question left to a permanent board of engineers, whose business it should be to see that the best interests of the whole country were protected, and no individual caprice suffered to throw obstacles in the way of either river or railway transportation. I am yours, very respectfully, G. K. WARREN, FRED. A. CHURCHILL. Major Engineers and Brevet Major-General, United States Army. LETTER OF MR. R. B. MASON, CIVIL ENGINEER. CHICAGO, January 28, 1869. DEAR SIR: Your circular of December 21, 1868, owing to my absence for the most part of this month, only reached me a few days since. I was not present, and did not participate in the discussion at the convention of engineers at Saint Louis. I, however, approved generally of their doings; I have never understood from any of those gentlemen that I have talked with since that convention that they considered a 500-feet span impracticable. But a span of 350 feet was considered long enough to meet the requirements of business at that point, and it being cheaper to build with shorter spans, they therefore recommended the shorter spans. This was about the view I took of it. I could not exactly see the necessity of a 500feet span, and if not necessary, then why build it? I am not willing to place myself on record that it is impracticable to build a bridge of 500-feet span. Yours, truly, G. K. WARREN, R. B. MASON. LETTER OF GENERAL WILLIAM SOOY SMITH, CIVIL ENGINEER. Council Bluffs, Iowa, February 8, 1869. SIR: I have just received your printed circular, dated December 21, 1868, asking how far the convention of engineers assembled to consider the subject of the construction of a railway and highway bridge across the Mississippi River at Saint Louis, intended to decide the question of bridging the great navigable western rivers. As I have no opportunity to confer with other members of the convention, I can only venture to give you my individual impressions. I think the spirit with which our inquiries were conducted is very well indicated in one of the sentences quoted in your circular from the opening address of our president, viz: "Although we have been convened at the request of one of the rival companies, yet we are to consider all of the questions bearing on this subject without reference to any particular plan or company." And while the facts and opinions were elicited by a single case, they have a legiti mate application to all similar ones, and so to the general subject of "Bridging the great navigable western rivers;" though I don't think it was any part of the duty or intention of that convention to extend its inquiries to that general subject. Perhaps I have now answered all that you intended I should, and yet I cannot refrain from tendering the following considerations upon a subject of so much importance to the engineering profession and to the communities we serve. I know of but one span (not suspension) that nearly approaches 500 feet in the clear, and that is the long span of the Kuilenburg viaduct, over the river Leck, in Holland, recently completed. This span is 492 feet in the clear, and at American prices for iron and steel would have cost $892,311, three and three-tenths times the cost of the 262 feet span, and more than the whole 7 spans of 187 feet each of the same viaduct. The great additional cost of the span proposed (500 feet) will, in my judgment, make the act if passed "practically prohibitory to bridges," except in cases of such necessity as to justify almost any expenditure of money. I do not think that a 500-feet span is mechanically impracticable. The most serious difficulty in the way of the construction and erection of such a span would be experience in shaping and uniting parts of sufficient dimensions to resist the great strains to which they would be subjected, and in providing false work on a bottom that may be shifting and unreliable; that could not be depended upon for supporting so costly a superstructure during the time necessarily consumed in putting together in place over two thousand tons of iron and steel consisting of so many parts. The steamboat interest demands that bridges over our navigable rivers shall be placed 50 feet in the clear above high-water surface. This requirement greatly increases the difficulty and expense of falsework to sustain a structure so ponderous and costly. While, therefore, it may be granted that a single span of 500 feet in the clear can be built, it must also be conceded that the construction and erection of such a span over some of the rivers which the public interest requires us to bridge, would demand all the resources of the very best mechanical and engineering talent and experience. It is to be regarded as an extreme to which the engineer should be driven only by an absolute necessity. Moreover, the question of safety involved in the adoption of spans of such great length can scarcely be regarded as settled by a single example so recently furnished. Of course it is easier to build a short bridge than a long one, the difficulty increasing nearly in proportion to the squares of the distances to be spanned; and whether the limit of safety, with the plans and materials at hand for the use of the engineer is not reached far within the extreme length of span 500 feet no sufficient experience has yet been had to determine. The passage of an act making it necessary for the American engineer to exceed with all his bridges over our navigable rivers the greatest length of span yet attained would be viewed with surprise by the engineering profession throughout the world. Legislation requiring us to go beyond the limit of all reliable experience would be unwise, dangerous, and only calculated to prohibit bridges altogether wherever it might apply, except in cases of extreme necessity. The navigable channels of