Demonstrations by means of models

William Hamilton
1847 Journal of the Franklin Institute  
At the monthly meeting of the Franklin Institute, held May" 20th, 1847, the following statements and computations relative to Mr. Herron's model of a railroad bridge, which the inventor then exhibited, were made by Prof. It/. R. Johnson: Mr. Herren had stated that his bridge was intended to have a clear opening of one hundred and fifty feet, a height of sixteen,and a breadth of twenty.two feet,and to bear a load equivalent to two trainsoftwentyton locomotives placed close together throughoutits
more » ... length, and which he supposed would be equivalent to two tons per foot of length of the structure. He also stated that the model was constructed on the scale of one inch to a foot, and that the proportion of parts had been preserved throughout. The model had been weighed just before the meeting, and 150 inches of its length were found to weigh 1~6.3 pounds. He also mentioned that it had been subjected for many hours to a weight of 3o80 pounds distributed over its length, and to more than 1700 pounds accumulated near the centre. This latter load had been accompanied by rather violent motions, on the model, of a part of the load, consisting of the weight of two or three heavy persons, who, by rising a short distance and then coming do~ta with nearly their whole weight, were able to commtmieate shocks more than proportional to those which the large structure could receive from the passage of trains. Prof. Johnson observed, at the commencement of his remarks, that the plan of solving questions in practical mechanics and engineering by faithfully constructed models, presented the very obvious advantage of substituting the moderate cost of experiment for the often burthensome, sometimes ruinous, expense of experience. This method has wisely been adopted by the engineer entrusted with the building of a railroad bridge over the celebrated Menai Strait; and the liberal expenditure understood to 'have been made under the control of Messrs. Fairbairu and Hodgldnson, to prove experimentally the value of different plans of construction for rigid iron bridges, proves the estimation in which this method of solving practical questions is held among the engineers and practical mechanics of Great Britain. The eonditions to be fulfilled il~ constructing models so as to give reliable information in regard to the action or the stability ofstruclures, were stated as fol[ows'~ 1. An entire correspondence in the model, at least of all essential parts, to the scale of dimensions and weights on which it is proposed to represent the str~tcture. 2. Identity not only in the nature, but also in the condition, of materials employed in the model and structure respectively. 3. Proportional accuracy in forming junctures; and proportional tensions given by tightening-screws, keys, wedges, and other mechanical means by which the parts are compacted together.
doi:10.1016/0016-0032(47)90501-2 fatcat:nsn53ochljdq3ogq4gknwhzkxe