14(5 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Mat, 



With the exception of the first two, nearly the whole of the tubes were 

 ruptured bv tearing asuniler at the bottom through the line of the rivets. 



Findion the cylindrical form comparatively weak, the next experiments 

 were upon tubes of the rectangular shape, which gave much better results. 

 For the present it may, however, be more convenient to take the elliptical 

 kind, as being the nearest approximation, as regards both form aud strength, 

 to the cylinders recorded above. 



ELLIPTICAL TUBES. 



It will be observed that the whole of these experiments indicated weak- 

 ness on the top side of the tube, which, in almost every case, was greatly 

 distorted by the force of compression acting in that direction. It is proba- 

 ble that those of the cylindrical form would have yielded in like manner, 

 had the rivetting at the joints been equally perfect on the lower side of the 

 tube. This was not, however, the case, and hence arise the causes of rup- 

 ture at that part. 



The next experiments, and probably the more important, were those of 

 the rectangular kind ; they indicate a considerably increased strength when 

 compared with the cylindrical and elliptical forms : and, considering the 

 many advantages which theypossess over every other yetexperimented upon, 

 I am inclined to think them not only the strongest but the best adapted 

 (either as regards lightness or security) for the proposed Bridge. 



RECTANGULAR TUBES. 



On consulting the above table, it will be fou nd that the results as respects 

 strength are of a higher order than those obtained from the cylindrical aud 

 elliptical tubes; and particularly those constructed with stronger plates 

 on the top side, which, in almost every experiment where the thin side was 

 uppermost, gave signs of weakness in that part. Some curious and inte- 

 resting phenomena presented themselves in these experiments, — many of 

 them are anomalous to our preconceived notions of the strength of mate- 

 rials, — and totally diflerent to any thing yet exhibited in any previous re- 

 search. It has invariably been observed, that in almost every experiment 

 the tubes gave evidence of weakness in their powers of resistance on the 

 top side, to the forces tending to crush them. This was strongly exempli- 

 tied in experiments 14, 15, 16, &c., marked on the drawings and the table. 

 With tubes of a rectangular shape, having the top side about double the 

 thickness of the bottom, and the sides only half the thickness of the bottom, 

 or one-fourth the thickness of the top, nearly double tlie strength was ob- 

 tained. In experiment 14, (marked in the margin of the above table,) a 

 tube of the rectangular form, 9 J inches square, with top and bottom plates 

 of equal thickness, the breaking weight was 3,738 lb. 

 Rivetting a stronger plate on the top side, 



the strength was increased to .. 8,273 1b. 



The difference being 4,535 lb., — considerably more 

 than double the strength sustained by the tube when the top and bottom 

 Bides were equal. 



The experiments given in No. 15 are of the same character, where the 

 top plate is as near as possible double the thickness of the bottom. In 

 these experiments, the lube was first crippled by doubling up the thin 

 plate OQ the top side, which was done with a weight of . . 3,788 lb. 



It was then reversed with the thick side upwards, and by 



this change the breaking weight was increased to . . 7,118 



Making a difference of .. .. .. 3,360 1b. 



or an increase of nearly double the strength, by the simple uperatioo of 

 reversing the tube, and turning it upside down. 



The same degree of importance is attached to a similar form, when the 

 depth in the middle is double the width of the tube. From the experiments 

 in No. 16, we deduce tlie same results in a tube where the depth is 18J, 

 and the breadth yj inches. Loading this tube with 6,812 lb. (the thin 

 plate being uppermoai), it follows precisely the same law as before, and 

 becomes wrinkled, with a huramoc rising on the lop side so as to render it 

 no longer safe to sustain the load. Take, however, the same lube, and 

 reverse it with the thick plate upwards, and you not only straighten the 

 part previously injured, but you increase the re^isti^g powers from C,812lb. 