even our largest western rivers are rarely 500 feet widegenerally not more than half that width. Then why compel engineers and those interested in the vast trade that seeks transit across our rivers to span sand-bars and unnavigable water when such compulsion is so detrimental to these great interests, and involves such nice questions vitally affecting the public safety? Nothing but gross carelessness or criminal intent will bring a steamboat in contact with piers placed from 250 to 350 feet apart in the clear, and parallel with the direction of the current. Otherwise, how can steamboats be kept in the narrow tortuous channels they now navigate? It so happens that many of our great navigable rivers lie directly across the lines over which our surplus products must move to reach their best markets. This movement is more important in winter than in summer time; but in winter nearly all of our most important western rivers are closed by ice. So that even if they ran in the most desirable directions they would not be available at the most favorable season of the year for the transportation of our products. So it happens that the great bulk of the inland carrying-trade is done by railroads, and the interests of the whole producing population of the West are thus vitally affected by any enactment which will so seriously impede the operations of those roads. As the trade obstructed by the rivers, increases, the hinderances thrown in its way unnecessarily by the less important river commerce will become more and more objectionable to the people interested. And have they not a right to demand now that while guarding a minor interest a greater one shall not be sacrificed? It is, of course, of first importance that our great natural channels of inland commerce should be preserved. They are free to all, and can never become the property of powerful combinations and monopolies. But they are not seriously obstructed by an occasional bridge with spans of from 250 feet to 350 feet in the clear, and these in their turn serve to perfect other lines of still greater importance to the great mass of our people. I think the opinion not only of the Saint Louis convention, but of the great body of engineers in this and all other countries, is adverse to 500 feet spans, except in cases of absolute necessity. They are barely practicable. They may or may not prove permanently safe and reliable, and they are enormously expensive. Yours truly, General G. K. WARREN. WM. SOOY SMITH, Superintending Engineer, Omaha Bridge. LETTER OF MR. J. B. MOULTON, CIVIL ENGINEER. NORTH MISSOURI RAILWAY COMPANY, DEAR SIR: Your printed circular dated at Saint Paul, Minn., addressed to the members of the convention of civil engineers, held in Saint Louis, Mo., in August, 1867, to consider the subject of a railway and highway bridge across the Mississippi River, has been read by me with much interest. The whole tone of your letter goes to show that your great desire is to collect correct information, to the end of doing the greatest good you can to the railway interests of the West, and at the same time to preserve the integrity of navigation on the Ohio, Mississippi, and Missouri Rivers. These rivers are the great competitors of western railways, as carriers, and the unobstructed, free navigation of which is a matter of great importance to the country. It is equally important, however, that the railway interest be not overburdened, or be forced to greater outlay of money in building bridges than is necessary to protect the boating interests. Believing that you wished to get the opinions of practical men as well as scientific, I addressed a note to Col. C. Shaler Smith, of the firm of Smith & Latrobe, Baltimore, who combines both these qualities, requesting him to give his views of spans which can be adopted over western navigable rivers without injuring railroad interests too much by great outlays, which I herewith inclose. I do not fully agree with the views expressed in it as to the necessity of the very long spans mentioned. For while I admit the possibility of constructing long spans, it seems to me that Congress would not be justified in going beyond the requirements of western navigation, and I am one of those who believe that it does not require spans reaching beyond 350 feet. Indeed it seems to me that spans of 300 feet in the clear will accommodate all possible contingencies, except perhaps when made directly opposite our largest cities, where boats of the different classes of tonnage exchange freight. Colonel Smith admits, and in this I agree with him, that the cost of superstructure is nearly as the square of the span; which will make a span of 500 feet cost within 25 per cent. of three times as much as a span of 300 feet. This is more than most western railways can spare to build bridges over the rivers named. Draw-bridges, built and proposed in some instances, have been and will be built by independent companies, because railroad companies are too poor; these companies charge toll, which nets a profit, and will continue to do so, at the cost of the shipper, no doubt, for a long time. Owners of railroads will contend that they should not be required to build bridges beyond a span which will not obstruct navigation. Steamboat-owners will contend for long spans, as it is manifestly their interest to keep back the bridges as long as they can, while railroad owners for a parallel reason will contend for the cheapest bridge. A proper discussion will bring the minds of Congress to see the propriety of taking a mean that will do justice to both sides. Pilots will be slow to testify that 300 feet is not enough; a contrary opinion would be refuted by the fact that on every trip at ordinary stages of water they are compelled to steer their boats through channels