heat, of 42° C. or 107° F. in the body; much as 5° C. or 9° F. But, for the while the extreme practicable air tem- mean temperature throughout any conperature, still not free from the certain siderable length of the tunnel, the calcuprospect of impaired health and prema- lated result agrees exactly with actual ture death, would be that under which observation, however varied be the mountthe heat of the body should not rise ain profile overhead. In confirmation above 40° C. or 104° F. The latter limit the Author reproduces from the St. being applied to the atmospheric condi- Gothard Company's eighth report a Table tions affecting the two ends of the St. of observations made in 1878 through Gothard tunnel, the Author calculates 1,500 yards' length of the southern half that the practicable limit of air tempera- of the tunnel, between 5,000 and 6,500 ture should not exceed 453° C. or 114° F. yards in from the mouth. The variations in the Goschenen end, and 37° C. or of mountain profile may even have the 100° F. in the Airolo end, for the labor- effect of producing a practically uniform ers clearing the advanced headings. He temperature for miles of tunnel, notwithadds, in a tabular form, the corresponding standing hundreds of yards difference in figures for assumed cases both of harder the heights of rock overhead. For the and of easier work than that of removing temperatures to be expected at the midthe spoil from the ends of these particular headings.

dle of the tunnel, the minuter calculations are given which had been made last year by the Author, at the time when each end had been driven about 7,700 yards in, leaving then about 1,000 yards intervening to be driven. His estimate, with

II.-Limit to Depth of Tunnel below Summit of Ridge. The scale for increase of temperature with depth, which obtains in mines and bore holes sunk below level ground, is inapplicable to mountains, a margin for error of 24° C. or 4° F. owing to the cooling effect of their ex- was 31° C. or 89° F. For the rock; the posed sloping sides. From Borelli's same for the air about 150 yards behind temperature observations in the southern either forebreast, prior to the two driv portion of Mont Cenis tunnel, Ansted ages holing through into each other; deduced an increase of 1° C. per 50 meters, or 1° F. per 91 feet depth below the summit of the mountain profile over the line of the tunnel; but that rate did not hold good for lower elevations in the same profile, and was therefore not applicable as any guide for other tunnels. From his own observations in the St. Gothard tunnel, from 1873 to 1877, for distances of 4,800 yards in from the northern mouth and 4,500 yards in from the southern, the Author deduced empirical formulæ for the temperature to be expected during the further progress of the work. These having been found to hold good for the 2,000 to 3,000 yards next driven, he now reproduces some of the principal. For the temperature of the rock itself, the general average result in its simplest form is an increase of 1° C. per 48.4 meters, or 1° F. per 88.1 feet of vertical depth below the surface of the mountain. This scale gives too low General Remarks.-Having entered a temperature for parts of the tunnel minutely, and at great length, into the that are under valleys and plains on the various conditions affecting the execution mountain sides, and too high for parts of the work in the two ends of the St. under peaks; the variations from the ac- Gothard tunnel, the author refrains from tual temperatures noted at individual attempting to lay down a fixed limit, points in the tunnel amount to nearly as either of temperature or of depth, for Vol. XXIV.-No. 1-3.

and 333° C. or 93° F. for the water issuing from the rock. In March, 1880, after the holing through, the actual mean temperature of the rock for the middle length of 1,000 yards was found to be 301 C. or 87° F.; and this was also the air temperature 150 yards behind the forebreasts prior to holing through. At the two forebreasts themselves the air temperature was brought down as low as 27° C. or 82° F. while boring, and 30° C. or 86° F. while clearing away spoil : which was about 3° C. or 5° F. lower than calculated, the reduction being due to an extra supply of compressed air during the last 1,000 yards' drivage. These several temperatures, though high enough to retard the progress materially in the middle of the tunnel, and render the work more laborious, was not so excessive as to entail either stoppage or danger.

any future tunneling operations which ward, it would have been attended with may enjoy the advantage of resources several advantages, which he here resuperior to those hitherto available. But capitulates. It is not so easy from surthe means to be employed for dealing face indications to form an idea about with higher temperatures than have the quantity of water likely to be met hitherto been encountered would have to with, and the solidity of the rock to be be very carefully matured beforehand, both theoretically and practically.

