62 4 



NA TURE 



[Oct. 26, 1882 



sine difficulties as Sir George Airy, I propose with your per- 

 mission to offer a few explanations. 



Sir G. Airy summarises his remarks under six heads, but I 

 think two would have sufficed, viz. that the bridge was too big 

 to please Sir George, and that the engineers were presumably 

 incompetent. As to size, for example, Sir George considers the 

 fa:t of the cantilever being " longer than the Cathedral by 175 

 feet is in itself enough to excite some fear," and even to "justify 

 great alarm." But when I look for some justification for this 

 bold statement I find that Sir George doe; not advance any 

 reason whatever, nor make use in any way of his high mathe- 

 matical attainments, but simply shi'ts the resp msibility for this 

 alarm on to the shoulders of the " citizens of London," asking, 

 '•would they feel themselves in perfect security? I think not"; 

 and I claim the same privilege of entertaining the sense of 

 insecurity for the proposed Forth Bridge." 



If Sir George had alleged that the stresses on the cantilever 

 could not be calculated, or that the strength of the steel ties and 

 Struts could not be predicted, or that the cantilever could not be 

 erected, I might have replied by publishing diagrams of stresses, 

 results of experiments, and the names of the firms who have 

 te idered for the work. I cannot, however, answer an argument 

 based upon the supposed fears of the "citizens of London." 



To prove that Sir George's criticisms imply a charge of in- 

 c impetency on the part of the engineers, I need only point out 

 that in one sentence he remarks that " experienced engineers 

 must have known instances in which buildings have failed from 

 want of consideration of buckling," and in another, that "there 

 appears to be a fear of its occurrence in various parts of the 

 bracket," when " the bridge will be ruined." Sir George's con- 

 clusions on this head are, however, as he fairly enough states, 

 "made in the total absence of experiment or explanation," and 

 in ignorance whether " a theory of buckling finds place in any 

 of the books which treat of engineering." To assume, how- 

 ever, that an engineer is similarly ignorant, clearly amounts to 

 a grave charge of incompetency. Again, how incompetent 

 must the engineer be who required to be informed that 

 the "horizontal action of the wind on the great projecting 

 brackets depends not simply on the wind's pressure, but also 

 on its leverage," or who neglected to provide for the conse- 

 quent stresses. Yet Sir George does not hesitate to say. in 

 reference to this, that "in the proposed Forth bridge there is a 

 risk of danger of the most serijus kind, which may perhaps 

 surpass all other dangers." 



As Sir George in the whole of his letter does not produce a 

 single figure or fact in support of his very serious charges, I 

 must, injustice to Mr. Fowler and myself, explain that it was 

 from no want of data. At Sir George's request he was fur- 

 nished with every necessary detail for ascertaining the maximum I 

 stress on each member, and the factor of safety. I stated in 

 the paper referred to by Sir George at the commencement of his 

 letter, that under the combined action of an impossible rolling 

 load of 3400 tons upon one span, and a hurricane of 56 lbs. per 

 square foot, the maximum stress upon the steel would in no case 

 exceed 7^ tons per square inch. Any useful criticism must be 

 directed to prove that such load is not enough or that such stress 

 is too great. Nothing can be decided by appeals to the citizens 

 of London. 



Sir George's remarks about what he terms "buckling," and 

 the "total absence of experiment," I can hardly reconcile with 

 his having read my paper, because I have there devoted six 

 pages to the question of long struts, and have given the results 

 of the most recent experiments on flexure by myself and others. 

 When he asks whether a tubular strut 340 feet long would be 

 safe against buckling, he has evidently overlooked the twenty 

 years' existence of the Saltash Bridge, which has a tubular 

 arched iron strut 455 feet long, subject to higher stresses than 

 are any of the steel struts in the proposed bridge. Reference is 

 made to the fill of the roof of the Brunswick Theatre, which is 

 attributed to buckling. This accident occurred about fifty-four 

 year., ago, and consequently considerably before my time ; never- 

 theless I have heard of it often, and if I am not mistaken, the 

 verdict of the jury was to the effect that the fall of the roof was 

 due to a carpenter's shop weighing about twenty-five tons having 

 been built on the tie-rod, which sagged under the weight, and so 

 pulled the feet of the principals off the wall. However that may 

 be matters little, as engineers are in possession of more recent 

 and trustworthy data than the personal reminiscences of Sir 

 George Airy. American bridges invariably have long struts, 

 pjcntly there is no lack of practical experience on the 

 subject. 