much less than 300 feet wide, which is often done at night with no moon in sight. It is for the interest of all railroad companies or bridge companies to build high bridges, and they ought to be compelled to do so, as a safeguard against accidents to both bridges and steamboats. Besides, a river carrying almost the whole year an immense amount of sand and silt is likely, and does, in fact, alter its channels, which, with a draw-bridge, will give trouble, while under high bridges it will be avoided. This will apply with much force to these three rivers, except to the Mississippi above the mouth of the Missouri. Another reason why high bridges should be adopted is that the requisite spans are not attainable with draws. I am aware that I have not yet answered your question asked in the printed circular, nor is it necessary if what I have said is noticed. The convention was, as you suppose, called in the interest of the rival bridge company. You mention a fact I did not happen to know when I accepted the call to attend, and many of the civil engineers whose names are appended to the report did not attend, although I think they all saw the report and signed it before the printed form had much circulation. I did not have much to do in the matter, except to contribute to the "Report on the regimen of the river near Saint Louis and Saint Charles." The gentlemen composing that convention with whom I became acquainted, many of them for the first time, while in Saint Louis, are engineers much older and better skilled in bridge-building than I am, and to whose opinions I defer. My opinions as to spans best adapted to bridges on the Ohio, Mississippi, and Missouri rivers are contained in this communication. Under different circumstances I should have much preferred to have been silent; as it is I write with a full knowledge of the inadequacy of my experience and judgment in treating a subject properly, so vital to the commercial industry of the West. Your obedient servant, General G. K. WARREN, U. S. A. J. B. MOULTON. LETTER OF GENERAL C. SHALER SMITH, CIVIL ENGINEER. NORTH MISSOURI RAILWAY, ENGINEER'S OFFICE, SAINT CHARLES BRIDGE, Saint Charles, Mo., January 23, 1869. DEAR COLONEL: In reference to the letter of General Warren received from you, I would say that in regard to the proper length of spans on navigable streams, I have come to the following conclusions: First. That the location should be such that there is a clear distance both above and below the bridge, of two and a half boat lengths (longest boat in the trade) of straight current, and that where this is the case the span should be sufficient to allow the longest boat on the river to float through sidewise, and the longest and broadest towfleet to go through at an angle of 30° with the line of the current. The location of the piers, in this case, should be such that the natural axis of the current is through the middle of the span, without reference to the deepest water. Where this is attended to, the normal action of the current will carry an unmanageable boat or tow through without danger. Where the location cannot be made over a straight current, the principles which should govern are these: Ascertain the distance in the line of the bridge between the axis of the current at the lowest navigable stage, and the same at the highest water, and add this distance to the lengths of span previously determined for a straight current. This will carry a boat through as before, by the natural action of the current alone, whether the river be low or high. You can judge of the principle from the following sketch. I have made a good many experiments on the lee-way made by boats in turning a bend and from wind, but I find the foregoing principles come more near to covering all possible points than any plan I have yet seen or thought of. In applying this to practice on the Missouri River, we have, at the Saint Charles bridge, 1,000 feet of straight current above the bridge and 1,800 feet below. From the regimen and character of the river, tow-boats will never become profitable on it; so the longest boat fixes the span; this is 285 feet, and allowing 15 feet at each end of the boat for clearance, we have 315 feet as the proper span. The arrangements here suit the river interests so well that not only have they not attempted to enjoin us, but a number of pilots have voluntarily communicated with me for the purpose of expressing their entire satisfaction. On the Mississippi and Ohio Rivers the longest tow-fleet woull fix the spans. This would be 7 barges wide and 3 barges long, or a rectangle of 480 by 182 feet. This, at an angle of 30°, will require 400 feet net, and 430 feet with the proper clearances. This for a span over a straight current. In the curved current in the bend just at Saint Charles, where the varying axis of the current equals, I should think, the variations likely to take place at any point on the Ohio or Mississippi Rivers which would be selected for a bridge, the distance between the high and low water axis is 80 feet; add this to 430 feet, and we have 510 feet as the widest span which would probably be required on the rivers in question. In relation to the practicability of these long spans, this is simply a matter of cost. Any one of the four or five of the best forms of truss in use in this country can be extended to 700 feet span in iron or 1,000 feet in steel, without either difficulty or danger. One of our firm has just successfully extended the common Howe truss, without an arch but with a wrought-iron lower chord, to 300 feet span, and this without exposing the wood at any point to more than 1,000 pounds per square inch, with the truss fully loaded. When this can be done with a material like wood, it is easy to see the extended limit which can be