driven through, as it is from the height and contour of a mountain to calculate Projected Simplon Tunnel.-The ex- the increase of temperature likely to be tent of guidance, furnished by the light encountered. The principles long estabof present experience, is illustrated by the lished by general mining experience may, author's application of his calculations to however, be properly applied to tunnelthe principal projects for tunnelling ing; and in the longitudinal sections through the Simplon. Of the longer compiled by the author for the St. Gothroutes plotted, for piercing the mountain ard tunnel, it is seen how the surface innearest its base, from near Brieg on the dications held good down to that depth: northern slope, those of MM. Favre & fissures, slides, and faults, noted on the Clo, and M. Lommel, each nearly 12 mountain profile, severally made their miles, would appear to be impracticable of appearance without fail in the tunnel, execution with the resources at present most of them in a very unwelcome manavailable. According to their mountain ner. A recent illustration from near the profiles, the depth of these tunnels below middle of the tunnel is cited. The author the ridge would be about 7,300 feet; thinks it safer and easier to avoid difficulwhence the temperatures to be expected ties of this sort by careful selection of midway in the tunnels would be about route beforehand, than to try to get over 47° C. or 116° F. for the rock, and for them afterwards during the execution of the air 150 yards behind the forebreast, the work. For the projected Arlberg and about 53° C. or 127° F. for water tunnel in Austria, he is glad to find the issuing from the rock. In the shorter route laid down with a view to avoiding, and higher tunnel proposed by M. Stock- by a curved course, rock that seems likely alper, about 10 miles long, piercing the to let out streams of water. Tunnels range at about 2,560 feet above sea-level and beneath a rather lower portion of the ridge, the author estimates the depth below the ridge at about 5,900 feet, and the rock temperature midway in the tunnel at about 40° C. or 104° F.; here therefore the extreme limit of practicable execution would be reached. In two still shorter and higher tunnels, each about 7 miles long, proposed by MM. Clo & Venetz, and M. Jacquemin, the temperature encountered would be much the same as has been met with in the St. Gothard tunnel; but their greater height above sea level would somewhat detract from the value of both these tunnels, as compared with those nearer the

through high mountains should, as far as possible, be driven in places where they will have the least height and bulk of mountain overhead, and where they will be likely to be most free of water.

Mode of Driving.-The mode adopted for driving the St. Gothard tunnel is not considered by the author, from the experience there gained, to be the most eligible for long tunnels, where not more than two ends can be driven, and these not faster than a certain limit of speed, and where it is impossible to foresee all the difficulties that may arise. In respect of the atmosphere pervading the working places, that mode of driving seems indeed decidedly objectionable. In the St. Gothard tunnel the long succession of Selection of Route.-Supposing its trenches and steps, by which the small height above sea level be already deter- advanced heading was gradually widened mined by commercial or political consid- and deepened to the full size of the finerations, the route selected for a tunnel ished tunnel, caused the working places should be such as to offer the greatest to stretch back as far as a couple of miles facilities for its execution. In the case of the St. Gothard tunnel, it was pointed out by the author in 1877 that, if the route had lain 1 mile or 1 miles west

foot of the mountain.

behind the forebreast. Hence, even admitting the practicability of artificially supplying the advance heading itself with air so cold that, where the rock tempera

Artificial Cooling.-This is the most difficult question of all, and is by no means solved by the use of extensive appliances for supplying compressed air. The heat from the rock is practically in

ture was from 40° to 50° C. or 100° to 120° from the forebrcast. As soon as the two F., the men could work a hundred yards headings met, a good current could be behind the forebreast, the author considers established right through from one end the same means would be altogether in- of the tunnel to the other, which would adequate for such a length as a couple be largely efficacious in keeping down of miles of tunnel, full of men at work the temperature at the working places, and smoke from blasting. Moreover, in during the subsequent enlargement of the usual plan of driving the advance the central length. heading along the crown of the tunnel, Haulage.-Manual or animal labor in the water issuing from the forebreast has the working places should as far as posto flow all along the heading, and then sible be replaced by mechanical power; down along all the following steps and though how to manage this practically trenches of the successive enlargements, for the most fatiguing portion of the before reaching the completed culvert in work-the clearing away of the spoilthe finished portion of the tunnel. The yet remains to be found out. Endlessauthor therefore suggests the desirability rope hauling, as already carried out at of proceeding by an inverted method, collieries, would be more convenient for somewhat as in metalliferous mines: removing the stuff than the use of comdriving the advanced heading along the pressed-air locomotives, the size of which very bottom of the tunnel, and carrying prevents their penetrating so far into the the culvert forwards promptly with it, so advancing end, and also interferes more that all water should drain at once into with the ventilation. a narrow channel, presenting small surface for evaporation. Less moisture would thereby be imparted to the air, even when driving through very wet rock, than is evolved in drier rock, from the vast extent of wet surface exposed exhaustible; and, whether all the airby the puddles and mud that are inevitable with the ordinary mode of driving. Such an inverted method would doubtless present difficulties in regard to ventilation; but more attention might advantageously be paid in tunneling to the improvements that have already been realised in mine ventilation by the adoption of systematic arrangements. For long tunnels that will take years to execute, instead of improvising foul-air outlets temporarily and at random, or opening cocks here and there on the compressed-air main, a regular system of ventilation should be carried out by means of brattices, air-doors and trunks, and should be pushed forward continuously to follow up the progress of the work. A further suggestion for facilitating the execution of tunnels through hot rock is to complete each end to the full size as far in from the mouth as can be done without the heat becoming too oppressive, say up to 30° C. or 86° F.; and then to push forward and complete the advance heading alone, without continuing the work of enlarging behind it. The practical difficulty would then be reduced to the efficient ventilation of the heading for a length of 100 yards back

