Improvement in the Construction of Bridges, ^'c. 2S9 



ber of different principles in bridge building used, than at present ; 

 for although a very indifferent principle, or execution of a princi- 

 ple, or even both, will answer a considerable purpose for a time, 

 for bridges of 75 to 120 feet spans, yet, it is always apparent very 

 soon, and beyond all question, whether the principle or execution 

 of bridges having spans of 200 feet or more, are sufficient or in- 

 sufficient ; here is no room for doubt — no disguise ; the principle 

 and practice, both, must be good, or the defect will soon exhibit 

 itself in some shape not to be misunderstood. The reason of 

 this difference between large and small spans is evident — it is for 

 the same reason that a model of some modes of building bridges 

 may have considerable strength, and appear to many to be good, 

 yet when executed fidl size, will either fall down when the 

 stages are removed, or soon thereafter. Perhaps the most obvi- 

 ous explanation of the reason cf this fact may be thus explained, 

 viz. suppose a piece of pine wood, half an inch square and 15 

 feet long, supported at the two ends, and resting in a horizontal 

 position ; it is easy to perceive that it would have strength to siis- 

 tain its own weight, and probably something more. Conceive 

 this to be an exact model of another stick of the same kind, the 

 dimensions of which should be every way increased in a twenty 

 fold ratio, viz. 300 feet long, and 10 inches square ; let this stick 

 be supported at the ends, as the model of it was, and what would 

 be the result ? Nay, cut it into three pieces of 100 feet each, 

 and would they, if supported in the same manner, bear their own 

 weight ? Most certainly not. 



Thus, then, the idea or belief that models are good representa- 

 tions of the strength of bridges when built, is erroneous in the 

 extreme, and leads to sure disappointment and the destruction of 

 property. Models of bridges only show the relative strength, or 

 merit of diff"erent modes or principles ; this they show pretty ac- 

 curately, when made to the same scale, to the same width of 

 spans, of the same materials, and in all other respects similar. 

 Perhaps no one error has done more mischief, in the hands of un- 

 scientific and ignorant mechanics, than the misunderstanding of 

 the nature and real use of models, in illustrating the strength and 

 goodness of bridges. Millions have been sacrificed in this coun- 

 try, either in this manner, or in a way so similar as not to need a 

 nicer distinction. '^ 

 Vol. xxsviii, No. 2.— Jan .-March, 1840. 37 