reached with such a material as steel, which is good for 25,000 pounds per inch. Up to within the limits I have given, the cost of a truss is very nearly in proportion to the square of the span, the girders of the arch and bowstring types, however, increasing in relative economy with the truss-girders proper, as the spans are lengthened. Yours, very truly, Col. J. B. MOULTON, Chief Engineer North Missouri Railroad. [Indorsement.] C. SHALER SMITH, Chief Engineer Saint Charles Bridge. Respectfully referred to General G. K. Warren, U. S. A. LETTER OF MR. W. W. EVANS, CIVIL ENGINEER. NEW YORK, 47 Exchange Place, February 15, 1869. GENERAL: I am in receipt of your circular letter in reference to the report of the convention held in Saint Louis in August, 1867, to consider the subject of a railway and highway bridge across the Mississippi River at Saint Louis. In answer I would say that, although my name appears as one of the convention, I was not present, and did not see the report until after it appeared in pamphlet form. The president of that convention was my chief for a number of years, and having always had the most exalted opinion of his engineering ability, I told him I would sign the report, and would sign anything he would put his name to. In this way, my name became attached to the report. 1 beg now to say, in reference to the questions asked, that I fully believe in the practicability of building girder-bridges of 500 feet clear span, but I do not believe in the propriety of them in any case or under any circumstances, but I do believe in the propriety, the economy, the reliability, and the usefulness of bridges of suspension for railway and highway purposes of spans ranging from 500 to 3,000 feet. Why the law should have been framed to exclude the suspension principle is, to me, inexplicable, when we have in existence a railway-bridge of much greater span which has given satisfaction. That bridges on the girder principle of 500 feet span can be built there is now no longer reason to doubt, with the knowledge we have of the Britannia bridge, 459 feet 3 inches span, and the bridge just constructed over the Leck at Kuilenburg, in Holland, of 492 feet span, and another over the Moldau at Prague, in Austria, of the same span. But I would ask, what engineer is there in the United States bold enough to recommend such bridges for construction here, when he has staring him in the face the figures 2,123 as the tons of iron and steel used in the construction of one span of 492 feet, and the figures 3,000 as representing the tons of iron used in one span of the Britannia bridge, of less than 460 feet, and then compare these figures with the 400 tons of iron and less than 600 tons of wood used by Roebling in the construction of the Niagara Suspension bridge, of over 800 feet span. I am fully convinced, after years of investi gation, that bridges of 500 feet span of iron and steel for railway purposes, having no curved lines in any of the chief members, and having as much rigidity as the Britannia bridge, can be built, and successfully built, on the truss suspension principle with much less than 700 tons of metal. Fink's patent suspension truss-bridge, as now built by an eminent engineering firm of Baltimore, and very extensively used throughout the country in spans of 250 to 400 feet, combines many of the features required in a Roebling bridge. This plan may, in my opinion, be improved for very long spans, by introducing a plan for keeping the main suspension members in straight lines, by auxiliary chains, as supporters of the sag, which would naturally take place. This feature in bridge-building will, before long, be understood, appreciated, and used for railway bridges, of very long spans. I see no good reason why railway bridges of 1,000, 1,500, or even 3,000 feet span should not be built, when occasion and circumstances call for them. But spans of these dimensions evidently cannot be built where the chief members are arches, depending on compression for strength. Bars, or beams, or columns, beyond a certain length, cannot be depended on for strength, without much lateral bracing and propping on every side, and even then no man can tell when and where buckling and doubling up will take place, while a bar or chain of iron or steel, or cable wire, has a known strength, no matter what the length, and can be relied upon. The suspension principle is fully understood and appreciated by the leading engi neers of the United States; so it is a folly for legislators and others, at the present day, to ignore its merits. Mr. Roebling deserves much praise and lasting honors for the boldness exhibited in design and the skill developed in execution, in railway bridges of suspension, and that, too, after the matter had been discussed and condemned by the Institution of Engi neers in England. It is shown by actual results that the deflection of the Niagara suspension-bridge, which has a span of 821 feet 4 inches, is about the same in proportion to the span as that of the Britannia bridge, a rigid iron-plate girder, of 459 feet 3 inches, while each are loaded with 300 tons. When the Niagara suspension railway-bridge became a fixed fact, European engi neers made it the subject of much discussion. Many of the routine engineers, those that had been brought up in a groove, could not believe in it, and do not believe in it to this day; but one among them, a well-known writer on the strength of materials, Peter W. Barlow, packed his valise, came to this country on purpose to look at this wonder after it had been in existence over five and a half years. It may be interesting to quote some of his opinions and conclusions, taken from a pamphlet published on his return to England. On page 6 he says: "The suspension principle, if correctly |