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bars, and it is to be greatly desired that of plate smiths or when built into struc similar observations should be made on tures, they behave very differently to all kinds of modern metallic structural iron with the same apparent mechanical materials. The mild bars to which Dr. properties. The toughness of good Siemens' table refers shows a mean elas- iron plates enables them gradually to tic limit of 17.37 tons when annealed, dissipate any internal differential molecuthough the same bars showed the lower lar strains of tension and compression mean elastic limit of 16.62 before being that may be resident in them as they annealed, and curiously the elastic ex- leave rolling mills, by differential comtension is also greater in the annealed pression and tension. Steel plates bars, the mean extension in bars 5 feet which are comparatively hard, but which, 0 inch, and 4 feet 11 inches, being 0.086 when torn asunder in test-strips, indiinches, the value of the Te of the bars cate an ultimate extension of as much being by the before-mentioned formula, as 15 per cent., might be expected to do respectively 57.04 and 53.348. This de- the same. Such, however, is not the notes a high structural value, although case, for the plate has often behaved, this indication must be taken with the when built up into a structure, as though value of Tr which cannot be gained its ultimate extension was not more than from the table referred to. Such mate. one or two per cent., and, like glass, posterial, however, is not to be had or can- sessed of a high elastic limit, but no not be safely used in plates, and even in toughness. Why this should be is not bars has been very little used in bridge- known, but it may be suggested that work. Steelmakers have yet to satisfy such being the behavior, the following engineers that it can be safely used. may afford some clue to the fact that
The most of the recorded experiment- thick plates, at least of such material, al results of tests of mild steels do not have fractured in various directions, give the extension of the elastic limit, after the structure of which they have so comparisons with other materials can- formed an integral part has been comnot at present be made. From what has pleted. Most plates before being built been said, however, it will have been into a structure are annealed, but the seen, that it is very necessary that these following remarks apply equally whether figures should be obtained, and what has annealed or not. often been observed may be here re Plates when taken from the rolls or peated, namely, that in order to the pro- from the annealing oven are generally duction of a satisfactory series of tables laid on a flat surface to cool, but whether of the mechanical properties of different laid down or stood on edge, cooling structural metals, experiments should be takes place somewhat more rapidly toconducted on a uniform basis, and with wards the corners and edges than at the uniform lengths of tests pieces, or if not middle. At first, the whole plate is of of uniform length, they should be of a the same temperature, which may be minimum length of either ten inches or that of redness. The exterior parts one foot.
first assume the rigidity of cold iron, Such experiments would be perhaps and contraction takes place on the intecostly, but there is at least one wealthy rior parts which remain at a higher temengineering society, by a committee of perature, and therefore the contraction which such experiments might be use. has taken place under a tensile strain. fully carried out on the structural mate- Thus, if a plate of 1 inch in thickness is terials of to-day.
| considered in illustration, it will be seen In conclusion, a few words may be that a corner of, say 6 inches on either said upon the behavior of the harder edge, has an area on the two sides of 36 kinds of steel plates in which they differ square inches, but it has also the addi. from those of iron. It has been ob- tional effective cooling area of the edges, served by many that some of the steel which adds 12 square inches, making for plates with an elastic strength as high a surface area of 36 square inches a as 18 tons per square inch combined total of 48 square inches of effective coolwith a ductile extension as high as 15 ing area by radiation and evection. If, per cent., as obtained by the tests of on the other hand, an area on the two strips of such plates, that in the hands sides at the center of the plate, of 36
square inches of such surface, be taken these internal strains are aggravated, into consideration, it will be seen that and they may possibly be of such magnithe edge surface can only be considered tude that extraneous strains, that would as cooling by conduction. Thus, the not materially affect a tough iron plate, effective cooling area of the outer parts may be sufficient in a hard steel to cause of the plates is much more efficient than rupture. Further, when a plate of such the central parts. These outer parts a character is being riveted up, every having, then, become rigid and con- rivet is compressed under a very high tracted under tension, exert a corre- strain to make it fill the holes, and thus, sponding compressive strain upon the acting as a viscous fluid, adds to the interior parts still at higher tempera- strains already tending to destroy the ture and thus more or less amenable to plate. compression. This tensile strain upon These remarks are only made as sugthe outer parts or borders of the plate gestions, and should perhaps have been is gradually eliminated as the interior put in the form of a question, as they parts cool, and is finally changed into are somewhat aside the object of this one of compression, as the inner parts note, which is to invite discussion on contract in cooling under a molecular what seems to be the tendency in the tensile strain, due to the incapacity of production of very mild steel plates, the rigid border to follow the inner parts namely, that in the endeavor to remove in their contraction. In the cold plate the difficulties which have attended the put into a structure, there is thus initial use of steel plates of high elastic limit, molecular strain differentiating from com- we seem to be in danger of losing the pression at the edges to tension towards facilties for producing lighter structures the center. If the plate is cooled under which the application of steel seemed circumstances inducing unequal cooling at one time to afford.
