TTIBULAB GIRDER BRIDGEl 181 



as correctly as possible, to apply those principles; and having 

 a strong conviction of the great superiority of strength^ 

 durability, and cheapness which the system offers in com- 

 passing large spans, I have not hesitated to advocate its 

 extension. It, however, becomes necessary, from time to 

 time, to submit the bridges to a rigid examination ; and, 

 before opening any one of them as a public thoroughfare, 

 it is essential to submit them to severe and satisfactory tests. 

 These tests and examinations have been various and fre- 

 quent ; and I believe we may venture to affirm, that in no 

 case where the Tubular Girder Bridge has been duly pro- 

 portioned and well executed, has there been the least reason 

 to doubt its security. 



" The first idea of a Tubular Girdier Btidge originated in 

 the long preliminary experimental research which I con- 

 ducted, in connection with the great bridges on the line 

 of the Chester and Holyhead Railway ; and, during its first 

 application to railway constructions, the utmost precaution 

 was observed in the due and perfect proportion of the parts. 

 These proportions were deduced from the experiments made 

 upon the model of the Britannia Tubular Bridge at Mill- 

 wall, London ; and after repeated tests upon a large scale 

 (full size), the resisting powers, and t)ther properties of the 

 bridge, were fully established. From these experiments, 

 a formula was deduced for calculating the ultimate strength 

 of this tubular description of bridge, having spans of from 30 

 up to 300, or even 1000 feet; and, as that formula is now 

 before the public, I believe it may be relied upon as practically 

 correct To relieve it, however, from any thing like ambi- 

 guity, I shall endeavour to state as briefly as possible, 

 certain points which, in my opinion, should be taken into 

 consideration in its application." 



Experiments were made on a large scale to determine 

 the accuracy of my views, and to ascertain the best and 

 strongest form of tube as a means of supporting the Chester 



