186 MEMOIR OF THE LATE PROF. E. HODGKINSON, F.R.S. 



tubular bridges^ speaks in the highest terms of the import- 

 ance of Mr. Hodgkinson's labours in fixing the proper di- 

 mensions of the bridges. 



We are indebted to him also for nearly the whole of the 

 mathematical calculations in reducing the experiments 

 which were made into a form fit for application to a large 

 structure. But we are also indebted to Mr. Fairbairn 

 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 (/) 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 J tons when 

 the thickness of metal varies from '525, '272, to •124 

 of an inch. The determination of (/) is the chief obstacle 

 to obtaining a formula for the computation of the strength 

 of tubes of every form. 



The strength of the Conway tube was calculated to bear 

 1084 tons when the value of (/) 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 