 to 12,188 lb. Let us now examine the tube in the 2'Jlh experiment, where 

 the lop is composed of corrugated iron, as per 

 sketch, forming two tubular cavities extending lon- 

 gitudinally along its upper side. This, it will be 

 observed, presents the best form for resisting the 

 ** puckering," or crushing force, which, on almost 

 every occasion, was present in the previous experi- 

 ments. Having loaded the lube with increasing 

 weights, it ultimately gave way by tearing the sides 

 from the top and bottom plates, at nearly one and 

 the same instant after the last weight, 22,469 lb., 

 was laid on. The greatly increased strength indi- 

 cated by this form of tube, is highly satisfactory, 

 and provided these facts be duly appreciated in the 



construction of the bridge, they will, I have no doubt, lead to the balance 

 of the two resisting forces of tension and compression. 



The results here obtained are so essential to this enquiry, and to our 

 knowledge of the strength of materials in general, that I have deemed it 

 essential, in this abridged statement, to direct attention to facts of immense 

 value in the proper and judicious application, as well as distribution, of 

 the material in the proposed structure. Strength and lightness are deside- 

 rata of great importance, — and the circumstances above stated are well 

 worthy the attention of the mathematician and engineer. 



For the present we shall have to consider not only the due and perfect 

 proportion of the top and bottom sides of the tube ; but also the stitfening 

 of the sides with those parts, in order to effect the required rigidity for 

 retaining the whole in shape. These are considerations which require 

 attention : and till further experiments are made, aad probably some of 

 them upon a larger scale, it would be hazardous to pronounce anything 

 definite as to the proportion of the parts, and the equalization of the forces 

 tending to the derangement of the structure. 



So far as our knowledge textends, — and judging from the experiments 

 already completed, — I would venture to slate that a tubular bridge can be 

 constructed, of such powers and dimensions as will meet, with perfect 

 security, the requirements of railway traffic across the Straits. The utmost 

 care must, however, be observed in the construction, and probably a much 

 greater quantity of material may be required, than was originally con- 

 templated before the structure can be considered safe. 



In this opinion Mr. Hodgkinson and myself seem to agree: and 

 although suspension chains may be useful in the construction in the first 

 instance, they would nevertheless be highly improper to depend upon as 

 the principal support of the bridge. Under every circumstance, I am of 

 opinion that the tubes should be made sufficiently strong to sustain not 

 only their own weight, but in addition to that load, 2,000 tons equally dis- 

 tributed over the surface of the platform, a load ten limes greater than 

 they will ever be called upon to support. In fact, it should be a huge 

 sheet iron hollow girder, of sufficient strength and stiffness to sustain those 

 weights ; and, provided the parts are well proportioned, aud the plates 

 properly rivetted, you may strip off the chains, and leave it as a useful 

 monument of the enterprise and energy of the age in which it was con- 

 structed. 



In the pursuit of the experiments on the rectangular as well as other 

 description of tubes, I have been most ably assisted by my excellent friend 

 Mr. Hodgkinson ; his scientific and mathematical attainments render him 

 well qualified for such researches; and I feel myself indebted to him for 

 the kind advice aud valuable assistance which he has rendered in these 

 and other investigations. I am also deeply indebted to yourself and the 

 Directors for the confidence you have placed in my efforts, and for the en- 

 couragement I have uniformly received duringthe progressive development 

 of this enquiry. 



But, in fact, the subject is of such importance, and the responsibilities 

 attached to it are so great, as to demand every effort to demonstrate, cal- 

 culate, and advise what in this case is best to be done. Both of us have 

 therefore laboured incessantly at the task, and I am indebted to my friend 

 for the reduction of the experiments which I would not attempt to weaken 

 by a single observation. 



IVm. FaIRBAII!N. 



Mr. Hodgkinson's Eeport. 

 Summary of Results offered, in conjutiction with one by William Fair- 

 bairn, Esq., M.Inst. C K , to Robt. Stephenson, Esq , M. Inst. C.E , &c., 

 &c.,/or the Directors of the Chester aud Holyhead Railway, on the subject 

 of a proposed Bridge across the Menai, near to Bangor. — By Eaton Hodg- 

 kinson, F.li.S. 