VERY high pressures are now carried it. The pressure was to be found by the at sea on the outside of tubes of com
2 st paratively large size. These tubes are well-known rule
where s is the
d the cylindrical furnaces of marine
strain in pounds per square inch to boilers, and reach in some cases a diam
which the iron is to be subjected, t the eter of 4 ft. It is of the utmost import- thickness of the plate, d the diameter of ance that very definite rules should be the boiler, and p the working pressure. laid down for the guidance of those who design such furnaces, in order that no half that which would suffice for a shell.
Then the proper thickness for flues was mistakes may be made. We
We say “defin- This practice is, we are happy to say, no ite rules,” because the whole subject has longer followed. It answered fairly well been fully investigated. There is appar, while moderate pressures not exceeding ently nothing more to learn about it, and
about 45 lbs. on the square inch were there should, therefore, be no trouble in constructing a simple formula which used, but it was totally unfit to deal
with such pressures as are now carried would enable an engineer to tell, with very little calculation, what is the proper following formula for the strength of
at sea and on land. Wilson gives the thickness for a furnace tube of any given
262.4 Xt length and diameter, intended to sustain flues: p=
where p is the cola stated pressure. Something of the
lxd kind will be found in almost all treatises lapsing pressure in pounds per square on steam boilers. The practice thirty inch, t the thickness of tube in thirtyyears ago was to treat the fues as though seconds of an inch, I the length in feet, the pressure were to be exerted inside of and d the diameter in quarter feet;
other rules may be found in other haves very vexatiously and insists on a treatises. Fairbairn has shown that the much lower pressure being carried than strength of a flue to resist collapsing is necessary. Before any opinion on pressure varies directly as the 2.19 this point can be properly pronounced power of the thickness, and inversely as it is necessary to call in some other the diameter and length. Thus: authority. Referring to the tables given
in Wilson's treatise on steam boilers, we P=33.6 X (100t)2.19 :LXd
find that the collapsing pressure of a and p=5.6 x (100+)2,19-LXd. Hlue 7 ft. long, x'inch thick, and 40 These are very ungainly formulæ, and inches in diameter is nearly 400 lbs. on useless without logarithms. It may not the square inch. Lloyd's factor of be superfluous to say here, however, for safety is consequently about 9 to 1, the guidance of those who would like to while the Board of Trade factor is nearly use Fairbairn's rules as a check on their 15 to 1. If one margin be enough then own practice, that in using the formula the other must be too great.. In dealthe thickness of the plate is to be multi- ing with this part of the question we can plied by 100. The log. of the result is only arrive at anything like a satisfactory to be found and multiplied by 2.19. conclusion by resorting to the result of This gives a log. the natural number of experiment. Mr. Wilson's figures refer which is the 2.19 power of 100t.
to very perfect tubes, such as may or It is not to be supposed that the may not be met with in practice. Mr. Board of Trade, which is so precise in D. K. Clark has investigated many cases its instructions and rules for marine of collapsed tubes, and he has prepared engine builders, would allow this subject the following formula: to pass without consideration. Accord
(50,000 ingly the Board has proposed a rule, P:
d 60,000 to which runs thus: p=
Here Applying this rule to the case stated, we (1+1)xd
have 105.4 lbs. as the collapsing press60,000 is a constant for furnace tubes In this case the Board of Trade with longitudinal seams, lapped joints, factor of safety is nearly 4 to 1, and and punched holes, single riveted; ? is Lloyd's factor is a little over 2 to 1. It the length of the furnace in feet; d is must be remembered, however, that Mr. the diameter in inches; and p the work- Clark's rule applies to flues of considering pressure. To illustrate the applica- able length without any strengthening tion of this rule, let us suppose that a rings; to these he has attached no prefurnace is 40 inches in diameter and 7 ft. cise value. But it may be taken for long, and that the plates are .375 inches granted that a marine boiler furnace 60,000 X.140625
tube well secured at each end and not thick. Then = 26.3 lbs. more than a few feet long, is much
very (7+1) X 40
better able to stand up against a collapsas the working pressure. But the Board ing strain than a tube 25 ft. or 30 ft. of Trade rule is not the only one with long. We may, we believe, take a mean which engineers have to deal. "Lloyd's" between Mr. Wilson s figures and Mr.
