Suspe7ision Bridges and other Structures. 389 



niercial importance, but also because, in these very light and 

 slender structures, we are so near to the limit of possible strength, 

 that the mere addition of weight to the parts, instead of 

 strengthening, is certain to weaken them, by increasing the load 

 to be borne, and the momentum of the mass, and so increasing 

 equally the danger of oscillation instead of diminishing it ; and 

 therefore it is peculiarly necessary in this case to obtain the 

 greatest possible strength to resist injury, with the smallest 

 amount of weight. The means of doing this is, therefore, the 

 principal object of this paper. 



The original structure ^ig- 3. 



was of red pine scantling, 6 

 in. X 6 in., and about 80 

 feet high. Figure 3. will 

 illustrate its construction. 

 There are four beams cut 

 from a log, as vertical 

 supports at the corners ; 

 these being stayed in the 

 middle by cross bars, and 

 the whole diagonally fram- 

 ed for the purpose of sta- 

 bility and stiffness. A B G 

 D is one of the sides of the 

 framing, and the stays are 

 at mnop, the middle of 

 the structure ; and the 

 top of the structure was 

 fixed by ropes attached to 

 surrounding trees. 



This structure was perfectly adequate to the temporary pur- 

 pose for which it was erected, and its extreme slenderness en- 

 abled me to watch the nature of its oscillations. The under 

 part of tlie structure was sheltered by trees and high banks, 

 while its upper part was exposed to the wind in every direction. 

 When the wind began to act on the top of the framing, it pro- 

 duced an oscillation between A and ?/?, as I have represented in 

 fig. 4, and not only there, but it propagated a simultaneous 

 and almost equal current of oscillation to the part B m below 



