894 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



either succeed or fail in ansv/ering them. We must bear in mind 

 that a bridge at any site must cross a broad river with rapid and 

 variable currents, separating two large cities with concentrated 

 and growing populations, in constant and intimate intercourse Avith 

 each other, besides bearing upon its surface a vjtst commerce 

 which involves in its prosecution myriads of vessels, from the small 

 oyster boat to the stately, vessel employed in the China trade. 

 From these considerations, we deduce four leading points to start 

 from. These are : Firsts that the navigation of the river shall 

 not be interfered with, thus determining the height of superstruc- 

 ture above high water. Second, that the river channel shall be 

 contracted to a minimum amount, involving the necessity of long 

 spans. Third, that property shall be interfered with and des- 

 troyed as little as possil)le in both New York and Brooklyn. 

 Fourth, that there must be at least two carriage ways, and tAvo 

 broad footwalks, one on either side. We might add aJiftJihj, the 

 minimum cost that will accomplish these results. That bridge 

 which will satisfactorily answer all these requirements is clearly 

 the one that should be adopted. It is an easy matter to sit down 

 with a sheet of paper before one, and with a few strokes. of a pen- 

 cil jumping the river with a single arch, or perhaps strike oft' a 

 suspension bridge of fabulous span. I must remind projectors of 

 such Quixotic schemes that there are certain laws that will not be 

 violated in actual practice, however they may be set at defiance 

 upon paper. Do what we will, we cannot change the fixed prin- 

 ciples of statics and the equilibrium of forms, nor the physical pro- 

 perties of materials of construction, any more than we can change 

 the four seasons into their opposites. The engineer has to use 

 what nature has given him, and can only bend to his service the 

 maximum faculties of the workshop and foundry. So in a mea- 

 sure he is limited at the start by the knowledge of the practical 

 part of his profession, and he has more sense than to retire to his 

 office or study and prepare a design, no matter how correct theo- 

 reticall}', that transgresses what he knows to be feasible. 



For the sake of convenience, if you please, Ave will consider our 

 subject in the folloAving order : The Tubidar Girder; The Trussed 

 Girder; The /Suspension Bridge; and lastly. The Suspension 

 Trussed Girder. You will notice in this classification I have left 

 out the arch system altogether, since its want of adaptibility to the 

 crossing of the East river strikes even the unprofessional man at 

 once, and I do not care to enter into a useless discussion. 