The late Astronomer Royal thinks that "the proposed con- 

 struction is not a safe one," and hopes to see it withdrawn. 

 When he wrote his letter it probably did not occur to him that 

 rival railway companies might be only too glad to seize hold of 

 anything which might prejudice the Forth Bridge project and 

 alarm the contractors who were preparing their tenders for the 

 work. I do not complain of Sir George's action, as it involves 

 a matter of taste of which he is sole judge. I would only 

 mention that when he penned the above sentence he had been 

 furnished by the engineers with the Parliamentary evidence 

 and other documents necessary to inform him of the follow- 

 ing facts:— (1) That a wind pressure of 448 lbs. per square 

 foot upon the front surface would, as stated in my paper on the 

 Forth Bridge, be "required to upset the bridge, and under this 

 ideal pressure, though the wind bracing would, it is true, be on 

 the point of failing, none of the great tu< es or ten-ion mem- 

 bers of the main girders would even be permanently deformed." 

 (2) From the evidence given before the Tay Bridge Commis- 

 sioners, Sir George, being a witness, would know that, even 

 supposing the workmanship had been good, a wind pressure oi 

 about one-tenth of the preceding would have sufficed to destroy 

 the Tay Bridge. (3) He would also remember, no doubt, his 

 own report of 1873, wherein he says that "the greatest wind 

 pressure to which a plain surface like that of the Forth Bridge 

 will be subjected in its whole extent is 10 lbs. per square foot." 

 (4) The Parliamentary evidence would have informed him that 

 the proposed design was the outcome of many months' considera- 

 tion by the engineers-in-chief of the companies interested, 

 representing a joint capital of 225 millions sterling, and that it 

 was referred to a Special Committee ol the House of Commons 

 and to a special Committee of the Board of Trade inspecting 

 officers for examination and report, and that the reports of 

 engineers and committees were alike unanimous in testifying to the 

 exceptional strength and stability of the proposed bridge. As a 

 sample of foreign opinion, I would quote that of Mr. Clarl e, the 

 eminent American engineer and contractu-, who has built more 

 big bridges himself than are to be found in the whole of this 

 country, and who has just completed a viaduct 301 feet in 

 height, by far the tallest in the world. Referring to the pro- 

 posed bridge, he writes : " If my opinion is of any value I wish 

 to say that a more thoroughly practical and well considered 

 design I have never seen." I need hardly say that the opinion 

 of such a man has far more weight than that of an army of 

 amateurs. 



Sir George Airy refers "unhesitatingly to die suspension 

 bridge "as the construction which he should recommend. He 

 has clearly learnt nothing on that head during the past ten 

 years. In a report on the late Sir Thomas Bouch's design for 

 the Forth Bridge on the suspension principle, dated April 9, 

 1S73, he says : " I have no doubt of the perfect success of this 

 bridge, and I should be proud to have my name associated with 

 it." Chiefly on this recommendation, and in spite of numerous 

 w arnings from practical men, the bridge was commenced, but it 

 had to be abandoned after spending many thousands, because 

 having reference to the fate of the Tay Bridge, it was pro- 

 nounced by the Board of Trade and every engineer of experience 

 at home and abroad to be totally unfit to carry railway trains 

 in safety across the Forth. 



Sir George Airy stands alone in his advocacy of a suspension 

 bridge for high speed traffic, and in his views as to the force 

 and action of the wind on such a structure. That being so I 

 may be permitted to say that I should have felt no little mis- 

 giving if he had approved of the substituted girder bridge, 

 because it has been the aim of Mr. Fowler and myself to design 

 a structure of exceptional strength and rigidity, differing in every 

 essential respect from that with which Sir George evidently 

 would still be proud to have his name associated. 



B. Baker 



The alarming observations in Sir George Airy's paper on the 

 stability of the Forth Bridge as proposed by Mr. Fowler, which 

 appeared in your last issue, seem to call for a reply, and I think 

 I am in a position to make an unbiassed reply, as I had nothing 

 whatever to do with the design, and moreover do not approve of 

 it. I disapj rove of the adopted system as one in which the dis- 

 tribution of the material can be economical only in a moderate 

 degree, and I object to it from an aesthetic point of view, and 

 also on account of some practical reasons of minor import, but I 

 have no hesitation in asserting that the material may be so 

 arranged in it— and very probably is so arranged— that the sta- 



