222 MEMOIK OF EATON HODGKINSON. 



more confidence than I othenvise sJiouJd liave done." (See vol. i, p. 35, of the 

 " Britannia and Conway Tubular Bridges/' by E. Clark, esq.) 



This declaration of Mr. Stephenson completely disarms all future praise or 

 detraction with respect to the part which Mr. Hodgkinson took in the execution 

 of the tubular bridg-es. It places him before the public in his right position as 

 a most important contributor to the success of an enterprise Avhich will represent 

 the engineering skill of the present time, and will be the admiration of future 

 ages. E. Clark, esq., who superintended the building of the tubular bridges, 

 speaks in the highest terms of the importance of Mr, Hodgkinson's labors in 

 fixing the proper dimensions of the bridges. 



We are indebted to him also for nearly the whole of the mathematical calcu- 

 lations in reducing the experiments which were made into a form fit for applica- 

 tion to a large structure. But we are also indebted to Mr. Faii'bau'n for a great 

 portion of the practical construction of the bridges. 



The answers given by Mr. Hodgkinson to his inquiries, and which rendered 

 such signal service to the engineer in the execution of his novel design, are as 

 follows : 



1. The value of ffj the strain upon a square inch at the top or bottom of the 

 tube is constant in material of the same nature, while it varies from 19, 14, to 

 7| tons when the thickness of metal varies from .525, .272, to .124 of an 

 inch. The determination of ffJ is the chief obstacle to obtaining a formula for 

 the computation of the strength of tubes of every fonn. 



The strength of the Conway tube was calculated to bear 1,084 tons when the 

 value of ffJ was taken at 8 tons, and the deflection about 15^ inches in the 

 middle. 



2. The strength of similar tubes was somewhat lower than the square of their 

 linear dimensions, being about 1.9 power instead of the square. 



3. The tubes may be reduced in strength and thickness towards the ends, 

 corresponding to the ratio indicated by theory, viz., that the strain at any point 

 of the tube is proportional to the rectangle of the two parts into which that 

 point divides the length of the tube. 



4. The power of the tube to resist a vertical strain is to its power to resist a 

 strain on its side, as from the wind, as 26 to 15, nearly. 



5. The resistance of tubes to crushing follows the law of cast-iron pillars 

 when the crushing force is not more than 8 tons per square inch. It appears, 

 however, that cast iron was decreased in length double what wrought iron was 

 by the same weight ; but the wrought iron sunk to any degree with a weight of 

 12 tons per square inch, while cast iron required double the weight to produce 

 the same eflFect. 



6. The power of plates to resist buckling varies nearly as the cube of the thick- 

 ness. Mr. Clark refers to this property as being most useful in the construction 

 of the tubular bridge . 



7. The tube bent by pressure had borne a deflection of five inches without 

 serious inj my; but its riveting was destroyed by repeated impacts deflecting it 

 through less than one inch. 



8. Resilience is perceptible, but very small. 



9. The introduction of cast iron on the top of the tube would be attended with 

 advantage in resisting the force of compression. Practical objections, however, 

 of a serious nature prevented Mr. Stephenson from availing himself of the power 

 of cast iron to resist compression. He thought it advisable to increase the thick- 

 ness of wrought iron to resist compression, rather than use a combination of 

 wrought with cast iron. It may be stated that Mr. Stephenson has used cast 

 iron, for the purpose recommended by Mr. Hodgkinson, with success in tubes of 

 smaller dimensions than the Conway tubes. 



In 1847 Mr. Hodgkinson was appointed one of the commissioners to inquire 

 into the application of iron to railw.ay structures ; and during the space of two 



