, and this makes the average temperature of the inclosed space more equal; but on the other hand, may be considered the greater liability of the very fine holes, necessary for illuminating flames, to be choked with dust and dirt. This may, to a great exent, be obviated by using very small union jets, and setting them horizontally, so as to make a flat horizontal sheet of flame. Burners placed this way are practically safe from the interference of falling dust or dirt, but not from splashes. Falling dirt or splashes must always be considered in the arrangement of any burners, and the ventilation must be no greater than is absolutely necessary for the required work. In cooking, this limit of ventilation may be exceeded, as most things are better cooked with a free ventilation, the extra cost of fuel being well compensated for by the better quality of the result.

The air in an oven or inclosed space heated by flames inside is similar in character to highly superheated steam. It contains a large proportion of moisture, and yet has the power of drying any substance which is heated to near its own temperature. A mass of cold metal placed in the oven is instantly bedewed with moisture, which dries up as the temperature of the metal rises. This is, for many purposes, an objection, and the remedy is to close the bottom of the oven and place burners underneath. If for drying purposes and a current of air is necessary, the simplest way is to place in the bottom of oven the a number of tubes hanging downward in such a position that the heat of the flame acts both on the bottom of the oven and the sides of the tubes, which, of course, must be long enough for the lower opening to be well below the level of the flame. The exit may be at any level, but for drying purposes it is better at the top, and it should be controlled by a damper to prevent cooling by excessive currents of air. If not otherwise objectionable, the arrangement of flames inside the oven is far the most economical in use.

Where an oven or drying chamber is used continuously, it should be jacketed with slag wool or boiler composition, but for many purposes this is no advantage. As an example both ways, I will instance the drying of founders' cores where there is only one blow per day. The cores of an ordinary foundry can be dried by gas in a common sheet iron even in about half an hour; any accumulation of heat after that time would be useless, and a jacketed oven would be of no advantage.

For the disinfection of clothes in vagrant wards and hospitals for infectious diseases, on the contrary, a continued heat is necessary, and in this case the accumulation of reserve heat, which takes place slowly in a jacketed oven, becomes of value, as the gas can be turned low or out, and the ventilators closed, insuring a more complete disinfection with a much smaller gas consumption. Where an oven or heated chamber is much used for periods of over half an hour at once, a non-conducting casing pays well by reduced gas consumption.

For albumen and glue drying, leather enameling, tobacco drying, and purposes where a large space has to be very slightly and equally warmed when the weather is unfavorable, steam-pipes are generally used, but, not being always available, an exceedingly good arrangement may be made by placing at intervals in the room gas burners, of any construction, close to the floor, and surrounded with a sheet-iron cylinder, say 2 ft. or 3 ft. high. The top of these cylinders must be connected throughout with a fairly large flue, which will take the products of combustion from the whole, and this flue must be carried either horizontally, or with a slight rise, so as to utilize all the waste heat. The reason for having a number of stoves at intervals is that the heat in a flue will not carry, for any useful purpose, more than about 8 ft. or 10 ft., and a single stove would give an irregular temperature in any except a very small room. If all are not used at once, the flues of those not in use may be closed by a damper to prevent down draught. The use of hot water pipes heated by gas may also be occasionally advisable, but, unless for some special reason, it is much more economical to use coal or coke, as the bulk of water makes an exceedingly good regulator, and makes a fire practically as steady and reliable as gas, thus superseding the more costly fuel.

For one of my own purposes I need hot-water pipes, having very little variation in temperature night and day; and using coke for economy's sake, I get a regular temperature by heating a large quantity of water, about 200 gallons, with the fire, and inclosing this in a tank jacketed with slag wool. My circulating pipes run from this tank, and a practically steady temperature, night and day, can be obtained with the most irregular firing, and occasional extinction of the fire for several hours at once.

For the heating of liquids, the greatest economy is to be obtained from one single flame, of as high a temperature as can conveniently be obtained, and the flame must be in actual contact with the vessel to be heated. In jacketing vessels, to prevent draughts, care must be taken that the jackets do not cause currents of cold air to rise rapidly up the sides of the vessel, and so cool it. If this is the case, the use of a jacket, instead of being an economy, is a positive expense, and waste of heat. Many processes, such as making oil and turpentine varnishes, require a heat under instant control, and in these the use of gas is an important matter, as the loss and risk of fire are very serious elements of expense, more especially in small works where special and costly preparations for contingencies cannot be afforded. I have here a burner which, for its power, is, perhaps, the most compact and gives the highest temperature of any burner yet known, and it is easily made in almost any size; it has, I think, many special advantages. The use of gauze, which is its only weak point, is more than compensated for by the very high duties obtained in practice with it, owing to the compactness and concentration of the heat obtained. The following extract from my communication to the Gas Institute will give all particulars as to the constructive detail of this burner. Those who wish to go further into the matter will find the paper referred to in the publication of the Gas Institute for the current year, and also in the _Journal of Gas Lighting_, June 26, 1883, and the _Review of Gas and Water Engineering_, June 16, 1883.

"The first and most important part is the mixing chamber or tube, one end of which is supplied separately with gas and air, which at the other end are, or should be, delivered as a perfect mixture. It may be taken as a rule that this tube, if horizontal, should not be less in length than four and a half times or more than six times its diameter. It is a common practice to diminish or make conical-shaped tubes. All my experience goes to prove that, excepting a very trifling allowance for friction, the area of the smallest part of the tube rules the power, the value of the mixing-tube being no more than that of the smallest part. If the mixing-tube is upright, new sources of interference comes in; notably the varying specific gravity of the mixture. Except with one definite gas supply, the result is always more or less imperfect, and regular proportions cannot be obtained. This is now so well known that the upright form has been practically discarded for many years, and is now only used where the peculiar necessities of the case give some special advantage.