PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 899 



eventuully leading to destruction. This stiffness must, in some 

 way or other, be supplied in oi-der to make the suspension bridge 

 useful, and it is done in various ways, depending, of course, upon 

 the nature of the Avork that it has to do, whether it is to be used 

 for the purpose of the railroad, viaduct, or aqueduct. Weights, 

 girders, trusses and stays all come into play to attain this end, 

 and accomplish it too successfullj-, when used together in a judi- 

 cious combination. Mr. Roebling's bridge, over the Niagara river, 

 is the best example extant of what may be done, and was a grand 

 triumph in the face of the sneers of all the European engineers 

 in its adaptation to railway purposes. I would that I had time to 

 take you through all its beautiful construction and show how 

 exquisitely Mr. Roel:tling has provided for every contingency, for 

 it is a Avork of which all Americans should be proud ; Init it would 

 lengthen out a paper that, perhaps, is already too long for you, and 

 I forbear. For hick of the above mentioned combination, the 

 Wheeling bridge of 1,000 feet span was blown completely over 

 by a gale a few years back. Here weight alone was relied upon, 

 which acquired such a momentum by the constant heaving and fall- 

 ing of the phitform, that it became really an element of weakness. 

 In the Niagara bridge, above mentioned, there are trussed girders, 

 under and overfloor stays, which assist in multiplyhig stationary 

 points. This idea of stationary points must always be kept in 

 view in designing a suspension bridge, for the most rigid bridge 

 is the one that has the greatest number of permanent points. 

 Although a suspension bridge satis factor ih'- answers most of the 

 requirements of our case, still there arc grave considerations to 

 be disposed of before it could be recommended for the end in 

 view. Suspension bridges usually cross ravines, the high ground 

 on either side forming the base of the land towers. In the case 

 before us we have no natural height thus provided for us, the land 

 on either side being very low, so that we must have enormous 

 masses of masonry to withstand the great pressure of the towers 

 themselves, and the load that they have to support. Then, again, 

 the land caljles strike their anchorage a long distance into the 

 heart of the city, necessarily destroying much invaluable property. 

 Not alone in their angle do they a1)sorb room, but if you will 

 think for a moment of the weight and extent of masonry required 

 to hold them fast and secure, you Avill readily understand that this 

 encroachment is a matter of considerable moment. Let us take a 

 span of 1,350 feet, with double roadway and footwalks, and see 



