VI 11 PREFACE TO FIRST EDITION. 



When engineers Bhall have become convinced of the fact that 

 there is in the continuous girder a theoretical saving of mate- 

 rial amounting usually to from 25 to 30 per cent, per truss, and 

 in the extreme case even reaching as high as 50 per cent., as 

 compared with the simple girder ; and that the main objection 

 which can be urged viz., the influence of small variations in 

 level of the supports has, when properly considered, no force 

 whatever, we shall probably hear less often of designs contem- 

 plating many successive and independent spans of considerable 

 length such as, for instance, for a bridge over the Hudson at 

 Poughkeepsie, consisting of five separate spans of 525 ft. each. 



The present work contains the only complete graphical and 

 analytical presentation of this subject in English professional 

 literature, and should it succeed in directing more general 

 attention to what has been done, will not have been in vain. 

 Almost the whole of Chap. XIII. is entirely new and consti- 

 tutes an important advance in the treatment of the subject. 

 Those acquainted with the old method will, we think, be pleased 

 with the simplicity and comprehensiveness of the new formulae. 



By their aid, indeed, we can solve with ease problems which 

 could hardly be attempted otherwise. In this connection the 

 list of literature upon the continuous girder appended to Chap. 

 XIII. may also be of service. 



We notice with pleasure in this direction the admirable little 

 treatise of Clemens Herschel^ C.E., upon draw spans.* This 

 subject is at least of admitted practical value, and we have 

 treated it with a fullness which, in our opinion, leaves little to 

 be desired. We have borrowed from the above work the con- 

 ception of the "Tipper" or draw with secondary span, which 

 is both new and, as it would seem, most adequately represents 

 the true state of the case, and alluded to the idea, also original 

 with Mr. Herschel, of weighing off the reactions at the supports 

 of a continuous girder, instead of measuring the differences of 

 level. In this case, as in that of the continuous girder gene- 

 rally, we have clearly brought out the met/tod of calculation by 

 apex weights, and here, indeed, lies the whole secret of thorough 

 practical solution. In fact, from this point of view, the com- 

 plete solution of a continuous girder for any number of spans. 



* Continuous, Revolving Drawbridges. Little, Brown and Company, Boa- 

 ton, 1875. 



