182 >IR. WILLIAM FAIRBAIBN ON 



and Holytbead Railway, across the wide spans of the estuary 

 of the Conway and the Menai Straits. The original 

 cooception of a huge wrought-iron tube, of a circular or 

 elliptical sectional form, suspended in mid-air, and of dimen- 

 sions calculated to allow of the passage of the locomotive 

 and its accompanying train through its interior, yielded 

 before the facts which these experiments brought to light, 

 to the still more extraordinary nud daring project of 4 

 colossal hollow beam, having within itself not only self- 

 supporting powers, but a sufficient excess of strength to 

 carry the weight of nearly a dozen railway trains. Beyond 

 this, the experiments gave the rough outline to the system 

 now under consideration, and which has already received, 

 in an extended application, the sanction and approval of 

 practically scientific men, and the confidence of the public. 



The Millwall experiments not only successfully realized 

 those objects, but they made us acquainted with other 

 constructions of equal if not even greater importance, in the 

 jievelopment of the tubular girder system, which is admi- 

 rably adapted for almost every description of bridge ; and, 

 beyond comparison, infinitely more extended and more 

 general in its application, than the form of tube which now 

 spans the depths of the Menai Straits. It is this girder con- 

 struction which I am anxious to bring before the meeting, 

 in order to explain its peculiar adaptations, and to receive 

 those suggestions for its improvement, which, I am satisfied, 

 will be freely given by the members of this Society. 



It waiS determined by the experiments, that, in order to 

 balance the two resisting fwces of tension and compression 

 in a wrought-iron tubular girder having a cellular top 

 {as shown in the plate)^ that the sectional area of the bottom 

 should be to the sectional area of the top, as 11 : 12; 

 and the proportional of these parts being thus established, 

 it therefore follows, that any increase to one or other of 

 them will not materially affect the strength of the bridge. 



