186 MR. WILLIAM FAIRBAIKN ON 



in order to meet all the requirements of varied strain to 

 which horizontal girders of this kind are exposed. 



Viewing the question in this light, it appears preferable 

 to adhere to a general formula, and to give to the artificer 

 a simple rule of extensive application, such as he may 

 safely use without entering upon theoretical investigation, 

 which more properly belongs to the mathematician than the 

 man of practical science. 



In offering these remarks, I am far from underrating the 

 manifold advantages which we derive from the theoretical 

 disquisitions of the mathematician. Every investigation for 

 the elucidation or correction of existing formulae by the test 

 of the exact sciences must be highly valuable ; but having 

 corroborated certain facts by repeated trials and experiments 

 on a large scale, and having found the formula from which 

 the calculations were made, apply with remarkable precision 

 to almost every extent of span, I am strongly inclined to 

 adhere to its truth, and to place implicit confidence in the 

 construction obtained from such a source. I hope, however, 

 that the time is not far distant when we may receive from 

 some able mathematician a preferable and more accurate 

 formula, if such can be obtained. 



It may, however, prove instructive if we examine this 

 question more closely, and endeavour to ascertain the real 

 value of the additional strength thus imparted to each suc- 

 cessive span by the continuous girder, and, for the sake of 

 illustration, let us take the design of the bridge before us,* 

 which has three spans, the middle being double the width 

 of the two end ones, and consequently required to support 

 double the weight. Now, it is evident that any considerable 

 weight laid upon the centre of the large span of 250 

 feet, will cause a deflection ; and, supposing the depth of 

 the girder at the pier to be 14 feet, we then have 125 feet, 

 or half the span, as the distance of the point of greatest de- 



* The design for a Tubular Girder Bridge for supporting the Dublin and 

 Belfast Junction Railway across the Bojae at Drogheda. 