cocks at the forebreast are open, or all shut, the air temperature 200 yards back is practically constant and equal to that of the rock, and thenceforth gradually rises as the air travels further back along the working places. The utmost cooling at the fore breast, while four rock-drills are working together and discharging their exhaust air at 2 to 4 atmospheres pressure, does not exceed 4° C. or 7° F. at most, the average being only 1° C. or 2° F. below the rock temperature; and during the clearing away of the spoil the air may rise as high as 4° C. or 7° F. above the rock temperature, the average being 14° C. or 3° F. above. A table is given of periodical observations made by the author at both ends of the St. Gothard tunnel while the drivage was suspended for surveying the line of driving, the working places being then deserted and the air cocks left open. These records show that with a rock temperature of 47° C. or 116° F. it would be no easy matter to keep the air down to 35° C. or 95° F. in the working places while work was being carried on. There is no great difficulty in delivering highly compressed air in large quantities at the forebreast; the difficulty lies in arranging its distri.

bution. Men cannot work within a dis- with skill and perseverance, form as valtance of 5 or 6 yards in front of an uable a class of workmen for European aperture not more than 4 inches diameter; tunnels as the Indians of Mexico or South and often put a basket over it to break America for tunneling operations in the blast. The most serious obstacle to those countries. Under such circumthe use of compressed air, as the sole stances as obtained in the St. Gothard means of cooling, lies in the low specific tunnel, the author considers that, having heat of air, whereby vast volumes become regard both to economy and to health, heated immediately to the temperature the average duration of shift should not of the rock, without perceptibly cooling exceed eight hours. Two hours must be the rock itself. A jet of cold water has allowed for walking in to the working very little cooling effect on the air, and places, when at a distance of 4 to 4 the mist thereby produced is decidedly miles in from the mouth; then some rest objectionable; on the contrary, indeed, is needed before commencing work; and the compressed air should itself be sup- two to three hours' work is all that can plied as dry as possible. To cool the air be managed before starting on the two before delivering it into the tunnel is of hours' return journey, which should also little use, because its cooling effect de- be made leisurely on foot. The men at pends much more on its sudden expan- the boring machines, and those clearing sion; and also because, however cold it away spoil just behind, having the coolest may have been at the tunnel mouth, by and driest air, often remained fourteen the time it reaches the inner end of a hours in the St. Gothard tunnel. The very long main, the air is found to have men should strip themselves of clothing acquired within 2° to 3° C. or 4° to 5° F. as far as possible, so long as the air temas high a temperature as the surrounding perature is lower than that of the body; atmosphere in the tunnel. As a means if ever tunneling came to be done at a of cooling in combination with other higher temperature, it would of course plans, the adoption of compressed air be desirable to work with clothes on. points to the use of compressing appara- Wherever artificial cooling of the air is tus on a much larger scale than that at required, the working places should also either Mont Cenis or St. Gothard. A be followed up closely by suitable refuges further problem will be to devise better or chambers, fresh and well ventilated, methods for drying the air. The air re- and even filled with dry air at a slight servoirs at Mont Cenis and St. Gothard excess of pressure, where any one overanswered the purpose to a certain extent, come with exhaustion can be promptly and were supplemented by additional restored, and where the men can dry and appliances at St. Gothard. For com- change before leaving work. Cooling pletely drying the air, Professor Dubois- beverages are recommended, for avoiding Reymond's suggestion to employ quick- the danger of drinking too much cold lime might be adopted; and the author water while at work.

