18 J5.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



365 



of iheir span much beyond a foot, would be geometrically possible if the 

 lengtli of these chains be supposed constant. 'We know that if the chains 

 were absolutely straight, their tension would be iulinite,and as an appruxi- 

 matiun to the straight line must produce an enormous increase of tension, 

 there nerd be no fear that the range of the saddles of Hungerford Bridge 

 would be insuflicieiit for the requisite adjustment. 



Approximalious similar to those used above would scarcely be suitable for 

 calculation of the variations of the side chains, of which the tension alters 

 !io rapidly for slight alterations of span that the approximate expression 

 must be continued to a much greater number of times to be at all salisfac- 

 tory, This objection does not however obtain respecting the main chain, 

 in which the vari.Uious of tension are not so rapid. 



We have also fjllowed Sir Howard Douglas in assuming the chain to 

 be a common catenary, and ihe load to be distributed uniformly along the 

 curve. These suppositions might be dangerous, if the strength oftliecbain 

 were being discn-seil, but fur the purpose of estimating roughly the eft'ect 

 of variations of the span, they seem sufficiently accurate. We may as wel 

 remark that the calculation of the strength of the main chain, which is 

 quoted in the last volume of this Journal, Vol. VIII, p.05, is exceedingly 

 incorrect and unscieniific. 



In ihe tract on iMetropolilan Bridges, and in the Reply to our remarks, 

 several analytical ditficultjes have been suggested which certainly appear 

 to miliiate against the conclusions here arrived at. These difficulties are 

 not however insuperable, as the consideration of the following passage, 

 which we quote from the original pamphlet will show. 



" In the case of the Hungerford Bridge, however, Ihe catenaries at the 

 two ends when completed, have a consider*ibly less span than the central 

 curve, but they have the same droop or deflection. Hence if2y be the 

 span of the ceutre arc, i y' the span of the curves at the two extremities 

 when completed; and «,a', the corresponding tensions at the lowest poiuis, 

 we have 



3y+T- 



3 y'- + r2 



And as y' is considerably less than y, a' will evidently be less than a in 

 a still higlier ratio. Hence there will be a constant horizontal strain, equal 

 to ((I — 0') acting at the top of each pier." 



The supposition that the side chains " have the same droop or deflection" 

 as the central chain, is a misapprehension as to a point of fact — tliey have 

 a considerably greater deflection. Hence in the second of these equations 

 we must put for x, a larger quantity x'. This correction is a satisfactory 

 solution of the difficulty suggested. The extracts quoted last month from 

 the Reply, contain the following clause in the sentence explaining the dia- 

 gram, 



" If the tangents of the curves formed by the heads of the piers do not 

 mftke equal angles with the vertical line A V, even if the tensions of A B, 

 A C, were equal (which is not the case in Hungerford Bridge), the re- 

 sultant of the strains at A would be in the direction of a line A U," ice. 



It seems to have been overlooked that the fact alluded to in the paren- 

 thesis, and the fact suggested just previously, are of a compensating nature 

 If there be an inequality, not only of the amount of the two tensions, but 

 also of their angles of inclination, the resultant may be vertical — if only one 

 of these inequalities exist, the resultant cannot be vertical. 



To Sir Howard Douglas belongs the merit of having suggested for dis- 

 cussion a subject of great interest to engineers and the public generally ; 

 and of having considered the question in that purely argumentative manner 

 which always ensures respect in a scientific controversy. He will, we are 

 sure, give us credit for having the same object in view as himself— that of 

 ascertaining the truth ; and if our reasoning should appea- erroneous, we 

 shall be quite ready to be set right. Of the strength of the main chains of 

 the bridge we have not as yet said anything, as we are not in possession of 

 ail the data. These however are promised to us, and a separate paper on 

 this part of the subject shall, if possible, appear in our next number. 



AM\TEUR ENGINEERING. 



The English Government is distinguished from those of other countries by 

 its reluctance in assisting or rewarding the etforts of mechanical inventors. 

 There is no doubt that from the operation of this rule many valuable dis. 

 coveries are lost to the country, which has a direct interest in promoting 

 those scientific labours which tend to the benefit of the whole community. 

 At the sHme time, it is certain that an indiscriminate encouragement of in- 

 ventors would suliject the country to an overwhelming numlier of claims on 

 the behalf of futile immature and valueless schemes. 