89,600 Xť Clark's, and assume that the collapsing have a rule also, which is, p=
lxd pressure of our tube would be about 200 Applying this rule to the furnace whose lbs. on the square inch. Under these dimensions we have just stated, we have circumstances Lloyd's formula gives a
factor of safety of 4.4 to 1, while the 89,600 X.140625
-45 lbs. We thus find Board of Trade rule gives 7.7. 7X 40
Now, it is evident that the disparity that one great authority on marine en- between the rules of the Board of Trade gineering allows nearly twice as great a and of Lloyd's ought not to exist. It pressure to be carried as the other. If places engineers and shipowners alike in the Board of Trade be right, then a very unpleasant position, and will Lloyd's must be wrong, and dangerously some day cause a good deal of trouble. wrong. If, on the other hand, Lloyd's A case may be cited which recently ocare right, then the Board of Trade be- Icurred. Two wing furnaces on board a
North-county steamer collapsed with 20 ing spectacle presented for consideralbs. of steam pressure and plenty of tion. water. The collapsed flues are round The engineers of a great public detopped, with flat stayed sides, and by partment asserting that a boiler is the Board of Trade rules, the working strong enough, while trose of the Govpressure was 22 lbs. and by Lloyd's 42 ernment assert that it is too weak, will Ibs. It is probable that when the case not be a satisfactory display in any comes to be investigated it will be found sense of the word. The question at that the metal was either over-heated by issue is one really of very great importthe presence of deposit, or that the flues ance to engineers and shipowners. Is it
Be this as it may, it is not too much to ask in the interests of cominconceivable that the engineers of the mon sense that the two bodies should Board of Trade and those of Lloyd's put themselves in communication and may some day come into collision in a agree on so apparently simple a matter court of law over such questions, and we as the preparation of a rule for the shall then have anything but an edify-thickness of furnace tubes?
INFLUENCE OF TEMPERATURE IN TUNNELING THROUGH
By DR. F. M. STAPFF.
Translated from Revue universelle des Mines, for Abstracts of Institution of Civil Engineers.
As the difficulties encountered from keeping perfectly still and in quite pure elevated temperature in tunneling air. In the author's experience on minthrough high mountains require the ing and railway works in Mexico and the adoption of appliances and resources south of the United States the temperavery different from those hitherto em- ture was as high as 40° C. or 104° F. ployed, the author, as Engineering In tropical seas the temperature in the Geologist of the St. Gothard Tunnel, stoke-holes of steamers occasionally Airolo, bas during the past six years reaches even 69° C. or 156° F., and is been collecting voluminous materials for aggravated by the dust raised in the act the purpose of endeavoring to solve the of stoking. Unfortunately there is not two following questions:
sufficient information at present avail1st. What is the highest temperature able as to the practicable limits of temat which men can work underground? perature in underground workings. In
2nd. At what depth below surface is upcast shafts used for winding, and for this temperature likely to be reached in raising and lowering the men, a tempertunneling?
