306 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[October, 



table") every tliird of wliicli is trussed with a central rod of 1 in. dia- 

 meter and' 4 rods of | in. diameter. Tlirougli tlie cast trusses, or 

 boxes,' in which tliese rods are connected, pass two internal longitu- 

 dinal beams, 4 in. by | in., supporting the untrussed transverse beams, 

 and reducing their lengtli between the bearing points. 



The planks are 3i in. thick, teak timber, spiked down to the table 

 of T-iron, and protected by metalling of Kunker, 4 in. thick. 



Capt. Goodwvn.in a letter to Mr.Dredge, dated Jnly last, observesp 

 "With the assistance of a very able and first-rate mathematician here, 

 I have studied the theory of these bridges most thorouglily: and the 

 model that I have made, 22 feet long, and 4 feet width of platform, is 

 on so large a scale, that I have been able to test it in every possible 

 way, and it has withstood the utmost efforts to derange its parts. The 

 Governor-General, and all the scientific people here, have perfectly 



satisfied themselves of the efficiency of the system, and all these proofs, 

 with my models, assure me that the theory is correct. 



It is in contemplation (o erect immediately two other bridges on the 

 same plan, one across the wet docks at Kudderpoor, near Calcutta, 

 and the other over the Hoogly." 



Without pledging ourselves to the accuracy of the theoretical cal- 

 culations on which the method of constructing the bridge seems to 

 have been based, we wish to call especial attention to that part of 

 the description wdiich states that " at the centre the sectional area of 

 the chain is reduced to two bars ith inch diameter." This statement 

 appeared to us so surprising that we wrote to Mr. Dredge to ask for 

 any explanation which he could furnish respecting it. The point is 

 most important, as it appears it was at the centre that the bridge 

 broke. Mr. Dredge favored us with the following reply : — 



Mr. Dredge presents his compliments to the Editor of the Civil Engineer 

 and Architect's Journal, and in answer to his enquiries begs to say that 

 the extract he quotes from the Mechanic's Magazine is a misprint, it 

 should have been " is reduced to two bars each If in. in diameter." And 

 by referring to Capt. Goodwyn's specification he also sees an error with 

 next sentence, where it is stated " they are J in. in diameter," it should 

 have been " they are 1^ in. in diameter. The first of these errors is of but 

 little consequence, because as there is no strain at the centre, no iron is 

 absolutely required there, and consequently two bars of J in. would be as 

 efficient, if the bridge were constructed propirly, as ten of 1| in.; the lat- 

 ter one is of more importance and ought to have been corrected at the time. 



We have also received some valuable information from Mr. Guppy, 

 of the Great Western Iron Ship Works, Bristol, who has sent an account 

 of the supposed causes of the fall of the bridge which he received by 

 the same overland mail that brought the news of the accident. He 

 has furnished us also with copies of some sketches sent to him at the 

 same time. The following engravings from these sketches materially 

 tend to the explanation of the subject. 



We rode over the bridge an hour after it was finished, and within three 

 days it tumbled down. 



After the ironwork of the bridge was completed, 3 inch teak planks were 

 laid on the bearing bars — these bearing bars, 6 inches deep and f inch 

 thick, having previously been keyed into stroug cast iron blocks built into 

 the abutments ; over the teak planks a flooring of bricks, was laid IJat ; and 

 over this 4 inches of stone concrete were laid, liut the workmen began 

 to lay the plauks at one end, and others followed up with the bricks and 

 concrete, so that before the last third was planked the first third was fully 

 leaded. The consequence was that the bridge assumed this shape — 



expedients they finally got the roadway tolerably level, hut the bridge still 

 showed twists in many places, particularly a dip iu the middle. It was, 

 however, considered safe, and preparations were making for a ceremo- 

 nious opening when it suddenly fell. Fortunately only one boy was on il, 

 who jumped into the river and swam ashore. Il appears to have parted 

 in the middle, as its appearance afterwards was— 



— which they attempted to reduce liy commencing to load at the other end 

 and cutting ihe bearing bars loose from the blocks. By this and various 



Fig. 6. 



We will last insert in this place a paper from Mr. Dredge, (though 

 in part referring to the strictures published by Mr. Bashwortli; in 

 order to bring together all the information we we have received 

 respecting the Bailee Khali Bridge. Our thanks are due to Mr. 

 Dredge for bis readiness to give all the information he possessed on 

 the subji'ct. 



Sir — I am not at all anxious to defend Mr. TurnbuU's Treatise, I have no 

 interest in common with it, and what I said in my last letter was no more 

 than under the circumstances was in justice due to the author. I should 

 quite agree with you Sir, that the whole would be obviously beyond all de- 

 fence, if the charge of such absurdities were correct, but I am sure no one 

 after a perusal of page IV, would accuse Mr. Turnbidl of such rediculous 

 blunders. I am aware of the objections against the treatise, and of the im- 

 perfect solution of the problem, but considering the difficulties (as regards 

 data, &c.) under which the author in I84I must have laboured, 1 think cen- 

 sure in 1845 might have been spared. I have said this much because it was 

 the first essay, and the author's endeavours deserve to be viewed with some 

 leniency on this account. As I am not at all interested in this treatise, I do 

 not compromise my position by having said what I have. 



You observe the question at issue, is, are " Mr. Dredge's bridges stronger 

 weight for weight, than those of ordinary construction." This point has 

 been so often proved hy practice, and demonstrated by experiments and cal- 

 culations, that I thought no doubt existed upon it. If a strain equal to 1875 

 tons be removed from the centre of the chains of such a bridge as the Menai, 

 the section of iron to resist this may be taken away also, thus not only re- 

 ducing the quantity of material, but increasing the absolute strength of the 

 bridge in extent equal to the weight of that material. You may inquire what 

 has been done with tliis enormous horizontal force .' Why, the greater part 

 is altogether vanished hy the reduction of material at the centre, and the 

 rest is transferred to, and diffused over the whole horizontal line of platform, 

 where it becomes essentially serviceable for the stability of ttie structure, by 

 producing a rigidity of the platform without injuring its transverse strength 

 to resist the transit loads. 



Fall of the Bridge at Calmfta. 

 In your notes to correspondents, you inquire if any one can give yon in- 

 formation of the bridge that has lately fallen at Calcutta. What little I 

 know of the affair is verv much at your service. In a letter I received from 

 Captain Goodwvn, R.B.E., dated Fo'rt WiUiam, July 10, 1844, he says, '■ On. 



