824 Transactions of the American Institute. 



the application of pendulums to support the cal)les on the pillars, 

 allowing expansion and contraction of cables, are ideas of the 

 famous engineer Brunei. These applications are made at Brunei's 

 bridge, on the Island of Bourbon, and are copied at the suspension 

 bridge across the river Meuse, at Liege, in Belgium, and at the 

 Allegany bridge near Pittsburg. 



Telford, the engineer of the Menai Straits suspension bridge in 

 England, as far as I can learn, added the improvement of saddles 

 on rollers supporting the cables or chains, to allow for the move- 

 ments in case of change of temperature. The railing of this bridge 

 was proposed six feet high, the span being five hundred and sixty 

 feet. 



I have mentioned that by the French government, in the year 

 1821, Navier Avas sent to England to study the existing suspension 

 bridges and to report on the same. He collected the diflerent 

 historical notices, described Finley's and the English bridges, and 

 gave the theory of movable suspension bridges in his classical book, 

 published in the year 1823. The curve of the suspension cables 

 when loaded, Navier first regarded as parabola, rejecting the com- 

 plicated older formulae as given by Jacob and John Bernouilli, 

 Huyghens, Euler and others. It is to be regretted that this cele- 

 brated and highly scientific man, overlooked the influence of wind 

 acting from below on the floor, and thus becoming the most dan- 

 gerous element to destroy suspension bridges. Indeed, Navier's 

 theory and his reputation contributed much to the false judgments 

 about Finley's improvements, and caused the error of introducing 

 small deflections and of omitting the American longitudinal beams 

 and railings, these important stiffening elements, which, in the 

 right proportions, are inseparable from any good supension bridge. 



Finley expressly advocated the construction of platforms of one 

 piece, but Navier and his followers supposed a perfectly movable 

 floor. Finley applied 4, Navier -^^ to j^g^ of the spans for deflec- 

 tions. Finley prevented the undulations by his stiflf platform; 

 Navier believed he could prevent greater deplacements by flat 

 catenaries. This is the great antithesis of these two celebrated 

 engineers, and every scientific bridge builder must concede that 

 Finley stood much nearer to the perfect solution of this problem 

 than Navier. 



But notwithstanding Navier's book was of great value to science, 

 and he opened for investigation many interesting questions. 



Navier paid attention to the eflfect of change of temperature on 