The restrictive rule observed by our Government is however occasionally 

 relaxed, and it happens, unfortunately, that the occasions chosen for this re- 

 laxation are those where the least benefit results to the country, and the 

 consideration shown to favoured individuals is least deserved. We allude to 

 public encouragement of amateur ship. building and marine engineering. It 

 would be invidious to point out all the instances in which the public money 

 has been recently squandered in constructing war steamers upon plans sug- 

 gested by persons whose occupations and previous studies by no means quali- 

 fied them for the task undertaken by them. Of course the inventors them- 

 selves stoutly deny the failure of their plans, and can overpower us with 

 proofs of their success ; but the professional engineer will immediately recall 

 numerous instances in which persons possessed of Parliamentary influence, 

 or Ihe advantage of titles to the peerage, have been able to waste many 

 thousands of pounds of public money in the manner alluded to. We may, 

 however, take one instance where failure has been so obvious that denial 

 would be impossible. The Sidoti, the offspring of Sir Charles Napier, was 

 intended to have been a lesson to all future engineers ; and such indeed it 

 has proved — in the way of salutary warning. 



If it be determined that the rule of discouraging inventors is to have ex- 

 ceptions, they ought not surely to be so selected that the possible failures 

 shall be productive of the greaten ' imnaat of loss. Judging a /^r/on', we 

 might suppose that naval construction on which the strength of Great Bri. 

 tain depends, and of which the operations are necessarily of the most costly 

 kind, would be the very last branch of the industrial arts chosen to be sub- 

 jected to the experiments of inexperienced amateurs. We might suppose 

 that the building of steam and other vessels of war, involving as it does, in 

 every case, a vast expense to the country, would be conducted with the ut- 

 most caution, and would be intrusted solely to careful and experienced hands. 

 It would seem a matter of common prudence that if amateurs must be en- 

 conraged in their taste for practical mechanics, they ought to try their 'pren- 

 tice hands on some work a little less costly than ship building. 



But the Government is not alone responsible for the errors in question, 

 which have been largely participated in by companies of private individuals. 

 A clever busy man, with a good deal of intrigue and energy, may often get up 

 a company, which he can coax into an encouragement of his own private 

 schemes, or those of the cliqtte to which he belongs. The Great Northern, 

 built at Londonderry, by some one who, from modesty (or more probably, 

 from prudence), has not trumpetted forth his name, was to have been a 

 model of Irish steamers for English engineers ; but alas, turned out but ill 

 The restorative applications of London engineers — Miller, Ravenhill and Co., 

 were found requisite intus et in cute before the unfortunate victim of me- 

 chanical dabblings could be brought to a state of vigorous healthy action. 



The Great Britain again was a woeful example of amateur patchwork. 

 The public were invited to inspect and admire this gigantic specimen of ship- 

 building ; and attracted by her magnitude and showy decorations, they in- 

 spected and admired accordingly. Could a promiscuous crowd of visitors be 

 supposed likely to scrutinise the proportions of the engines, the size, shape, 

 and arrangement of the small and inconspicuous parts, and all those minutia 

 which, as the professional engineer knows, constitute the excellence or 

 worthlessness of a marine engine .' The history of this vessel has been a 

 series of unfortunate mistakes from beginning to end. First, she was huilt in 

 dock? of which the opening was too narrow to permit her exit. The chrysa- 

 lis grew and increased in stature, and when the time came for quitting its 

 wingless state, and going forth freely into the world, was found unable to 

 emerge from its integument. However, these difficulties, the result of gross 

 bungling, were at last got over. But the troubles had only just commenced. 

 The engines were found to be— what engines might have been predicted to 

 prove, which were constructed by men who had never undergone the disci- 

 pline of an engineer's workshop. Professional engineers were again called 

 to the rescue, and this time, Mr. Field, of the firm of Maudslay and Field, 

 was the re-operator. 



At present, the Great Britain lies upon the coast of Ireland, beaten and 

 buffetted by the wind and tide. No one knows how she got into her presnet 

 position ; this, like all her previous adventures, has the charm of mystery 

 about it — a mystery which baffles human penetration. There is a convenient 

 complexity about the story, which averts condemnation by puzzling Ihe judg- 

 ment. Bristol charts not drawn as they ought to be, the Isle of Man 

 appearing where it ought not to be, the captain's notions of time and space, 

 the advantages of an untried course to the north of Ireland, the directors 

 complaisance, and barrels of gunpowder placed behind the vessel to blow 

 her — not up — but off: — these are the materials of the story ; of which the 

 only certain part is the event — namely, that the Great Britain lies ashore iu 



