TUBULAR GreDER BRIDGES. 191 



at a high velocity, is a subject of such vital importance to 

 the permanency and stability of the structure, as to require 

 the most careful investigation. It cannot therefore be sur- 

 prising that it should have occupied a considerable portion 

 of the time of the commissioners, and that it should have 

 found a prominent position in their report. 



It must, however, be observed, that the deflection of a 

 girder bridge arises from one of two causes, or from both. 

 First, from the weight of the bridge itself, which is a 

 constant producing a permanent deflection ; and, secondly, 

 from the passing load, whether viewed as a dead or a rolling 

 weight, acting as an antagonistic force to the resisting power 

 of the bridge. 



In some parts of the commissioners' report, the experi- 

 ments do not appear to me to bear out the facts of increased 

 deflection produced by a body, such as a railway train moving 

 at great velocity, and the same body remaining stationary, 

 upon the bridge. In several carefully conducted experi- 

 ments on tubular girder bridges of different spans, some 

 of them upwards of 150 feet, I found the deflection as 

 nearly as possible the same at all velocities; and, although 

 the experiments recorded by the commissioners are highly 

 valuable, they do not afford to the general practitioner 

 those conclusive results which seem to be essential for the 

 attainment of sound principles of construction. It is true, 

 the commissioners in their report have qualified the results 

 obtained from these experiments by others upon existing 

 cast-iron railway girder bridges, where the deflection was 

 reduced from an increase of the statical deflection, amount- 

 ing to T^ths of an inch, as produced upon the nine feet bars, 

 at 30 miles an hour, to | upon a bridge of 48 feet span, at 

 50 miles an hour, clearly showing that the larger the bridge, 

 and the greater the rigidity and inertia of the girders, the 

 greater will be the reduction of deflection to the passing 

 load. In the tubular girder bridges composed of riveted 