points out that this mode of drying In conclusion the author points out would be more efficaciously applied to that, from the nature of the work in long the compressed air before its delivery tunnels under high mountains and through into the tunnel, than by conveying the hot rock, the question of cost, and even lime itself to the working places. The of time, may often become a consideracooling mixture of ice and salt should, tion of secondary importance. however, be conveyed in the spoil-wagons to the working places, and there dis- HEIGHT OF WAVES.-From the result charged on the off side; as it gradually of ten thousand observations, it appears melted away, the water would run off, that the cubes of the heights are proporbut without imparting so much moisture tional to the squares of the velocities of to the air as if there were no salt mixed the winds. A wave of 2.20 meters in with it. The combination of these two height corresponds, according to the plans appears to the author to offer the author, with a wind of 5 meters per best means of enabling tunnels to be driven at high temperatures.

Workmen and Shifts.-Italians, used to a warm climate, and taking to mining

second velocity; and as the relation (2.20m)'=x (5)', .. x=0.426, and consequently (2 H)=0.426 v2. Hence 2 H, the total height of the wave,=0.75 vk.

AN ADAPTATION OF BESSEMER PLANT TO THE BASIC

PROCESS.

By A. L. HOLLEY, Memb. Inst. C. E., &c.

From Papers of the American Society of Mechanical Engineers.

THE maintenance of refractory linings in Bessemer converters, in such a way as to promote regular and maximum production, has been the subject of more experimenting than any other feature of the Bessemer system, and it is still the least perfect and satisfactory feature, excepting perhaps the casting of steel. Linings are not only eroded by the mechanical action of the charge, but they are chemically decomposed by its various slags. The silica linings usually employed have, indeed, been so improved, that an average of say 60 charges per 24 hours can be got out of a pair of converters, and the shifting of interchangeable converter bottoms (containing the tuyeres) is so rapid that it does not delay production; but the repairing of the fixed lining just above the tuyeres, where both mechanical and chemical action are most severe, is frequently the cause of delay, and the operation rapidly performed between heats is tedious and costly. The accumulations of slag on other parts of the lining must also be quarried out, else the converter will become too small for the charge.

These are the conditions of maintaining silica linings; but the difficulties are increased, probably about threefold, when the linings are made of lime, for the basic process. The basic process consists in removing phosphorus from the iron under treatment, by retaining the phosphorus oxidized by the blast, in a basic slag formed of say 20 per cent. of lime added to the charge. An acid (silica) lining would vitiate the basic slag, and would also be rapidly destroyed by it. Lime containing some magnesia, and produced by burning magnesian limestone (dolomite), is at present the only basic material successfully used for converter linings. It is usually made into bricks, which are hard-burned and built up with mortar of similar material to form the lining.

Basic bottoms and tuyeres stand ten to fifteen charges, nearly equaling acid bottoms, and they may be readily changed; but basic linings, near the tuyeres, and also in other parts where abrasion is severe, wear rapidly and must be frequently repaired by cooling the converter and inserting new bricks, or patching in some suitable manner. The converter is thus put out of use for at least twentyfour hours-a very serious delay to production. From a wide observation the author feels safe in saying that a basic lining is rarely run above 60 charges without extensive repairs, and in some works repairs are made every time a bottom is set. With some irons there is also an accumulation of slag around the mouth of the converter; its removal sometimes also causes delay.

The output of a pair of converters in Europe averages about half that of a pair of converters of the same size in the United States, and is often less than half. The limited endurance of basic linings in Europe is therefore a less conspicuous defect than it is here, where one converter must make 25 or 30 charges in 24 hours, so that the repairs of basic linings, as at present conducted, would keep an American plant idle about half the time. This delay is really as important in Europe as it is here; the greater the output from a given plant, the cheaper the product.* In order, therefore, that the basic process may come into extensive use, basic linings must be so maintained that their output will nearly equal that of acid linings.

There are two reasonable conditions of improvement; the one is to prolong the endurance of basic materials, so that their repairs can be made with little de

*The statement sometimes made in England that the

rapid production in America impairs quality of product, is but a cover for inadequate plant. Steel is obviously no better because five hours instead of one are consumed in setting a vessel bottom, or because it may take twice as long in an English works to handle materials and product.