Improvement in the Construction of Bridges, §c. 289 
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 full size, will either fall down when the 
Stages are removed, or soon thereafter. Perhaps the most obvi- 
ous explanation of the reason of 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 sus- 
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 disappoifitment and the destruction of 
property. Models of bridges only show the relative strength, or 
merit of different modes or principles; this they show pretty ae- 
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, xxayn, No. 2.—Jan.—March, 1840. 37 