ature of from 27° to 32° C. or 80° to 90° I.—Temperature at which underground F. is allowed for the air current in work becomes impossible.--The limit of English collieries; and in Belgian from heat at which men can work depends 22° to 34ļ° C. or 72° to 94° F. At upon the length of their exposure to it, Fahlun copper mine in Sweden, in rethe amount of exertion they put forth, opening some old workings that had their acclimatization, the nature of the been stopped by a fire twenty years atmosphere, and, most of all, its degree previously, the clearing up was done at of moisture. Omitting instances of a temperature as high as 52° C or 125° momentary exposure to exceptional heat, F.; but there the natural ventilation was it is certain that men cannot accustom excellent. In another mine, stoping was themselves to stand for any length of abandoned at a temperature of 33° C. time more than 60° to 75o Centigrade or or 91° F., where the dust from the 140° to 165° Fahrenheit, even when decomposed pyrites was more intoler
able than the mere heat, and dyed the cious if covered with a layer of common miners as black as ink.* The highest salt, whereby not only would the temtemperature observed in the Mont Cenis perature be still further lowered, but tunnel was 30° C. or 86° F., at about 4 also increase of moisture would be premiles in from the south end. In the St. vented. The men would have to work Gothard tunnel full work was carried on short shifts, and the heat of the body at about 31° C. or 87° F. on the south should not reach 40° C. or 104° F., the (Airolo) side, in air surcharged with ordinary bodily temperature being from moisture; and on the north (Göschenen) 360° to 38° C. or 988 to 100° F. side at 29°C. or 84°F., in an atmosphere The Author goes minutely into the efnot quite saturated. The author gives fects observed to be produced upon the detailed particulars of the air-supply to health of the men employed in the St. each end of the tunnel, and of the Gothard tunnel. He also reports at number of men, animals, and lamps con- length a number of observations he made, suming it; and on the assumption that as to how hot the men got, and how much the dryness of the air is of equal import- work they could do, according to the ance with its purity, he deduces the state of the air in the tunnel. The diratio of 4 to 3 as about representing the minished effort instinctively exerted by superior ventilation of the northern sec- them, in proportion as they get hotter tion over the southern at the time of his from the heat and badness of the air they observations, when each end had been breathe in such situations, is shown to be driven about 4 miles in.
in accordance with physiological princiAccording to Professor Dubois-Rey- ples. Taking as the unit of bodily exermond of Berlin, men can stand 50° C. or tion the amount of effort put forth in 122° F. when the air is as dry as possi- merely walking along the tunnel at an ble, as in the case of blast-furnace work- easy pace—say about 4,550 foot-lbs. per ers; but in an atmosphere saturated minute—the Author considers this is with moisture even 40° C. or 104° F. somewhat exceeded by the ordinary exwould almost certainly prove fatal. Air ertion of the workmen employed in the feels very dry when only one quarter roomier portions of the tunnel, behind saturated, but becomes stifling before the advanced heading. Those engaged, complete saturation is reached; hence a however, in loading the broken rock into slight diminution of moisture may be of the wagons near the forebreast, in the the utmost value, and the principal confined space of the advanced heading means of rendering high temperatures itself, must in his opinion, judging from endurable would consist in drying the the shorter time they work and the higher
A wagon-load of quicklime, not- rate of their wages, be working about withstanding the heat evolved in quench- twice as hard ; and occasional intermiting, he considers would even give a tent efforts may be even four times as fresher feeling than one of ice, on ac- great. They have to fill the broken stone count of the latter rendering the air into baskets upon long narrow lorries still more moist. If at the working running on temporary rails of 1 foot place there were both a wagon of lime guage, laid alongside the main rails of and one of ice, and also à supply of 34 feet guage; the lorries have to be run fresh air direct from the tunnel mouth, back by hand through a distance averaghe is of opinion that work could be car- ing about 80 yards to the main trucks, ried on even where the temperature of into which the baskets have then to be the rock was as high as 50° C. or 122°F. emptied. A score of men formed the The ice would indeed be far more effica- gang occupied in this heaviest work.
The relative cost of the clearing per cubic * In the last working of the United copper mines in Gwennap, Cornwall, a hot spring of water at 115° F. was
yard of rock in the two headings is found met with in the bottom level about 1,500 foet below sur- to be represented by the very same ratio clothing the miners could not lean against it. (Sue Pro- as the absolute moisture of the air in ceedings Institution of Mechanical Engineers, 1873, p. them, namely, 1 to 1.18 in the Göschenen tare averages 130° F. at about 2,000 feet below surface, and and Airolo ends respectively. sometimes reaches 139° F., while in particular places even 157° F. has been observed. (See Transactions American
The highest limit theoretically possible Institute of Mining Engineers, vol. vii., pp. 45-78; and for the air temperature in tunnel work
, . Ivii., p. 393).
ings would be such as would induce fever